Abstract: Plants with potential therapeutic value have been used from time immemorial to cure various ailments and infectious diseases. Secondary metabolites or the bioactive compounds (phytochemicals) present in plants have been reported to be accountable for various observed biological activities. Consumer awareness of the possible side effects of using chemical-based antimicrobial agents has forced researchers to identify and explore natural plant-based antimicrobial agents (or preservatives) that are toxicologically safe, especially when used in food applications. Of late, scientific evidence has been provided on the potential antimicrobial activities exhibited by certain traditionally used flower extracts or their essential oils (edible and wild). This review focuses on providing and updating available information on the antimicrobial activities exhibited by flowers, which are envisaged to find potential applications as natural preservatives for foods or applications in the pharmaceutical industries to develop new and economical herbal-based products for treating various diseases.
Infectious diseases and foodborne illnesses can cause severe health effects and can even lead to death among the residing population, especially in the developing regions of the world. The continual emergence of antibiotic-resistant microorganisms has prompted researchers’ world over to search for new antimicrobial agents that are more effective against the resistant microbial pathogens (Nascimento and others 2000; Thaller and others 2010). Structural modification of the antimicrobials (against which microbial resistance has been developed) is reported to improve the effectiveness of antimicrobial agents against bacteria, fungi, and viruses (De Clercq 2001; Poole 2001; Jeu and others 2003; Zhang and others 2010). However, of late, research efforts have been put forth to improve the effectiveness of antimicrobial drugs by developing novel and a new class of antimicrobial drugs that can effectively work on multitargeted sites or organisms (Esterhuizen and others 2006; Alka and others 2010).
Traditionally, plants with potential therapeutic or medicinal values have been successfully utilized for preventing and treating various ailments and foodborne illnesses. Since time immemorial, various plants and their products have been used in traditional medicine to cure some of the common disorders and degenerative diseases in humans as well as in animals (such as Ayurvedic and traditional Chinese medicinal practices). The effectiveness of these procedures has been attributed mainly to the presence of active phytochemicals or bioactive compounds in plants (Quarenghi and others 2000; Ye and others 2004; Zhang and Zhang 2007; Dung and others 2008; Zhao and others 2009).
Given the scope of searching new antimicrobial agents, antimicrobials derived from plant materials are often regarded as natural and safe compared to industrial chemicals. Of late, plant-based medicine has become more popular due to the increasing concern of consumers with regard to the use of synthetic chemical preparations and use of artificial antimicrobial preservatives, especially in modern food protection practices (Marino and others 2001; Hamedo and Abdelmigid 2009).
Some of the hoped-for advantages of using natural antimicrobials include: reducing total dependence on antibiotics, reducing development of antibiotic resistance by pathogenic microorganisms, controlling cross-contaminations by foodborne pathogens, improvizing food preservation technology, and strengthening immune system in humans (Abou-taleb and Kawai 2008; Fisher and Phillips 2008; Tajkarimi and others 2010). Today, growing market trends indicate a rapid increase in the number of natural plant-derived products (such as green tea, herbal decoctions, or herbal medicines) that may include aerial parts, seeds, fruits, roots, rhizomes, and flowers. Among these, flowers have attained high priority and found various applications. Floral extracts and their isolated essential oils are traditionally believed to be rich in phytochemicals exhibiting rich bioactivity. These compounds are of interest to the local industry as well as to the general population and are actively being explored for various commercial applications (such as tea, bakery products, and more). Floral extracts and essential oils are also considered to be potential natural antimicrobial agents. Available reports indicate their efficacy and to possess a broad spectrum of antimicrobial activity against various spoilage and pathogenic microorganisms, which is attributed to their bioactive constituents (Quarenghi and others 2000; Ye and others 2004; Zhang and Zhang 2007; Dung and others 2008; Zhao and others 2009). Based on these facts, the present review focuses mainly on providing baseline information on exploring some of the common and wild (edible and nonedible) flowers possessing potential antimicrobial activities. The details on these aspects are hopefully expected to be useful for the commercial exploitation of flowers to develop natural preservative preparations with applicability in the food and pharmaceutical industries.
Solvent extraction is one of the most widely employed methods for preparation of flower extracts. Solvent extraction (solid-liquid extraction) involves the process of leaching (simple physical solution or dissolution process). Leaching is a separation technique that involves removal of soluble solids from a solid mixture by employing a suitable solvent or solvent mixture. Various factors influence the solvent extraction procedure, which includes: the rate of transport of solvent into the material, rate of solubilization of soluble constituents in the solvent, and the rate of transport of solution (extract) out of the insoluble matter. Solvent polarity, vapor pressure, and viscosity are also of importance for effective extraction. In case of plant materials, adequate time is required for diffusion of solvent via plant cell walls for dissolution of soluble constituents and for diffusion of the solution (extract) out to the surface of the cell wall (Houghton and Raman 1998; Singh 2008; Wijekoon and others 2011).
Flower extracts can be prepared either from fresh or dried samples. Prior to extraction, flower samples are subjected to air-drying or freeze-drying, followed by grinding, milling, or homogenization to reduce sample particle size. These procedures are followed in order to enhance the efficiency of extraction process and yield of the resulting extract. Various solvents, such as methanol, ethanol, hexane, acetone, ethyl acetate, chloroform are commonly used for extraction (either in the pure form or after dilution with distilled water) (Dai and Mumper 2010). Choice of selecting a solvent mainly depends on the solubility of the bioactive constituents, safety aspects, and potentials involved for artifact formations (Jones and Kinghorn 2005). Maintaining the stability of bioactive compounds is vital while selecting an appropriate and efficient extraction method as some of the compounds (mainly those of phenolics) tends to get oxidized and degraded at high temperature or on prolonging the extraction time (Robards 2003; Dai and Mumper 2010). Besides, an optimized value of “sample-to-solvent” ratio needs to be standardized, which involves equilibrium between avoidance of saturation effects, solvent wastes, and costs incurred (Pinelo and others 2006; Dai and Mumper 2010). Magnetic stirring and continuous rotary shaking are also employed in certain cases to enhance molecular interactions during extraction process. Usually, to ensure maximum extraction of bioactive compounds, the extraction process is repeated 2 or 3 times and the extracts are pooled together (Guillén and others 1996; Stalikas 2007). Followed by this, the extracts are filtered and centrifuged to remove any floating particulate matters. In order to prevent formation of artifacts and degradation or polymerization of phenolic compounds, flower extract should not be stored in the solvent at room temperature or exposed to direct sunlight for a long time duration. Once done, extracts are freeze dried or concentrated at reduced pressure (temperature preferably ≤ 40 °C) in a rotary evaporator to prevent degradation of heat-sensitive compounds.
Solvent extractions are classified into 2 methods: continuous and noncontinuous. In continuous extraction method (such as percolation, soxhlet extraction), solvent flow through the sample continuously and the saturated solvent is constantly replaced with a less saturated solvent. In noncontinuous method (such as maceration, infusion, decoction), the extraction is stopped when a suitable equilibrium is reached between the solute concentration (inside the flowers and the solvent), unless the solvent needs to be replaced with a new batch of solvent (Jones and Kinghorn 2005).
This is an efficient method wherein a percolator is used for extraction. Percolator is comprised of a wide opening (at the top) to accommodate addition or removal of a sample along with a valve at the bottom, designed to allow outflow of the solvent. With the valve held at a closed position, samples in powdered form are added and packed into the percolator leaving sufficient space to allow expansion. Then the samples are covered by addition of a suitable solvent, and are allowed to soak for few hours or overnight. Further, the solvent is allowed to flow out at a controlled flow rate from the bottom of the percolator through the valve. Fresh solvent is added at the top to replace the saturated solvent “flow-out” from the percolator (Jones and Kinghorn 2005; Singh 2008).
Soxhlet extraction is a common conventional method used for extracting heat-stable compounds. The Soxhlet extractor consists of a distillation flask, an extractor, and a condenser. The solvent in the distillation flask is heated and the resulting vapor is condensed in the condenser. The condensed solvent from the condenser fills into the thimble-holder containing the sample that needs to be extracted. When the solution in the extractor reaches the overflow level, a siphon aspirates the solution of the thimble-holder and unloads it back into the distillation flask, carrying dissolved solute into the bulk liquid. The solute is left in the distillation flask while the solvent is evaporated, condensed, and passed back into the sample solid bed. This process is repeated 3 to 5 times or until a complete extraction is achieved (Tandon and Rane 2008).
This method is routinely employed in the labs wherein a conical flask covered with aluminum foil or parafilm is used to prevent evaporation of the solvent to avoid batch to batch variations. The powdered sample is left to macerate for a known period after addition of a suitable solvent. The maceration process is considered to be rather slow, and sometimes requires occasional or continuous shaking (or stirring), as it works by molecular diffusion. Occasional shaking ensures dispersal of saturated solution around the particle surface, bringing fresh solvent to the surface of particle for further extraction. After maceration, the extract is filtered through an appropriate filter or screen. In certain instances, the solid residues are pressed and the occluded solutions are pooled with the extract before filtration (Jones and Kinghorn 2005; Singh 2008).
Infusion is a dilute solution that contains readily soluble constituents prepared by short period of maceration (steeping) of sample in cold or boiling water. Cold water is recommended to be used for extraction of heat-sensitive compounds. It is highly crucial to dispense the infusion within 12 h of its preparation as it is liable for microbial contamination (Singh 2008).
Decoction is the most widely used and popular traditional method for the preparation of aqueous extracts of medicinal plants. It is made by boiling the sample in water for a period of fixed time duration (Tandon and Rane 2008).
Extraction of Essential Oils
Hydrodistillation is the simplest and oldest method for obtaining essential oils from plants. In this method, samples are packed in a distillation unit with addition of water. This is brought to a boil by applying mild heat (water distillation); alternatively, live steam is injected into the sample (direct steam distillation). Essential oils are liberated from oil glands present in the plant tissues (due to effects of hot water and steam). The vapor mixture of water and oil is condensed, when it is carried over to the condenser. From the condenser, the distillate flows into a separator, where the essential oil is separated automatically from the distillate water. Laboratory-scale isolation of essential oil from flowers is accomplished by hydrodistillation with a Clevenger apparatus. In this method, water distillation is used wherein samples loaded in the apparatus are completely immersed in water, and brought to a boil (Handa 2008).
However, there are also other physical methods that are used in conjunction with these methods, such as ultrasound treatments, radiation treatments (UV, Gamma, or electron beams), supercritical carbon dioxide extraction and others, which have been found to be beneficial for better extraction of bioactive compounds.
Methods for determining antimicrobial activity of floral extracts and essential oils
Various conventional methods are routinely employed for determining the antimicrobial activity of floral extracts and essential oils. Generally, in vitro assays are employed. The agar diffusion method (paper disc or well) and dilution method (agar or broth) are the 2 most common techniques used.
Agar diffusion method
The agar diffusion method is one of the most widely employed techniques for evaluating antimicrobial activity. In this technique, agar plates are inoculated with test microorganisms (usually pathogenic microbes). Floral extracts or essential oils are applied directly onto paper discs, which are then placed on the agar medium or into wells made in the agar. The agar plates are incubated to allow the components of floral extracts or essential oils to diffuse into the agar medium. The diameter of growth inhibition zones around the discs or wells is then considered to be an indication of the effectiveness of the material being tested (Kalemba and Kunicka 2003; Holley and Patel 2005).
In this method, agar broth cultures (in Petri dishes or test tubes) and liquid broth cultures (in conical flasks or test tubes or by microtiter plate-broth microdilution method) are used for determining antimicrobial activities. The inhibitory effect of the extracts or essential oils are measured based on turbidimetry or the plate count method. The obtained result is expressed as growth inhibition index (percentage growth inhibition compared to the control cultures without extract or essential oil) or minimum inhibitory concentration, MIC (lowest concentrations of extract or essential oil that can inhibit the growth of microorganisms). In some of the reports (Dung and others 2008; Abdoul-Latif and others 2010) minimum lethality concentration (MLC, the lowest concentration of extract or essential oil that kills or totally inhibits a microorganism), minimum bactericidal concentration (MBC) or minimum fungicidal concentration (MFC) is computed. The microorganisms from agar broth or liquid broth where no growth occurs are observed and are transferred into a new medium and incubated for a certain fixed period of time. In some instances, MLC is considered to be a concentration that leads to >99.9% reduction in the number of microorganisms originally inoculated (Kalemba and Kunicka 2003; Holley and Patel 2005).
Floral Extracts and Their Essential Oils with Antimicrobial Activities
In Table 1, an overview is presented on some of the selected reports on edible flowers exhibiting antimicrobial activities. A schematic representation on the potential uses of edible flowers, their antimicrobial activities, and their applications as natural antimicrobial agents is depicted in Figure 1. Additionally, some common flowers with reported antimicrobial activities are shown in Figure 2. In the text below, the potential antimicrobial activities exhibited/reported on some floral extracts (in solvents) and their essential oil is discussed.
Table 1–. Some selected reports on antimicrobial activities of edible flowers.
Method of extraction/essential oil isolation
Major antimicrobial component
Allium spp. (Onion)
Blending at room temperature
S. flexinix, K. pneumoniae, B. subtilis, B. cereus, S. aureus, E. coli
Hexane, chloro-form, methanol, water: Not tested Ethyl acetate: T. mentagrophytes, T. simii, T. rubrum, E. floccosum, Scopulariopsis spp. Rhein: T. mentagrophytes, T. simii, T. rubrum, E. floccosum, Scopulariopsis spp.
Cleistocalyx operculatus (Roxb.) Merr and Perry (Water fairy flower)
Essential oil isolation: hydrodistillation with a modified Clevenger apparatus for 4 h. Solvent extraction: ethanol extraction (3×) at room temperature
Essential oil: B. subtilis, P. aeruginosa (FS); S. aureus, L. monocytogenes, E. aerogenes, S. Typhimurium, S. enteritidis, E.coli, E. coli O157:H7 (FB); S. aureus, S. epidermidis, E. coli, C. albicans (SP); MRSA; E. faecium (VRE); A. baumannii, E. coli, E. cloacae, K. pneumoniae, P. aeruginosa, S. marcescens, S. aureus (MARB) Ethanol: B. subtilis, P. aeruginosa (FS); S. aureus, L. monocytogenes, (FB); S. aureus, S. epidermidis (SP); MRSA; E. faecium (VRE); A. baumannii, S. aureus (MARB)
Allium spp. belongs to the largest genus (Allium, Alliaceae family) that is comprised of nearly 450 species and is found distributed widely in the northern hemisphere (Lonzotti 2006). However, most of the plants belonging to the Allium genus are consumed regularly in many Asia-Pacific regions. The plants and their parts are used in cooking because of their characteristic flavor, attributed due to sulfur-based compounds (Tada and others 1988). Evaluation of antimicrobial activity has been reported to support the therapeutic value of these species as anti-infective agents (Chehregani and others 2007).
The effective antimicrobial activities (of the aqueous extracts) of different parts of Allium spp. (bulbs, leaves, flowers, rhizomes) against pathogenic bacteria such as Shigella flexinix, Klebsiella pneumoniae, Bacillus subtilis, Bacillus cereus, Staphylococcus aureus, and Escherichia coli have been reported based on agar disc diffusion and serial dilution methods (Chehregani and others 2007). The reported diameter of inhibition zones for Allium atroviolaceum, Allium eriophyllum, Allium scabriscapum, Allium stamineum, Allium iranicum, and Allium shelkovnikovii ranged from 8.5 to 36.2 mm, 6.4 to 36.8 mm, 5.4 to 25.3 mm, 4.4 to 39.7 mm, 3.9 to 28.3 mm, and 0.0 to 27.8 mm. Moreover, the flower extracts of some Allium spp. (A. scabriscapum, A. iranicum, A. shelkovnokovii) exhibited much higher antibacterial activity than the bulb extracts with MIC values (ranging from 0.625 to 5.00 mg/mL, 2.50 to 12.50 mg/mL, and 2.5 to 10.00 mg/mL), indicating that the tannin accumulated in the flowers to have played a role in exhibiting the antimicrobial activities. While the bulbs of Allium spp. are known for their high antibacterial activities, the results of this study indicated that the floral extracts from Allium spp. also have high potential for use as antibacterial agents.
By employing the agar disc diffusion method, antimicrobial activity of galangal flower buds against both Gram-positive and Gram-negative bacteria have been tested. The effects of drying methods (oven drying and freeze-drying) and solvents (hexane and ethanol) on the antimicrobial activity have also been investigated (Hsu and others 2010). Galangal was shown to be effective against Gram-positive bacteria (Listeria monocytogenes and S. aureus) but exhibited little or no effect against Gram-negative bacteria (Salmonella spp., E. coli O157: H7, and Shigella spp.). Overall, antimicrobial activity of galangal was the highest for oven-dried samples extracted with ethanol (inhibition zone = 8.94 mm and MIC = 1.457 mg/mL) and the lowest for the freeze-dried samples extracted with ethanol (inhibition zone = 7.05 mm and MIC = 2.470 mg/mL). Due to its ability to inhibit the growth of Gram-positive bacteria, galangal flower buds have potential to be used as natural antimicrobial agent for preservation of perishable foodstuffs.
Anthemis cotula L. (Stinking chamomile)
Anthemis cotula (family: Asteraceae), a native of Europe and a weed that grows extensively in Argentina, is commonly known as Manzanilla del campo. Traditionally, A. cotula is believed to be effective for treatment of dysentery and gout. The decoctions made from the leaves and flowers are reported to exhibit insecticidal properties (Quarenghi and others 2000). The flavonoid constituents present in this flower have been reported to contain: quercetagetin, quercetagetin 7-glucoside, quercetin, quercetin 7-glucoside, patuletin, patuletin 7-glucoside, kaempferol, kaempferol 7-glucoside, and kaempferol 3-rutinoside (Quarenghi and others 2000).
Quarenghi and others (2000) employed the agar disc diffusion method to evaluate the antimicrobial activity of A. cotula (methanol extract) against some of the pathogenic microbes, such as S. aureus, Staphylococcus epidermidis, Micrococcus luteus, Streptococcus pneumoniae, E. coli, Pseudomonas aeruginosa, Proteus vulgaris, and Salmonella spp. Owing to the flavonoid compounds present in the flower, methanol extract at a concentration of 200 μg/mL was found to exhibit rich antimicrobial activities against the bacteria tested (except for S. pneumoniae and Salmonella spp.) with diameters of inhibition zones ranging from 6.0 to 9.0 mm.
Bombax buonopozense P. Beauv. (Gold Coast Bombax)
Bombax buonopozense (family: Bombaceae) is a large tropical tree in Africa (found in Ghana, Uganda, and Gabon). The plant grows up to 40-m high with large buttress roots, which are spread upto 6 m (Beentje and Sara 2001). This tree is also popular as “Vabga” in Ghana and “Kurya” in Northern Nigeria. The plant parts are used as food, as building materials, for extracting dye, and also as a source of clothing fiber. The decoctions prepared from leaves and roots are traditionally used to treat fever, muscle aches, pains, and stomach discomforts (Akuodor and others 2011).
Mann and others (2011) have evaluated antimicrobial activities of Bombax flowers against S. aureus, E. coli, and A. niger using the disc diffusion method. The results obtained clearly demonstrated antimicrobial activity of methanol, chloroform, hexane, and aqueous extracts against the 3 pathogens tested. However, the chloroform extract did not exhibit any activity against S. aureus and the aqueous extract showed no activity against A. niger. The results of this study were able to provide scientific evidence to support the traditional uses of B. buonopozense for curing microbial infections.
Cassia fistula Linn. (golden shower tree)
Cassia fistula (family: Leguminosae) is an ornamental tree found in various parts of China, India, Mauritius, South Africa, Mexico, the West Indies, East Africa, and Brazil. Various parts of this plant are used in the treatment of intestinal disorders, skin diseases, such as leucoderma, liver problems, tuberculosis, hemetemesis, diabetes, rheumatism, hypercholesterolemia, and diarrhea. The fruit pulp is used as a laxative, purgative, antipyretic, analgesic, and antimicrobial agent in Indian Ayurvedic medicine. Its flowers have been reported to exhibit antifungal activities (to treat skin infection) and are used to treat nasal infections in certain tribal sects (Perumal Samy and others 1998; Prashanth Kumar and others 2006; Duraipandiyan and Ignacimuthu 2007; Bhalodia and others 2011). Also, flowers have been reported to be useful in treating pruritus, burning sensation, dry cough, and bronchitis attributed to its demulcent, lubricating, cooling, and emollient effects (El-Saadany and others 1991; Duraipandiyan and Ignacimuthu 2007; Bhalodia and others 2011).
Duraipandiyan and Ignacimuthu (2007) have reported antimicrobial activity of hexane, chloroform, ethyl acetate, methanol, and water extracts (at 1.25, 2.5, and 5 mg/disc) of C. fistula flower against Gram-positive bacteria (S. aureus, S. epidermidis, B. subtilis, Enterococcus faecalis) and 1 Gram-negative bacterium (P. aeruginosa) with the inhibition zones ranging from 7 to 23 mm. The MIC for the sensitive microorganisms (S. aureus, S. epidermidis, B. subtilis, E. faecalis, P. aeruginosa) were found to be 0.039 to 2.5 mg/mL. The compound 4-Hydroxy benzoic acid hydrate, which was obtained from ethyl acetate extract, showed a MIC of 0.5 mg/mL for fungi, such as Trichophyton mentagrophytes and Epidermophyton floccosum. The compound rhein (1, 8-dihydroxyanthraquinone-3-carboxylic acid) isolated from the ethyl acetate extract was found to be effective against fungal pathogens, such as T. mentagrophytes, Trichophyton simii, Trichophyton rubrum, Epidermophyton floccosum, and a Scopulariopsis spp. with MIC values of 31.25, 125.00, 62.50, 31.25, and 250.00 μg/mL, respectively (Duraipandiyan and Ignacimuthu 2010).
Sangetha and others (2008) evaluated the antimicrobial activity of methanol extracts from different parts (leaves, flowers, stems, and pods) of C. fistula and Cassia surattensis against 12 bacteria and 3 fungi by using the agar disc diffusion assay. All the bacteria and fungi studied (except E. coli and Saccharomyces cerevisiae) were susceptible to the extract of C. fistula flowers with inhibition zones ranging from 12 to 20 mm.
Bhalodia and others (2011), in one of their studies, by employing the agar disc diffusion method, screened the antimicrobial activities of hydro-alcohol and chloroform extracts of C. fistula (5, 25, 50, 100, and 250 ìg/mL) against S. aureus, Streptococcus pyogenes, E. coli, P. aeruginosa, A. niger, Aspergillus clavatus, and Candida albicans. Results showed both the extracts to exhibit moderate to strong antibacterial and antifungal activities (inhibition zones of 12 to 21 mm for bacteria and 13 to 22 mm for fungi) at all the tested concentrations, except for 5 μg/mL. Preliminary phytochemical screening performed in this study showed that the chemical compounds of the hydro-alcohol extract contained tannins, flavonoids, saponins, triterpenoids, steroids, glycosides, anthraquinones, reducing sugars, and amino acids, while those of the chloroform extracts were found to contain high amount of glycosides, phenolic compounds, tannins, and anthraquinones.
Cassia surattensis Burm.f. (Sunshine tree)
Cassia surattensis (family: Leguminosae) is a flowering plant native to South Asia, and found growing abundantly in India, Myanmar, Southern Pakistan, and Sri Lanka. The plants are grown as ornamental trees in tropical and subtropical regions. The bark and leaves of C. surattensis are believed to exhibit antiblenorrhagic properties (Sangetha and others 2008).
In one of the experiments conducted by Sangetha and others (2008) on different parts (leaves, flowers, stems, and pods) of C. fistula and C. surattensis, all the bacteria and fungi studied (except Bacillus thuringiensis and S. cerevisiae) were susceptible to the methanol extract of C. surattensis flowers with inhibition zones ranging from 12 to 20 mm.
The antimicrobial activity of essential oil and methanolic extracts from the flower of C. macropodum against 9 bacteria and 2 fungi was determined by Ebrahimabadi and others (2010) who employed agar disc diffusion and mico-well dilution assays. From gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis, the chemical composition of essential oil was found to be comprised of 49 components representing 98.3% to 99.4% of the oil with trans-β-farnesene (27.5%), trans-β-ocimene (20.9%), β-pinene (2.8%), limonene (12.0%), spathulenol (8.6%), and myrcene (1.3%) as the major constituents. The essential oil was found to be active against all the tested microorganisms (except for Shigella dysenteriae and A. niger) with inhibition zones recorded as 8 to 26 mm and MIC recorded to be 125 to 500 μg/mL. However, the methanol extract did not show any inhibitory effects on the tested microorganisms.
Chrysanthemum morifolium Ramat. (Chrysanthemum)
Chrysanthemum morifolium (family: Asteraceae) is an important medicinal herb of the Asteraceae family consisting of 8 major varieties (Hangju, Boju, Gongju, Chuju, Qiju, Huaiju, Jiju, and Hang ju). C. morifolium is traditionally used in China to protect the cardiovascular system, to lower blood glucose and fat levels, to regulate blood pressure, excrete lead, and to scavenge free radicals. This plant has been reported to exhibit significant antibacterial, antioxidant, anti-inflammatory, and anticancer activities. The bioactive compounds of C. morifolium consist of flavonoids, sesquiterpenoids, chlorogenic acids, vitamins, and amino acids (Zhang and Zhang 2007; Zhao and others 2009).
The methanolic extract of C. morifolium (inflorescence) showed antimicrobial activity against B. cereus, L. monocytogenes, E. coli, and Salmonella anatum with inhibition zones in the range of 5.5 to 9.2 mm (Shan and others 2007). Besides, the antimicrobial activity of petroleum ether, ethyl acetate, and methanolic extracts of 7 species of C. morifolium flowers cultivated in Kaifeng, China were tested against S. aureus and methicillin resistant S. aureus (MRSA) by the disc diffusion assay (Zhao and others 2009). Petroleum ether extracts of Mailang, Chunrijianshan, and Lengyan, as well as ethyl acetate extracts of Mailang, Chunrijianshan, Lengyan, Jianliuxiangbai, Guohuawansheng, and Changhong varieties showed good antibacterial activity on S. aureus with MIC of 125, 250, 250, 250, 250, 125, 250, and 250 μg/disc, respectively. The petroleum ether extracts of Mailang, Chunrijianshan, and Lengyan were active against MRSA with MIC of 250 μg/disc. All the extracts of Baiyudai did not exhibit any activity against S. aureus and MRSA at the tested concentration (250 μg/disc). Also, the methanol extracts of all the species of C. morifolium did not show any antimicrobial activity. The authors have concluded that better antimicrobial activity was shown by yellow flowers compared to purple and white flowers.
Chrysanthemum trifurcatum (Desf.) Batt. and Trab.
Chrysanthemum trifurcatum (family: Asteraceae) is an herbal plant bearing small yellow flowers. This plant is widely distributed in Tunisia regions and the plant parts are used for treating constipation, intestinal transit problems, and postdelivery pains (Sassi and others 2008b). The antimicrobial activity of petroleum ether, ethyl acetate, methanol, and hot water extracts of Tunisian Chrysanthemum species against 5 Gram-positive and 9 Gram-negative bacteria and 4 yeasts were evaluated by Sassi and others (2008a) by employing agar disc diffusion and microdilution assays. The results obtained showed all the extracts to inhibit growth of the tested microorganisms (inhibition zone = 7.1 to 8.5 mm; MIC = 1.25 mg/mL) except for S. aureus, E. coli, K. pneumoniae, Aeromonas hydrophila, C. albicans, and Candida tropicalis.
Further, the same authors have reported on the antimicrobial activity of the essential oil from C. trifurcatum flower heads against 5 Gram-positive bacteria (S. epidermidis, Staphylococcus hoemolyticus, Staphylococcus hominis, Staphylococcus simulans, and B. subtilis) and 3 Gram-negative bacteria (E. coli, Hafnia alvei, and Proteus mirabilis) (Sassi and others 2008b). The broth microdilution method was adopted for assaying antimicrobial activities. The essential oil was found to exhibit better antimicrobial activity against Gram-negative bacteria compared to Gram-positive bacteria. At a concentration of 500 μg/mL, the essential oil inhibited the growth of S. epidermidis and B. subtilis by 66% and 64% with IC50 (concentration that inhibits 50% of growth) of 62.5 and 125 μg/mL, respectively. The authors reported the presence of 56 compounds representing 97.48% of the oil with limonene (20.89%), γ-terpinene (19.13%), 1,8-cineole (10.64%), β-pinene (8.77%), α-pinene (5.32%), 2-hexenal (4.85%), 4-terpenyl acetate (3.42%), β-myrcene (2.31%), germacrene-B (2.01%), β-spathulenol (1.62%), longifolene (1.39%), α-cadinol (1.39%), α-thujene (1.23%), and β-bourbobene (1.06%) as the major constituents that contributed to the antibacterial activity of the essential oil.
Cleistocalyx operculatus (Roxb.) Merr and Perry (water fairy flower)
Cleistocalyx operculatus (family: Myrtaceae), also known as Eugenia operculata or Syzygium nervosum, is a perennial tree, widely distributed in China, Vietnam, and other tropical countries. Traditionally, the leaves and flower buds of the plant have been reported to be used as an ingredient in preparing certain beverages (tea decoctions) for treating gastrointestinal disorders and antisepsis (Dung and others 2008). In vivo and in vitro studies have shown the potentiality of C. operculatus buds to exhibit anticancer, antitumor, antihyperglycemic, and cardiotonic properties (Anthony and others 2002; Ye and others 2005; Mai and Chuyen 2007; Dung and others 2008). Results on the phytochemicals screening of flower buds have shown the presence of sterols, flavanones, chalcones, triterpene acid, β- sitosterol, and ursolic acids in the buds (Ye and others 2004; Dung and others 2008).
Dung and others (2008), by using agar disc diffusion and microdilution susceptibility tests, have screened the effectiveness of the essential oil and ethanol extract of C. operculatus buds against 2 food spoilage bacteria (B. subtilis and P. aeruginosa), 9 foodborne pathogens (2 isolates of S. aureus, L. monocytogenes, Enterobacter aerogenes, Salmonella Typhimurium, Salmonella enteritidis, E. coli, and 2 isolates of E. coli O157:H7), 4 skin infectious pathogens (S. aureus, S. epidermidis, E. coli, and C. albicans), 3 methicillin-resistant S. aureus, 3 vancomycin-resistant Enterococcus faecium, and 15 multi-antibiotic-resistant bacteria (2 isolates of Acinetobacter baumannii, 3 isolates of E. coli, 2 isolates of Enterobacter cloacae, 2 isolates of K. pneumoniae, 3 isolates of P. aeruginosa, 2 isolates of Serratia marcescens, and S. aureus). The essential oil of C. operculatus buds showed inhibition zones and MIC/MBC, which ranged from 8 to 16 mm and 1 to 20 μL/ mL, respectively, effective against all the tested microorganisms. The ethanol extract demonstrated antimicrobial activity against all the Gram-positive bacteria and 1 food-spoilage Gram-negative bacterium (P. aeruginosa) with inhibition zones and MIC/MBC in the range of 8 to 22 mm and 0.25 to 32 mg/mL. Besides, in the cell viability assay of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant Enterococci (VRE), the authors found essential oil at the MBC (8 and 16 μL/mL) to possess potential inhibitory effects with the exposure time required for complete inhibition of cell viability to range from 10 to 40 min and 10 to 20 min, respectively. Scanning electron spectroscopy on the most sensitive methicillin-resistant S. aureus and vancomycin-resistant Enterococcus (MRSA–P249 and VRE–B2332) treated with essential oil at MIC (8 μL/mL) showed disruption and lysis of membrane integrity. The essential oil was found to contain 55 compounds representing 93.71% of the oil with γ-terpinene (5.76%), cis-linalool oxide (5.21%), camphene (4.12%), trans-carveol (3.93%), α-pinene (3.45%), β-pinene (3.07%), terpinen-4-ol (2.58%), and myrcene (2.4%) as the major monoterpenes as well as globulol (5.61%), acorenol (5.12%), β-himachalol (3.84%), cyclobazzanene (3.12%), 2,3-dehydro-1,4-cieol (3.01%), trans-dihydrocarvone (2.58%), presilphiperfol-1-ene (2.48%), and γ-amorphene (2.12%) as the major sesquiterpenes. The authors, for the first time, concluded the use of essential oil and ethanolic extract of C. operculatus to have applicability for the prevention and treatment of diseases caused by foodborne and skin-infectious pathogens, especially those of antibiotic-resistant strains.
Clitoria ternatea Linn. (butterfly pea, Asian pigeon wings)
Clitoria ternatea (Family-Liguminoceae) is a tropical, perennial twining herb bearing blue or white colored flowers (in single). This plant is extensively grown for ornamental and medicinal purpose in the Asian subcontinent (India, Bangladesh, Indonesia, Malaysia). In Malaysia, aqueous extract of the flower is used as a natural coloring agent for preparing dish from glutinous rice. The plant parts have been reported to exhibit anti-inflammatory, antipyretic, antihyperlipidemic, analgesic, tranquilizing, and immunomodulatory activities (Mukherjee and others 2008; Solanki and Jain 2010, 2011, 2012). Root contains flavonol glycosides, which exhibit rich antibacterial activity (Yadava and Verma2003). Cliotides (biologically active peptides) (present in flowers, seeds, and nodules) have been isolated from heat-stable fractions of Clitoria ternatea extract. These cliotides showed potential antimicrobial activity against E. coli and cytotoxicity against HeLa cells (Nguyen and others 2011).
Uma and others (2009) have screened the flower extracts (by maceration technique: solvents used methanol, chloroform, petroleum ether, hexane, and aqueous) of Clitorea ternatea against pathogenic microorganisms, such as uropathogenic, enteropathogenic, and enterotoxigenic E. coli, S. Typhimurium, S. enteritidis, K. pneumoniae, and Pseudomonas aureginosa. These microorganisms were isolated from patients with urinary tract infection and acute gastroenteritis. The method adopted for determining antimicrobial activity was disc diffusion method and minimum inhibitory concentration (two-fold serial dilution method). Results of this study revealed aqueous, methanol, and chloroform extracts to exhibit antimicrobial activity against uropathogenic, enteropathogenic, and enterotoxigenic E. coli, S. Typhimurium, K. pneumoniae, and P. aureginosa. However, no antibacterial activity was recorded for petroleum ether and hexane extracts.
Cnicus benedictus Linn. (blessed thistle)
Cnicus benedictus (family: Asteraceae) is the single species in the genus Cnicus; it is native to the Mediterranean region. This annual plant grows up to 60-cm high, and has leathery, hairy leaves (extending up to 30-cm long and 8-cm broad), with minute spines on the leaf margins. The flowers are yellow, which are produced in a dense flower head of 3 to 4 cm dia. The entire plant of C. benedictus possesses astringent, bitter, diaphoretic, diuretic, emetic, emmenagogue, galactogogue, stimulant, stomachic, and contraceptive properties. An aqueous infusion of the entire plant is reported to be used for the treatment of liver and gall bladder problems. The flowers, leaves, and stem of C. benedictus are traditionally used as a health drink (tonic) or used in other preparations taken orally to improve appetite and digestion (extracts are believed to stimulate gastric juices). This plant is known to contain ample amounts of sesquiterpene lactones, alkaloids, tannins, and volatile oil. Besides this, anti-infective, anticancer, and anti-inflammatory activities of C. benedictus have been reported through laboratory studies by Szabó and others (2009). In addition, the chemical constituents (such as cnicin and polyacetylene) have been reported to exhibit antibacterial activity (Szabó and others 2009).
The effects of ethanol extracts of C. benedictus flowers against American Type Culture Collection (ATCC) bacterial strains (S. Typhimurium, S. enteritidis, S. aureus, E. coli, S. pyogenes, P. aeruginosa, Bacillus proteus, and Shigella sonnei) and pathogens obtained from hospitalized patients (S. aureus, S. pyogenes, and E. coli) were assessed by using the agar disc diffusion assay (Szabó and others 2009). The antimicrobial activity of the C. benedictus flowers against all the tested bacteria were observed with inhibition zones of approximately the same values at different concentrations of the extracts (10% and 20%, respectively). The diameters of inhibition zones shown by C. benedictus mature flowers (16 to 30 mm) on ATCC bacterial strains were significantly different from those shown by immature flowers (18 to 32 mm). The test results on the microorganisms harvested from hospitalized patients treated with the extract of mature flowers showed diameters of inhibition zones to range between 10 and 24 mm.
Crocus sativus Linn. (saffron)
Crocus sativus (family: Iridaceae) has been used traditionally as a spice and as a food colorant in most of the countries over the world. Saffron, the world's most expensive spice is obtained from the flower (mainly the stigmata) of the C. sativus plant. In folk medicine, saffron has been used as aphrodisiac, antispasmodic, and expectorant (Nakhaei and others 2008). Saffron is also used to treat flatulence, colic, and abdominal pains, as well as to improve appetite and memory (Zhang and others 1994; Nakhaei and others 2008). Antitumor, radical scavenging, hyperlipemic, anticonvulsant, cytotoxic, antigenotoxic, and anti-ulcerogenic activities have been reported for C. sativus extracts or their chemical constituents (Nair and others 1995; Hosseinzadeh and Khosravan 2001; Abdullaev and others 2003; Al- mofleh and others 2006; Nakhaei and others 2008). The biological properties of C. sativus are mainly attributed to crocin and saffranal, which are isolated from stigmata, leaves, petal, and pollen. Other isolated chemical constituents include crocetin, picrotoxin, quercetin, and kaempferol (Nakhaei and others 2008).
According to Vahidi and others (2002), significant antimicrobial activity was observed against S. epidermidis, C. albicans, Cladosporium spp., and A. niger when an ethyl acetate extract of stigmata of C. sativus was used. The inhibition zones and MIC ranged from 12 to 19 mm and 6.25 to 50 mg/mL, respectively. The ethyl acetate extract of stamens exhibited antimicrobial activity against S. aureus, S. epidermidis, E. coli, M. luteus, Cladosporium spp., and A. niger with inhibition zones and MIC ranging from 15 to 21 mm and 12.5 to 50 mg/mL, respectively.
Nakhaei and others (2008) screened the anti-Helicobacter pylori activity of stigmata of C. sativus against 45 clinical isolates. Based on the results obtained from the agar disc diffusion method, the aqueous and methanol extracts of saffron exhibited antibacterial activity against all the isolates with inhibition zones being in the range of 10 to 23.5 mm. Based on the agar dilution method, the MIC of the methanol extract for all the isolates was 677 μg/mL. There was no significant difference in the activity of methanol extract at 80 and 121 °C, in comparison to the control, indicating that high temperature not to have any effect on the activity of the extract. The results on pH stability of the methanol extract in this study indicated that active compounds of C. sativus were stable at pH 5, 6, 7, and 8.
Crotalaria juncea Linn. (sunn hemp)
Crotalaria juncea (family: Leguminoceae) plant parts (flowers, buds, pods, and seeds) are commonly used as medicine and for culinary purposes (Bhatt and others 2009). The plant is widely distributed in tropical and subtropical regions, such as in India, Nepal, Sri Lanka, and Southern Africa. In Ayurvedic medicine, C. juncea has been used as an astringent, abortifacient, blood purifier, demulcent, emetic, purgative, and for curing anemia, impetigo, menorrhagia, and psoriasis (Sharma and others 2001; Chouhan and Singh 2010). The seeds of C. juncea have been reported to exhibit significant antispermatogenic, anti-ovulatory, and contraceptive activities (Vijaykumar and others 2004; Malashetty and Patil 2007). The chemical compounds isolated from the seeds of this plant were riddelline, seneciphylline, senecionine, trichodesmine, chodesmine alkaloids, galactose-specific lectin, and cardiogenin 3-O-[•]-d-xylopyranoside (Adams and Gianturco 1956; Chouhan and Singh 2010).
Chouhan and Singh (2010) have reported antibacterial activity of the ethanolic extract of C. juncea flowers against both Gram-positive and Gram-negative bacteria by employing the agar disc diffusion assay. The extracts were found to be effective against E. coli, K. pneumoniae, P. aeruginosa, S. aureus, and Vibrio cholare (inhibition zone = 13, 14, 10, 13, and 8 mm). However, the extracts did not exhibit any activity against Citrobacter freundi, E. faecalis, Shigella flexneri, and S. dysenteriae. Further, the authors have reported on the presence of steroids, triterpenes, flavonoids, phenolics, and glycosides in the ethanol extract.
Dendrobium nobile Lindl. (dendrobium orchid)
Dendrobium nobile (family: Orchidaceae) is a flowering ornamental plant encompassing nearly 35000 species. The flowers are very attractive and appear in various colors and forms. The opened flowers mimic bees, wasps, butterflies, moths, frogs, lizards, and even humans. Native inhabitants of the Eastern Himalayas (in India) believed that dendrobium flowers can cure eye diseases (Uma Devi and others 2009). Gigantel and moscatilin of D. nobile have been reported to exhibit antimutagenic activity and its 2-phenanthrenes to exhibit anticancer activity (Kong and others 2003; Uma Devi and others 2009).
Uma Devi and others (2009) used the “strip plate method” to evaluate the antimicrobial activities of different solvent (methanol, chloroform, and water) extracts of flowers and stems of D. nobile against pathogenic bacteria, such as E. coli, B. subtilis, Proteus spp., S. Typhimurium, and S. aureus. The extent of inhibition of floral extracts was high in the aqueous extract than in the other 2 extracts. The authors recorded the inhibition zones as 0.6 to 1.0 mm for ethanol extract, 0.3 to 1.0 mm for chloroform extract, and 0.53 to 1.2 mm for aqueous extract. Also, in aqueous extracts, the inhibitory activity was found to be significantly higher in flowers than that of stems.
Etlingera elatior (Jack) R.M. Smith (torch ginger)
Etlingera elatior (family: Zingiberaceae) is a perennial herbal plant (height of 3.6 to 4.7 m) found growing abundantly in parts of Malaysia, Indonesia, Vietnam, Sri Lanka, and Thailand. The flower (bud or inflorescence) is used both ornamentally and as a spice for culinary use. Rhizome and flowers of this plant are extensively used as a natural ingredient in cosmetics (as an ingredient of soap, shampoo, perfume) and also as a therapeutic agent for treating common ailments. Fruits of the torch ginger plant are traditionally used to treat ear ache, while leaves find use to clean wounds and to remove body odor (Chan and others 2007). Flowers and the mature inflorescence of torch ginger are used to prepare such popular dishes as asam laksa, nasi kerabu, nasi ulam (in Malaysia), arisk ikan mas (in North Sumatra, Indonesia), and sayur asam (in Thailand) (Lachumy and others 2010; Wijekoon and others 2011). Torch ginger inflorescence is reported to possess strong antioxidant activities (Wijekoon and others 2011).
Lachumy and others (2010) evaluated the antimicrobial activity (by agar disc diffusion and serial dilution methods) of an 80% methanolic extract of torch ginger flowers against 7 strains of bacteria, 1 strain of yeast, and 1 strain of mold. Results of this study showed methanol extract of the flowers to possess high amounts of flavonoids, terpenoids, saponins, tannins, and carbohydrates. Floral extracts were found to be active against the tested microorganisms (inhibition zone = 12 to 23 mm; MIC = 1.563 to 50.000 mg/mL). Results from the brine shrimp lethality test revealed absence of toxicity of the flower extract (LC50= 2.52 mg/mL against Artemia salina), and therefore are nontoxic to humans.
Eugenia caryophyllata (family: Myrtaceae) is commonly found growing in warm and humid climatic conditions, such as those encountered in tropical Asia (India, Sri Lanka, Malaysia, Indonesia). The handpicked, unopened, air- or sun-dried flower buds are used as spice. Traditionally, the floral buds have been used to treat tooth aches. The essential oil obtained from buds are extensively used as an ingredient of dental formulations, toothpastes, breath fresheners, mouthwashes, cosmetics, soaps, and insect repellents (Politeo and others 2010). The essential oils have been reported to exhibit good antibacterial, antifungal, cytotoxic, and antioxidative activities (Baratta and others 1998; Gayoso and others 2005; Prashar and others 2006).
Stonsaovapak and others (2000) have reported on the inhibitory effects of ethanolic extracts of E. caryophyllata flowers against pathogenic E. coli O157:H7 and Yersinia enterocolitica with inhibition zones of 17.75 and 18.00 mm. At 3.0 × 104 CFU/mL, the MIC for E. coli O157: H7 was 1250 μg/mL, while at 3.0 × 106 CFU/mL, the MIC was 2500 μg/mL. For Y. enterocolitica, the MIC was 625 μg/ mL at 6.0 × 104 CFU/mL and 1250 μg/mL at 6.0 × 106 CFU/ mL.
Shan and others (2007) have reported effectiveness of methanol extracts of E. caryophyllata against B. cereus, L. monocytogenes, S. aureus, E. coli, and S. anatum with inhibition zones being in the range of 10.1 to 21.3 mm. And Ushimaru and others (2007) have reported methanol extract of E. caryophyllata to effectively inhibit the growth of S. Typhimurium, S. aureus, Enterococcus spp. and E. coli (MIC 50%= 0.41% to 1.60% v/v and 0.39 to 1.52 mg/mL; MIC 90%= 0.49% to 1.76% v/v and 0.46 to 1.67 mg/mL).
Bansod and Rai (2008), reporting on the antifungal (against Aspergillus fumigatus and A. niger) assays of some Indian medicinal plants isolated from patients with pulmonary tuberculosis noted E. caryophyllata to exhibit antifungal activity. Based on the disc diffusion assay, the essential oil of E. caryophyllata was found to exhibit moderate antifungal activity with inhibition zones ranging from 8 to 15 mm. The MIC, determined by the agar dilution method, was found to be 0.12% (v/v) for both of the fungi, while the MIC/MLC (determined by the broth microdilution method) was found to be 0.06%/0.12% (v/v) for A. fumigatus and 0.12%/0.06% (v/v) for A. niger, respectively. The authors have concluded that the essential oil of E. caryophyllata might play a pivotal role in treating mycotic infections.
Euphorbia hirta Linn. (asthma weed)
Euphorbia hirta (familiy: Euphorbiaceae) is a small perennial herb that is found widely spread in tropical regions of the world. The plant is erect, bears a slender hairy stem, and grows up to 80 cm in height. Occasionally, the plant is also witnessed to grow as a semicreeper. The leaves are broad, elliptical, oblong, and lanceolate, darker on the upper surface with slightly toothed margins. Flowers of this plant are small, numerous, and crowded together in dense cymes (about 1 cm in diameter).
The stems and leaves contain milky-white latex. Rajeh and others (2010) have reported on the traditional use of E. hirta plant decoctions to treat amebic dysentery, diarrhea, peptic ulcers, heartburn, vomiting, respiratory problems (bronchitis, coughs, colds), kidney stones, and fertility-related problems (menstrual problems, sterility, and venereal disease). In certain instances, the plant parts have been recommended to be used as an antidote and to relieve pain from scorpion stings or snake bites.
Methanolic extracts from different parts of E. hirta (leaves, flowers, stems, and roots) were evaluated by Rajeh and others (2010) for antimicrobial activities against 4 Gram-positive bacteria (S. aureus, a Micrococcus spp., B. subtilis, and B. thuringiensis), 4 Gram-negative bacteria (E. coli, K. pneumoniae, Salmonella typhi, and P. mirabilis) and 1 yeast species (C. albicans). Results of this study, which were based on the agar disc diffusion method, revealed all the tested microorganisms, except C. albicans, to be sensitive to the flower extract, with inhibition zones formed ranging from 9 to 28 mm. The LC50 value (0.033 mg/mL) against Artemia salina, which was obtained from the brine shrimp lethality test, demonstrated that E. hirta flower extract might be toxic to humans.
Helichrysum gymnocomum DC.
Helichrysum gymnocomum (family: Asteraceae) is a perennial herb with long flowering seasons commonly encountered in regions of Kwazulu-Natal Drakensburg, Africa. The pleasant scented flowers and leaves are burnt by the indigenous people of this region to fumigate sick rooms and to invoke the goodwill of ancestors. H. gymnocomum has also been traditionally used for the treatment of wounds, coughs, and colds (Drewes and Van Vuuren 2008).
The antimicrobial activities of H. gymnocomum dichloromethane (CH2Cl2/MeOH) extract and isolated compounds against 5 Gram-positive bacteria, 3 Gram-negative bacteria, and 2 yeasts were evaluated by Drewes and Van Vuuren (2008) by the serial dilution method. From the results, it was noteworthy that the crude extracts demonstrated antimicrobial activities with MIC ranging from 312.5 to 1000 μg/mL.
All the isolated compounds (2′-hydroxy-4′,6′-dibenzyloxy-chalcone; 5,7-dibenzyloxyflavanone; an acylphloroglucinol derivative; 1-[2,4,6-trihydroxy-3-(2-hydroxy-3-methyl-3-butenyl)-phenyl]-1-propanone; 3-methoxyquercetin; a 4′-O-glucose derivative of 2′-hydroxy-6′-methoxy chalcone) were good inhibitors against the tested microorganisms with MIC values below 64 μg/mL. The findings of this study showed acylphloroglucinol derivative to be the most potent inhibitor for 8 of the 10 tested microorganisms (MIC = 6.3 to 45 μg/mL), including S. aureus (MIC = 6.3 μg/mL) and methicillin- and gentamycin-resistant S. aureus (MIC = 7.8 μg/mL). The results also revealed highest sensitivity of P. aeruginosa to all the compounds (except 5, 7-dibenzyloxyflavanone) with MIC being in the range of 45 to 63 μg/mL. According to the authors, the traditional use of H. gymnocomum in healing wound infections was supported by the notable antimicrobial activity of the plant, particularly against S. aureus and P. aeruginosa.
The floral extract (water and ethanol) of H. sabdariffa has been reported to show high inhibitory effects against B. cereus (Hamdan and others 2007). The inhibition zones against B. cereus were 2, 6, and 16 mm and 4, 9, and 12 mm at 1, 2, and 4 mg/mL for water and ethanol extracts, respectively. The authors also noted that as the content of water and ethanol extract increased (from 0.82 to 4.12 mg/mL), a corresponding increase in the inhibition on the growth of B. cereus occurred with complete inhibition (100%) attained at a concentration of 3.45 and 4.12 mg/mL, respectively. Besides, heat treatment at 70 °C for 3 min did not significantly affect the antibacterial activity of H. sabdariffa extract against B. cereus.
Jasminum sambac (Arabian jasmine/jasmine flower)
Jasminum sambac (family: Oleaceae) originated in India and Burma and is widely grown in Ambouli (Republic of Djibouti) for producing perfume. This plant is a perennial twining shrub (attaining height of 5 to 6 feet) and bearing small, white-colored scented flowers. The flowers are used ornamentally as well as to decorate hair. Skin care products are also formulated by using the essential oil extracted from the flowers. The essential oil of the flower is used to reduce skin inflammation, tone the skin, and lift up mood (Abdoul-Latif and others 2010). Extracts of flowers are also used to prepare herbal tea decoctions. The floral extract is reported to possess analgesic, anti-inflammatory, antidepressant, aphrodisiac, antiseptic, expectorant, sedative, and tonic properties. Besides, flowers and plant parts have been reported to have anticancer properties (Houghton and others 2007; Alka and others 2010).
Tsai and others (2008) have reported on the inhibitory activities of methanolic extract of the flowers against Streptococcus mutans and Streptococcus sanguinis. They adopted the broth microdilution method for evaluating the antimicrobial/inhibitory activities. From their study, they reported the MIC to be 1 mg/mL for S. sanguinis. However, the MIC of the extract for S. mutan was >8 mg/mL, which is an indication of “no activity” against S. mutans.
Lonicera japonica Thunb. (honeysuckle)
Lonicera japonica (family: Caprifoliaceae) is a native plant of eastern Asia and is widely seen in parts of Japan, Korea, northern and eastern China, and Taiwan. Flower buds of this plant possess anticancer, antimicrobial, and anti-inflammatory properties (Zhang and others 2008). Results on the phytochemical screening have reported the presence of iridoid glucosides and polyphenolic compounds in the flower buds (Kakuda and others 2000).
Based on the results obtained by the agar well diffusion method, methanol extracts of the flower showed inhibitory activities against B. cereus, S. aureus, and S. anatum with the diameters of inhibition zones ranging from 5.5 to 7.2 mm (Shan and others 2007). On another note, Tsai and others (2008), screening on the methanolic extract of different herbs against growth of S. mutan and S. sanguinis, found MIC of L. japonica to be 4 mg/mL for S. sanguinis, while MIC for S. mutan was >8 mg/mL (no activity).
In another study reported by Rahman and Kang (2009), the essential oil of L. japonica flower demonstrated inhibitory activities against L. monocytogenes, B. subtilis, B. cereus, S. aureus, S. enteritidis, S. Typhimurium, E. aerogenes, and E. coli with inhibition zones recorded in the range of 12.1 to 20.3 mm and MIC in the range of 62.5 to 500 μg/mL. The authors used the agar disc diffusion and broth dilution assays for the analysis. Their results of a GC-MS analysis showed the essential oil to contain 39 compounds wherein 92.34% of the oil was composed of trans-nerolidol (16.31%), caryophyllene oxide (11.15%), linalool (8.61%), p-cymene (7.43%), hexadecanoic acid (6.39%), eugenol (6.13%), geraniol (5.01%), trans-linalool oxide (3.75%), globulol (2.34%), pentadecanoic acid (2.25%), veridiflorol (1.83%),>br/> benzyl alcohol (1.63%), and phenylethyl alcohol (1.25%) as major components. However, antimicrobial activity results on the essential oil did not reveal any effects of the oil against E. coli O157: H7 and P. aeruginosa.
Recently, Rhee and Lee (2011) reported the antimicrobial activity of butanol extract from L. japonica flower against 104 clinical isolates of anaerobic bacteria (Bacteroides fragilis, Bacteroides ovatus, Clostridium difficile, C. perfringens, Propionibacterium acnes, and Peptostreptococci) (based on the agar dilution method). The butanol extract showed antimicrobial activity against all the tested bacteria with MIC ranging from 0.032 to 2.0 mg/L.
Mentha longifolia L. (horse mint)
Mentha longifolia (family: Lamiaceae) is a perennial herb commonly found growing in a hot and humid climate. Mint is widely distributed throughout South Africa, Botswana, Namibia, and Zimbabwe. The rhizomes creep below the ground and the erect flowering stems can grow up to 8-m high. The plant bears small white or pale purple flowers borne in elongated clusters on the tips of the stems. The entire plant exudes a unique mint aroma. The leaves are the most widely used parts of this plant. Leaf and stem decoctions are prescribed to cure common colds, cough, bronchial ailments, headache, fever, indigestion, flatulence, painful menstruation, urinary tract infections, diseases of the gastrointestinal tract, and bleeding problems (http://www.plantzafrica.com/medmonographs/menthlong.pdf, accessed on Jul 25, 2011).
In one of the experiments conducted by Pirbalouti and others (2010) on Iranian folklore herbs, the extract and essential oil from flowers of M. longifolia have been reported to exhibit strong antibacterial activity against all the tested bacteria (S. aureus, E. coli, P. aeruginosa, and K. pneumoniae) with inhibition zones and MIC values ranging from 9 to 17 mm and 0.156 to 10.00 mg/mL, respectively. Results of this study showed the essential oil of M. longifolia to exhibit stronger antibacterial activity than the ethanol extract.
Moringa oleifera (horseradish tree)
Moringa oleifera (family: Moringaceae) is a perennial timber yielding softwood tree. It is native to the sub-Himalayan tracts of India, Pakistan, Bangladesh, and Afghanistan. The plant is also found to be widely distributed in parts of Ethiopia, the Philippines, Africa, Latin America, the Caribbean, Florida, and the Pacific Islands (Fahey 2005). The whole plant is edible and possesses antispasmodic, anti-inflammatory, diuretic, obortifacient, emmenagogue, and ecbolic properties. The plant parts have been reported to possess therapeutic value and are used in the treatment of hysteria, tumors, leucoderma, and biliousness (Fahey 2005; Talreja 2010).
The potential antimicrobial activity of ethanolic extracts of its flowers against B. subtilis, S. aureus, E. coli, K. pneumoniae, and C. albicans has been reported by Talreja (2010). Results based on the agar disc diffusion assay showed the floral extract to have both antibacterial and antifungal activity with zones of inhibition formed in the range of 8 to 10.5 mm for bacteria and 6.5 mm for C. albicans.
Lotus (Nymphaea lotus Linn., Egyptian white waterlily; and Nelumbo nucifera L., Indian lotus, sacred lotus)
The antimicrobial activity of hot water and ethanolic extracts of 6 plants, utilized in Pakistan for the treatment of liver damage, against 7 bacterial strains (methicillin-resistant S. aureus, multidrug-resistant P. aeruginosa, enterohemorrhagic E. coli 0157 EHEC, S. typhi, P. vulgaris, K. pneumoniae, B. subtilis and 2 fungal species (C. albicans and A. niger), was determined by agar well diffusion and broth microdilution assays (Hassan and others 2009). Both extracts of N. lotus were active against all the microorganisms tested with zones of inhibition in the range of 16 to 36 mm and MIC/MBC in the range of 23.3/ 27.3 to 35.3/ 41.7 mg/mL. Antimicrobial activity of N. lotus was the stronger in ethanol extract than in water extract. This observation has been attributed to the enhanced nature of bioactive compounds in the presence of ethanol and the stronger extraction power of ethanol. The traditional use of both water and ethanol extracts of N. lotus against liver damage was supported by the results of this study.
With regard to Nelumbo nucifera, the entire plant has been reported to possess rich nutraceutical value (Sridhar and Bhat 2007). Flowers are white to pink, sweet-scented, single, and are 10 to 25 cm in diameter. Flowers are reported to be useful to treat bleeding disorders and to promote conception. Additionally, flowers are reported to be useful to treat diarrhea, cholera, fever, hepatopathy, and hyperdipsia. However, to our knowledge no reports are available on the antmicrobial activity of this flower or its extracts.
Plumeria alba Linn. (white champa)
Plumeria alba (family: Apocynaceae) is a small laticiferous tree that is a native of tropical America. The plant has also been found growing in India where it is popularly called Peru. The plant grows up to 4.5-m high bearing white and fragrant flowers. The fruits are edible, the seeds possess hemostatic properties, while the sap of the stem and leaves are often applied to heal ulcers, herpes, and scabies. The bark is bruised and applied as a plaster over hard tumors, which is also used as a purgative, cardiotonic, diuretic, and hypotensive agent (Radha and others 2008; Zahid and others 2010). The methanolic extract of the flowers of P. alba have shown antimicrobial activity against Bacillus anthracis and P. aeruginosa (Syakira and Brenda 2010; Zahid and others 2010). Different parts of the plant are traditionally used in the treatment of malaria, leprosy, rheumatism, and abdominal tumors (Syakira and Brenda 2010).
The essential oils isolated from flowers of P. alba have been evaluated against Gram-positive S. aureus, B. subtilis, and Gram negative E. coli, P. aeruginosa, and S. typhi by the agar well diffusion method (Zahid and others 2010); Gram-positive bacteria (S. aureus and B. subtilis) were found to be more sensitive to essential oils. The sensitivity has been attributed to the absence of an outer membrane surrounding the bacterial cell wall that restricts the diffusion of hydrophobic components of P. alba essential oil through the lipopolysaccharide covering.
Rosa spp. (rose flower)
Roses are indigenous to central Asia and are grown as ornamental plants. Rose flowers have been traditionally used as a medicine, for culinary purposes, and in the preparation of perfumes (due to a warm, intense, rich, and rosy fragrance). Rose flowers were used as medicine in ancient Assyria, China, Egypt, Greece, India, Persia, and Rome. The flower has 5 petals (or multiples of 5) with numerous stamens. Rose petals are aromatic and have various shapes and colors. They enclose androecium and gynoecium, which apart from facilitating pollination, possess antibacterial activity as a protection system. Perfumes prepared from rose petals are economically valuable and also have soothing effects (Hirulkar and Agrawal 2010). Rose petal jam (prepared in parts of Asia from Rosa indica L.) is considered to provide a cooling effect on mind and body. Oil obtained from rose flowers has been reported to reduce blood lipid levels in rats. The hydrating and anti-inflammatory properties of natural acids present in rose water are considered to be useful for skin and eye care. Rose petals are also recommended to be used as mouthwash. The tea decoction prepared from rose petals is recommended to heal breast pain, mastitis, menstrual difficulties, and restless fetus (Hirulkar and Agrawal 2010). The antioxidant and antimicrobial properties and the chemical compounds present in rose essential oil have been extensively published (Arıdoğan and others 2002; Basim and Basim 2003; Hirulkar and Agrawal 2010).
Petal extract of Rosa canina L. is reported to enhance the effectiveness of several antibiotics against methicillin-resistant S. aureus as well as to have strong inhibitory activity against C. albicans (Rossnagel and Willich 2001; Hirulkar and Agrawal 2010). Anthocyanins and proanthocyanidins, tellimagrandin I and rugosin B, carotenoids, plant acids, and essential oils are present in rose petals. Rose oil is reported to contain economically valuable alcohol, such as geraniol (a major constituent) and 1-citronellol (Hirulkar and Agrawal 2010).
Hirulkar and Agrawal (2010) used the agar disc diffusion method to study the antimicrobial activity of alcoholic, petroleum ether, and aqueous extracts of rose petals against various pathogenic bacteria. All the dilutions (1:1, 1:2, 1:3) of the 3 types of extracts showed inhibition against all the tested bacteria with inhibition zone diameters ranging from 12 to 30 mm. Among the tested bacteria, P. aeruginosa was the most sensitive to the petroleum ether extract of rose petals with an inhibition zone of 29 mm. Results of this study showed higher inhibitory activity of alcoholic extracts against S. pneumoniae (30 mm), E. aerogenes (28 mm), S. epidermidis (25 mm), B. subtilis (30 mm), and P. aeruginosa (32 mm) as compared to other bacterial strains. Aqueous extract showed higher inhibitory activity against E.coli (21 mm), E. aerogens (25mm), and B. subtilis (28 mm) as compared to other bacterial strains. It was found that alcoholic extract showed higher average antimicrobial activity (25 mm) when compared to aqueous extract (19 mm) and petroleum ether extract (18 mm). The authors concluded by stating that rose petals can be potentially used to treat diarrhea, opportunistic infection, and skin infections caused by various pathogenic bacteria.
Koday and others (2010) have reported on the bactericidal properties of methanol, chloroform, and hexane extracts of 40 different medicinal plants against Corynebacterium macginleyi using the agar well diffusion assay. From their study, the authors found methanolic extract of R. indica petal to possess better antimicrobial activity against C. macginleyi compared to chloroform and hexane extracts.
Rumex vesicarius Linn. (bladder dock)
Rumex vesicarius (family: Polygonaceae) is a wild edible plant that grows during spring time (Al- Quran 2009). It is native to southwest Asia and North Africa and is cultivated in India, especially in regions of Tripura, West Bengal, and Bihar (Khare 2007). The plant parts are used traditionally in the treatment of various diseases, such as tumors, hepatic diseases, indigestion, constipation, heart diseases, pains, spleen disorder, hiccough, flatulence, asthma, bronchitis, dyspepsia, piles, scabies, leucoderma, toothache, nausea, and dysentery. The plant also has cooling, laxative, tonic, antibacterial, analgesic, stomachic, appertizer, diuretic, astringent, purgative, and antispasmodic properties. Besides, this plant is used to reduce biliary disorders and to control cholesterol levels (Elegami and others 2001; Atiqur Rahman and others 2004; Lakshmi and others 2009; Mostafa and others 2011).
Antimicrobial activity of different plant parts of this plant against K. pneumoniae, S. pneumoniae, S. pyogenes, S. aureus, E. coli, and P. aeruginosa was performed using the agar disc diffusion method by Mostafa and others (2011). Results of this study demonstrated potential antibacterial activity (K. pneumoniae, Streptococcus pneumonia, S. pyogenes, S. aureus, E. coli, and P. aeruginosa) of the flowers extracted in different solvent extracts (petroleum ether, ether, chloroform, methanol, and ethanol). Chemical analysis of flower extracts revealed variations in the presence and amount of active compounds (flavonoids, anthraquinones, alkaloids, tannins, sterols and/or triterpenoids, carbohydrates and/or glycosides, chlorides, sulfates, and sublimable substances).
Santolina rosmarinifolia Linn. (green lavender cotton)
Santolina rosmarinifolia (family: Asteraceae) is a perennial shrub found in the Iberian peninsula and southern France (Ioannou and others 2007). The infusions from flower heads are reported to be used as an antipyretic and have been shown to posses hepatoprotective, antihypertensive, and intestinal anti-inflammatory properties (Novais and others 2004).
The antimicrobial activity of flower heads and leaves of S. rosmarinifolia against S. aureus, S. lutea, B. cereus, E. coli, and C. albicans was determined by Ioannou and others (2007) based on agar disc diffusion and broth dilution assays. From their study, the antimicrobial activity of essential oil from flower heads of S. rosmarinifolia at doses of 1, 5, and 10 μL was found to be stronger than those from the leaves, with inhibition zone diameters ranging from 8 to 23 mm. From the broth dilution assay performed on S. aureus, the MIC and MBC of 0.3 and 0.6 μL/mL were recorded indicating strong action of the flower head essential oil against this particular bacterium. The chemical composition of essential oil from flower heads of S. rosmarinifolia (assessed by GC-MS) was shown to be comprised of 42 components representing 92.3% to 94.0% of the oil, with β-eudesmol (13.5%), 1,8-cineole (12.9%), camphor (8.0%), borneol (5.1%), ar-curcumene (4.8%), terpinen- 4-ol (4.5%), and spathulenol (4.4%) as the main constituents.
Satureja bachtiarica Bunge. (savory)
The genus Satureja (family: Lamiaceae) contains more than 200 species of herbs and shrubs that are widely distributed in the Mediterranean region. Aerial parts of Satureja species are used as flavoring agents in a variety of food products as well as for herbal medicine preparations to treat gastrointestinal disorders (Sonboli and others 2004).
Pirbalouti and others (2010), by employing agar disc diffusion and serial dilution assays, determined the antimicrobial activities of some of the Iranian folklore herbs against S. aureus, E. coli, P. aeruginosa, and Klebsiella pneumonia. From the results of their screening tests, the ethanolic extract and essential oils of flowers of S. bachtiarica showed strong antibacterial activity against all the tested bacteria with the zone of inhibitions ranging from 12 to 23 mm. The MIC values ranged from 0.039 to 10.00 mg/mL. This study showed essential oils to exhibit stronger antibacterial activity than the ethanol extract.
Tamarix gallica L. (French tamarisk)
Tamarix gallica (family: Tamaricaceae) is a halophytic tree found growing in natural habitats ranging from coastal regions up to deserts. This plant is known to tolerate a wide range of harsh environmental conditions and can resist abiotic stress, such as salt, high temperature, and dryness (SaÏdana and others 2008; Ksouri and others 2009). In certain parts of Asia, the leaves, flowers, and galls of T. gallica are used as therapeutic agents, particularly as anti-inflammatory, antidiarrheic, cicatrizing, and antiseptic agents. They are also used for treating leucoderma, spleen trouble, and eye diseases (Ksouri and others 2009). Besides, the plant parts are used as astringent, aperitif, stimulant of perspiration, and diuretic (SaÏdana and others 2008; Ksouri and others 2009).
Based on the inhibition zone measured by the disc diffusion assay (Ksouri and others 2009), the methanolic floral extracts (2, 4, and 100 mg/mL) of this plant exhibited antibacterial activities against S. epidermidis, S. aureus, M. luteus, E. coli, and P. aeruginosa. The floral extracts also showed antifungal activity against Candida spp. with inhibition zones of 6 to 15 mm and 6 to 8.66 mm, respectively. The bioactive compounds present in the flowers were identified as phenolic acids (gallic, sinapic, chlorogenic, syringic, vanillic, p-coumaric, and trans-cinnamic acids) and flavonoids (catechin, isoquercetin, quercetin, apigenin, amentoflavone, and flavone).
Thymus daenensis Celak (thyme)
The genus Thymus (family: Lamiaceae), is a perennial herb that has its origin in the Mediterranean region. Leaves and flowers of Thymus species are traditionally used in Iran as tonic and herbal tea, antitussive, antiseptic, carminative, and a treatment for the common cold. Thymus oil and extracts (leaves and flowers) are widely used by the pharmaceutical, perfume, cosmetic, and food industries (Nejad Ebrahimi and others 2008; Pirbalouti and others 2010).
In one of the experiments conducted by Pirbalouti and others (2010) on Iranian folklore herbs, the extract and essential oil from flowers of T. daenensis have been shown to exhibit strong antibacterial activity against all the test bacteria (S. aureus, E. coli, P. aeruginosa, K. pneumoniae) with inhibition zones ranging from 8 to 22 mm. The MIC values of flower extracts ranged from 0.039 to 10.00 mg/mL. The results of this study clearly showed essential oil to have stronger antibacterial activity than the solvent (ethanol) extracts.
Zingiber mioga (Thunb.) Roscoe (Myoga)
Zingiber mioga (family: Zingiberaceae), a native to eastern Asia, is widely cultivated in Japan as a perennial herb. The stalks of this plant extend up to 1 m, with slender leaves reaching 30 cm having pine-cone-like flower buds ranging up to 7 to 10 cm in length. In Japan, “myogabochi,” a traditional food (buns filled with sweetened bean paste) is wrapped with the leaves of this plant to be preserved for a long period. Flower buds have a unique and pungent flavor, attributed to the presence of diterpene dialdehyde compound, 2-alkyl-3-methoxypyrazine and (E)-8-β-(17)-epoxylabd-12-ene-15, 16 dial (myogadial). Due to their pungent flavor, the flower buds have been used as a spice and to prepare pickles in Japan (Sakakibara and others 1991; Abe and others 2002).
Abe and others (2004) screened for the potential antimicrobial activities of the flower buds against a wide range of bacteria, yeasts, and molds by employing the agar disc diffusion assay. The results of their study revealed ethyl acetate extracts of flower buds to exhibit appreciable antimicrobial activity with an inhibition zone of 8 mm, while the methanol extract showed minimal activity with an inhibition zone of 2 mm. Additionally, MICs of the 3 diterpene dialdehydes (myogadial, galanal A, and galanal B) isolated from the ethyl acetate extract were measured using the serial dilution method. From the results, myogadial, galanal A, and galanal B were found to be effective against Gram-positive bacteria and yeast with myogadial (MIC = 25 to 125 μg/mL) showing higher activity compared to galanal A and B (MIC = 200 to 500 μg/mL). The reason for the antimicrobial activity has been attributed by the authors to the presence of galanal A and B that were present in the 1, 6 positions of the aldehyde groups, with the introduction of a hydroxyl group in their molecules.
Major Bioactive Compounds in Flower Extracts
Bioactive compounds isolated from flowers and their extracts have shown potential antimicrobial activities. The isolated compounds are classified into different groups, such as phenolics, terpenoids, essential oils, glycosides, and alkaloids. A few of these are discussed in the preceding text below.
Among the various phenolic compounds, flavonoids are found abundantly in plant-derived foods (Denny and Buttriss 2007). Flavonoids (consisting of a central 3-ring structure), are major phenolic compounds that play a pivotal role in plants, such as protection against UV, pigmentation, stimulation of nitrogen fixing nodules, and disease resistance (Pierpoint 2000; Denny and Buttriss 2007). This group of compounds can occur as glycosides too. Flavonoids are known to exhibit rich antioxidant, anticarcinogenic, and anti-inflammatory properties. Besides, flavonoids are known to possess antimicrobial activities due to their ability to form complexes with extracellular and soluble proteins and microbial cell wall (Tsuchiya and others 1996; Cowan 1999; Grotewold 2006).
Phenolic compounds as phenolic acids (gallic, sinapic, chlorogenic, syringic, vanillic, p-coumaric, and trans-cinnamic acids) are also found in flower extracts (Ksouri and others 2009).
Quinones are highly reactive compounds that have aromatic rings with 2 ketone substitutions. The antimicrobial activity of quinones involve forming of complex with proteins (surface-exposed adhesins), cell wall polypeptides, and membrane-bound enzymes, leading to inactivation of function of the proteins (Stern and others 1996; Cowan 1999). Besides, quinones are capable of rendering a substrate unavailable to microorganisms, thus inhibiting their growth. Mostafa and others (2011) described the presence of anthraquinones in various extracts of R. vesicarius flowers, which served as one of the antimicrobial compounds against the tested microorganisms (K. pneumonia, S. pneumoniae, S. pyogenes, S. aureus, E. coli and P. aeruginosa). In one of the experiments conducted by Bhalodia and others (2011) on hydro-alcohol and chloroform extracts of C. fistula flowers, anthraquinones present in the extracts was found to exhibit rich antimicrobial activities, which were found to be active against some sensitive bacteria and fungi (S. aureus, S. pyogenes, E. coli, P. aeruginosa, A. niger, A. clavatus, and C. albicians).
Essential oils are secondary metabolites containing a mixture of compounds that are based on 5 carbon isoprene structure (terpenes) occurring as diterpenes, triterpenes, tetraterpenes (C20, C30, and C40), hemiterpenes (C5), and sesquiterpenes (C15) (Cowan 1999). The antimicrobial activity of terpenes involves disruption of cell membrane by their lipophilic constituents. Preliminary phytochemical screening on essential oils extracted from the flowers of C. macropodum (Ebrahimabadi and others 2010), C. trifurcatum (Sassi and others 2008b), C. operculatus (Dung and others 2008), L. japonica (Rahman and Kang 2009), and S. rosmarinifolia (Ioannou and others 2007) have shown monoterpenes, sesquiterpenes, and their oxygenated derivatives to be major constituents, which contribute substantially to antimicrobial activities.
Plant sterols (also terpenoids), have the ability to reduce total and low-density lipoprotein cholesterol level in plasma (Denny and Buttriss 2007). Flowers of C.operculatus and R. vesicarius are reported to contain sterols (Dung and others 2008; Mostafa and others 2011). However, not much research reports are available on these aspects.
Glycosides are composed of a variety of secondary metabolites bound to a mono- or oligosaccharide or to uronic acid. The part of saccharide or uronic acid is known as glycone, and the backbone is known as aglycone. Cardiac glycosides, cyanogenic glycosides, glucosinolates, saponins, and anthraquinone glycosides, being different in their aglycone structures, are the main groups of glycosides (Bernhoft 2010). Investigations carried out on the flowers of C. fistula, C. Juncea, and R. vesicarius have clearly demonstrated the presence of glycosides in their extracts (Chouhan and Singh 2010; Bhalodia and others 2011; Mostafa and others 2011).
Alkaloids are heterocyclic, nitrogen-containing compounds, with potential clinical properties. Alkaloids have bitter taste and are present in threshold level in plants (Bernhoft 2010). Flowers of C. juncea and R. vesicarius have been reported to contain alkaloids (Chouhan and Singh 2010; Mostafa and others 2011).
Even though a wide range of bioactive compounds might be present in flowers, still there is a lack of detailed investigations carried out on these aspects. These needs to be explored further to confirm the antimicrobial activities exhibited.
Conclusion and Outlook
Edible flowers from ornamental, cultivated, as well as wild plants have high potential to be explored as natural resources of antimicrobial agents. Exploring these underutilized flowers by providing adequate scientific evidence might enhance the chances of developing new conventional and natural antimicrobial agents (drugs as well as food preservatives) and be good alternatives to synthetic chemicals. Screening for the potential bioactive compounds capable of exhibiting antimicrobial activities might provide more in-depth details. Further studies are warranted to identify various mechanisms involved in the antimicrobial actions exhibited by the bioactive compounds present in flowers (as to how they interact with a microorganism to cause inhibitory or lethal effects). As flower extracts and their essential oils have been proven to possess antimicrobial activities, these can be incorporated into developing new and novel biopolymer-based edible films, especially for preserving fresh produce.
Based on the reports available from in vitro studies conducted till date, high extraction yields and strong antimicrobial activities have been demonstrated by plant materials extracted in methanol (Quarenghi and others 2000; Annegowda and others 2011; Mann and others 2011). However, methanol can be highly toxic to humans and livestock and cannot be considered as a food grade solvent. In comparison to methanol, solvents such as ethanol and water, which have also exhibited appreciable antimicrobial activities, can be considered safe and results generated by using these solvents can be beneficial for food and pharmaceutical applications.
Flower extracts and their essential oils have many traditional uses, such as in the preparation of foods and herbal remedies, with minimal known “side-effects” on human health. Being natural, they are accepted to be highly safe for consumption. Of late, edible flowers are extensively being explored for commercial applications in food industries such as for development of floral teas, beverages, functional foods, and bakery products. However, to date, research works undertaken on issues pertaining toward safety and toxicity of flower extracts and their essential oils are scarce. The effectiveness of flower extracts against a wider spectrum of pathogenic microorganisms needs to be investigated before being used for food preservation or medicinal purposes. In addition, clinical effects of the floral extracts and their essential oils under “in vivo” conditions as well as in food systems need to be studied to evaluate in detail their potential effectiveness as antimicrobial agents as well as presence of any acute or chronic effects.
The sensory qualities of a foodstuff are related to consumers’ acceptance. With regard to flowers, sensory qualities are attributable to chemical constituents, particularly the volatile and colored compounds. The chemical compounds will affect the organoleptical attributes (desirable or undesirable with regard to appearance, colors, tastes, and odors) of food into which the flower extract or essential oil are being incorporated. The flowers with intense characteristic colors or pleasant aroma may be feasible to be exploited as a food colorant or food fragrance.
Based on the available literature, it is evident that still a wide gap persists in the scientific knowledge with regard to many other indigenous flowers (common and wild flowers) used for culinary and therapeutic purposes, which include: banana flowers, coconut flowers, cheddar pink, rosebay willowherb, champika, chickweed, Dutch clover, sweet snow, and others (just to name a few). This merits further investigation to search for potential antimicrobial activities and for prospective food industry applications.
The authors gratefully acknowledge anonymous referees and Scientific Editor (Prof. Dr. Manfred Kroger) for comments and constructive suggestions provided for improving the quality of this manuscript. First author thanks Inst. of Postgraduate Studies, Univ. Sains Malysia for the fellowship provided. Individual research fund provided as an RU grant (Nr 1001/PTEKIND/814139,USM) for the corresponding author is also gratefully acknowledged.