Eucalyptus spp. (family Myrtaceae) originated in Australia, but these plants now grow in almost all tropical and sub-tropical areas, and are cultivated in many other climates. Much research has been conducted on the medicinal properties of Eucalyptus spp. The leaf extract or essential oil from the leaves of Eucalyptus spp. has been reported to possess antifungal, antibacterial, mosquito repellent and antioxidant properties [1–3]. Against this background, the present study investigated the antibacterial effects of Eucalyptus globulus leaf extract against pathogenic bacteria isolated from patients with respiratory tract infection.
Clinical specimens from 200 patients (112 male, 88 female; aged 15–62 years) with pharyngitis and other respiratory tract infections who had been referred to the Imam Khomaini and Shariati hospitals (Tehran, Iran) were collected between September 2003 and September 2004. The specimens were cultured on blood agar and cefsulodin chocolate agar (Difco, Detroit, MI, USA) containing horse blood 7% v/v, and were incubated at 37°C for 24 h. Identification of bacterial isolates was based on standard biochemical tests [4–7]. The pathogenic bacteria isolated comprised 56 Staphylococcus aureus, 25 Streptococcus pyogenes, 12 Streptococcus pneumoniae and seven Haemophilus influenzae; no growth was observed for 100 (50%) specimens.
E. globulus leaf extract was prepared by the School of Pharmacy, Tehran University of Medical Sciences (Tehran, Iran) by extraction with methanol (80°C) using the percolation method [8,9], dissolved in sterile Mueller–Hinton broth (Difco), with or without horse blood 5% v/v, and then filtered using 0.5-µm filters (Millipore, Billerica, MA, USA). Different concentrations of E. globulus leaf extract were used to prepare a series of dilution tubes. For the broth dilution susceptibility tests, a standard inoculum of each microorganism (1 × 106 CFU/mL, equivalent to a 1:500 dilution of a suspension equal to a McFarland standard of 1.0) was added to an equal volume of each concentration of E. globulus leaf extract, and to a tube of the growth medium without added leaf extract to serve as a growth control. An uninoculated tube of medium was incubated to serve as a negative control. After incubation for 18–24 h at 37°C, the lowest concentration of the E. globulus leaf extract that inhibited growth of the organism was designated as the MIC. Following determination of the MIC, 0.1 mL from each of the tubes without visible turbidity was subcultured on to blood agar and cefsulodin chocolate agar, and then incubated at 37°C for a further 24–48 h. Following incubation, the number of colonies obtained was compared with the number of CFU/mL in the original inoculum. The lowest concentration of leaf extract that allowed < 0.1% of the original inoculum to survive was defined as the MBC[5,10]. The results obtained are summarised in Table 1. MIC50, MIC90 and MBC values ranged from 16 to 64 mg/L, from 32 to 128 mg/L, and from 64 to 512 mg/L, respectively, depending on the species.
|Pathogenic bacteria||Eucalyptus globulus leaf extract (mg/L)|
|Staphylococcus aureus (n = 56)||64||128||512|
|Streptococcus pyogenes (n = 25)||32||64||128|
|Streptococcus pneumoniae (n = 12)||16||32||64|
|Haemophilus influenzae (n = 7)||16||32||64|
Staph. aureus, Strep. pyogenes, Strep. pneumoniae and H. influenzae are the most important causes of bacterial infection of the human respiratory tract. The emergence of resistance to conventional antimicrobial agents poses a serious problem for physicians, and necessitates ongoing development of new antimicrobial agents that can inhibit the growth of or kill resistant organisms. Eucalyptal (1,8-cineole) is the active ingredient of eucalyptus oil that is responsible for its various pharmacological activities. Pharmacopoeial-grade dried eucalyptus leaf must contain a minimum of 2.0% v/w volatile oil, composed mainly of 1,8-cineole. Oil of eucalyptus has been used traditionally as an antiseptic and in the treatment of respiratory tract infections. However, scientific and toxicological data regarding its antibacterial action are lacking, and its current applications are focused on topical use as an antiseptic [11,12]. The results of the present study with bacteria isolated from patients with respiratory tract infections suggest that further studies to clarify the possible role of E. globulus leaf extract in the treatment of respiratory tract infections are warranted.