D‐tryptophan, an eco‐friendly natural, safe, and healthy compound with antimicrobial activity against food‐borne pathogens: A systematic review

Abstract Recently, the use of D‐amino acids as food preservatives has attracted considerable attention because these natural compounds do not have adverse effects on human health. In addition, D‐amino acids such as D‐tryptophan can reduce the harmful effects of other treatments. For instance, the use of D‐tryptophan in food reduces the requirement for high temperatures and their damaging effects on nutrients such as proteins and vitamins. The purpose of this systematic review was to investigate the antimicrobial effect of D‐tryptophan on food‐borne pathogens in vitro and in food models. To identify related studies, scientific digital databases such as PubMed, Science Direct, and Google Scholar were searched from January 2000 to February 2023. The results of the studies showed that when D‐tryptophan was used with other stresses such as using different salt concentrations, refrigeration, or high temperatures, it showed significant antimicrobial effects on Gram‐positive and Gram‐negative food‐borne pathogens, and antibiofilm impacts were also observed with D‐tryptophan. Since studies have shown that the antimicrobial activity of D‐tryptophan depends on several factors, including the pathogen strain, the type of stress, and the concentration of D‐tryptophan, and every article has focused on one of these factors, there is a need for a systematic review that summarizes and concludes the effect of all these factors on the antimicrobial activity of D‐tryptophan against food‐borne pathogens.

On the other hand, synthetic antimicrobial compounds are not only harmful to human health but may also cause off-flavors in foods.Moreover, the excessive use of these compounds has caused bacterial resistance (Afshari et al., 2022;Batiha et al., 2021;Chen et al., 2022).Therefore, the demand for healthy, natural, and high-quality processed foods is growing rapidly.One solution to reduce the use of chemical preservatives is to replace them with bio-preservatives (Elafify, Sadoma, et al., 2022;Mith et al., 2014;Moghimani et al., 2023;Mohan et al., 2021).
Bio-preservatives are produced naturally by plants, animals, and bacteria (Batiha et al., 2021;Liang et al., 2019;Moghimani et al., 2023;Salimnejhad et al., 2023).In this regard, different studies were conducted to find natural and safe antimicrobial compounds to be used in the food industry (Elafify et al., 2019;Erfani et al., 2023;Touranlou et al., 2023).D-amino acids are a group of these antimicrobial compounds (Martínez-Rodríguez et al., 2010).D-amino acids are enantiomers of L-amino acids, which were considered non-functional for a long time.However, further studies discovered that some D-amino acids, which are in mammalian tissues, contribute to critical physiological activities (Seki et al., 2022).D-tryptophan is one of these D-amino acids, and studies on it have approved its antimicrobial properties.For instance, Koseki et al. (2015) reported that D-arginine and D-proline had a mild antimicrobial effect at 40 mM and ≥3% NaCl, whereas D-tryptophan had a more significant antimicrobial effect under similar conditions (more than a 12-h delay in the growth of various pathogens) (Comai et al., 2020;Koseki et al., 2015).
Another characteristic of D-tryptophan is its antibiofilm activity.Biofilms are communities formed by one or more bacterial species in an extracellular matrix (Li et al., 2015;Rezaei et al., 2023).The formation of biofilms causes several problems.
The mechanism of antimicrobial and anti-biofilm activity of Dtryptophan has not yet been fully understood, but it seems that these functional properties are due to the prevention of initial cell adhesion caused by changes in the extracellular matrix (Elafify et al., 2019;Elafify, Sadoma, et al., 2022).
Since studies have shown that the antimicrobial activity of Dtryptophan depends on several factors, including the strain of the pathogen, the type of stress, and the concentration of D-tryptophan, and every article has focused on one of these factors, there is a need for a systematic review that summarizes and concludes the effect of all these factors on the antimicrobial activity of D-tryptophan against food-borne pathogens.

| ME THOD
This article is a systematic review based on PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analysis) (Figure 1) to explore the effect of the antimicrobial activity of D-tryptophan on food-borne pathogens in vitro and in food models.

F I G U R E 1
Flow diagram of the study selection process.

| Searchstrategy
To identify relevant studies, scientific digital databases such as PubMed, Science Direct, Web of Science, and Google Scholar were searched from January 2000 to February 2023.A total of 8912 articles were collected from all databases, 13 of which met the inclusion criteria (Figure 1).

| Dataeligibility,articleselection,and resultsynthesis
The articles were screened by MM (Minoo Moghimani) based on their titles and abstracts and were then re-examined by SN (Seyyed Mohammad Ali Noori), AA (Asma Afshari), and MH (Mohammad Hashemi).Articles that did not meet the inclusion criteria were removed.Finally, the full text of the related articles was assessed.

| Inclusionandexclusioncriteria
The included articles should investigate the antimicrobial and antibiofilm effects of D-tryptophan on food-borne pathogens, either in vitro or in food models, and should have been published in English.
The exclusion criteria were review articles, doctoral thesis, articles that had been published in a language other than English, articles that had investigated the antimicrobial effect of D-tryptophan in vivo, articles that had worked on clinical pathogens rather than food-borne pathogens, and articles that had studied the antibiofilm effect of D-tryptophan in an industry other than food.

| Riskofbiasassessment
Different sections of the included articles were qualitatively reviewed by the authors to assess the risk of bias.The results of this evaluation are shown in different colors, such as green (good), yellow (moderate), and red (poor), in Table 1.
Although the mechanism of antimicrobial activity of D-tryptophan is still not well known, it appears that D-tryptophan reduces the initial adhesion of pathogenic cells and changes the characteristics of the extracellular environment, which have a significant effect on the weakening of pathogenic cells.D-tryptophan also prevents the formation of biofilms by using the same mechanism and acts as an autoinhibitory compound, which prevents the germination of spores and the growth of pathogens (Elafify et al., 2019(Elafify et al., , 2023;;Elafify, Sadoma, et al., 2022).
According to studies, several factors affect the antimicrobial activity of D-tryptophan, as shown in Figure 2.

| Thestrainofthepathogen
The results of the studies revealed that the antimicrobial effect of D-tryptophan is strain-dependent.For instance, Chen et al. ( 2022)

| Osmoticstress
The results regarding the antimicrobial effect of D-tryptophan, along with osmotic stress on different food-borne pathogens, are summarized in Table 2.These results showed that co-administration of D-tryptophan and osmotic stress inhibited the growth of foodborne pathogens more than the control (without D-tryptophan) in all studies (Chen et al., 2018(Chen et al., , 2022;;Kan et al., 2018;Koseki et al., 2015;Radoshevich & Cossart, 2018).
The hypothesis justifying the effect of osmotic stress on the antimicrobial activity of D-tryptophan is that bacterial cells absorb a set of compatible solutes, such as glycine betaine, to manage osmotic stress.Therefore, the co-administration of osmotic stress and D-tryptophan causes more absorption of D-tryptophan by the bacterial cell, which is an incompatible compound, and subsequently causes disturbances in cell metabolism (Koseki et al., 2015).Kan et al. (2018) investigated the effect of ionic (sodium chloride and potassium chloride) and molecular (sugar) compounds on creating the osmotic pressure, which showed that the effective osmotic pressure with D-tryptophan to reduce E. coli was 2.4 MPa.They also reported that sugar, in amounts of 58.6%, created an osmotic pressure of 2.4 MPa, but together with D-tryptophan had no inhibitory effect, whereas potassium chloride and sodium chloride, with concentrations of 6.3% and 3% and osmotic pressure of 2.4 Mpa, had a weak inhibitory effect and a remarkable inhibitory effect with Dtryptophan, respectively (Table 2).Therefore, it can be concluded that the use of ionic components, especially sodium chloride, is more suitable for creating effective osmotic pressure because its small amounts can create enough osmotic pressure with D-tryptophan, which inhibits the growth of pathogens (Kan et al., 2018).

| Temperaturestress
The results regarding the antimicrobial effects of D-tryptophan and temperature stress on different food-borne pathogens have been summarized in Table 3.These results indicate that co-administration of D-tryptophan and low temperature (refrigeration) have a better antimicrobial effect compared to the control (without D-tryptophan).
Chen et al. ( 2022) showed that Shivanella baltica and Pseudomonas fluorescens, without D-tryptophan, tolerated refrigeration, and the presence of D-tryptophan could effectively reduce their growth (Chen et al., 2022).Kan et al. (2018) reported that D-tryptophan does not require high temperatures to eliminate pathogens.In this study, the presence of D-tryptophan at a temperature of 43-45°C reduced the number of E. coli below the detection limit (4.5 log cfu/mL reduction).In contrast, without D-tryptophan, higher temperatures were required to reduce E. coli (Kan et al., 2018).Therefore, the use of Dtryptophan, by reducing the temperature required to destroy pathogens, prevents the adverse effects of high temperatures on protein structure and the performance of essential amino acids and vital nutrients in foods (Abraha et al., 2018).
The mechanism of the effect of administration of D-tryptophan and temperature stress is such that when the bacterium is exposed to D-tryptophan as an environmental stress, it produces a series of defensive physiological responses by changing its metabolic activity, and protects the bacterial cell against D-tryptophan.One of the control factors of the change in metabolic activity is temperature, and changes in temperature lead to dysfunctions in the development of physiological defense responses, and finally, the bacterial cell becomes vulnerable to D-tryptophan (Figure 3) (Kan et al., 2018).

| ConcentrationofD-tryptophan
The results of the studies suggested that the concentration of Dtryptophan is another effective factor in the antimicrobial activity of D-tryptophan.The results also showed that the ideal concentration is 40 mM, and concentrations below 40 mM do not have an acceptable inhibitory effect (Chen et al., 2020;Elafify, Sadoma, et al., 2022).

| AntibiofilmactivityofD-tryptophan
Some bacteria can adapt to harsh chemical and physical conditions by forming a matrix consisting of exopolysaccharides, proteins, and extracellular DNA called biofilm in the environment (Sarkar & Pires, 2015).The capacity of food-borne pathogens to create biofilms is the main virulence factor in their pathogenesis and transmission.For instance, the formation of biofilm by Campylobacter jejuni, a food-borne pathogen, facilitates its transmission through the food chain.Unfortunately, there are no solutions to prevent it (Elgamoudi et al., 2020;Li et al., 2015).In addition, biofilms can damage both industrial equipment and the environment and, in some cases, can cause irreparable damage (Jia, Yang, et al., 2017).
In most cases, the only solution to solve this problem is to replace biofilm-contaminated equipment with new ones, which can be expensive and life-threatening.Fortunately, studies have shown that some D-amino acids have antibiofilm properties.The mechanism of the antibiofilm activities of D-amino acids is shown in the cell surface (Hochbaum et al., 2011;Li et al., 2015).Moreover, biofilm formation increases bacterial antibiotic resistance, while the use of D-amino acids increases the effectiveness of antibiotics due to the destruction of biofilms (Elgamoudi et al., 2020;Zilm et al., 2017).
Studies also indicate that D-tryptophan is one of the D-amino acids and can prevent the initial formation and further expansion of biofilms (Elgamoudi et al., 2020;Li et al., 2015) (Figure 4).In a study conducted by Elgamoudi et al. (2020), it was observed that Dtryptophan reduces the expression of the two genes; alanine racemase (alr) and D-Ala-D-Ala ligase (ddlA) in C. Jejuni.Both of these genes encode enzymes that participate in the metabolism of Dalanine, which is necessary for peptidoglycan synthesis in the bacterial cell wall.Therefore, D-tryptophan prevents biofilm formation by C. jejuni (Elgamoudi et al., 2020).
Interestingly, L-amino acids, unlike D-amino acids, increased biofilm formation because pathogens can catabolize L-amino acids, which increases their growth and biofilm formation (Elgamoudi et al., 2020;Leiman et al., 2013).The results of the articles that investigated the antibiofilm activity of D-tryptophan against food-borne pathogens are summarized in Table 5.The results showed that D-tryptophan had the highest antimicrobial activity in the first 24 h, and its effect  gradually decreased and reached its lowest level after 72 h.The reason for this is that D-tryptophan prevents initial adhesion and biofilm formation in the first 24 h.In addition.Studies have also indicated that the antibiofilm effect of D-tryptophan increases in nutritious environments because bacterial cells have more activity and movement in these environments, which leads to a decrease in cell adhesion and biofilm formation (Elgamoudi et al., 2020;Li et al., 2015).

| CON CLUS ION
In most studies, the use of 40 Mm D-tryptophan, along with other stress conditions, had excellent antimicrobial effects on Gramnegative and Gram-positive food-borne pathogens.Among the stresses, higher temperatures (43°C, 45°C) had bactericidal effects.Therefore, it can be concluded that since D-tryptophan is a safe natural antimicrobial compound, its use reduces the risks of chemical preservatives and, at the same time, it reduces the negative effect of thermal treatments on the nutritional value of food.D-tryptophan also does not change the organoleptic characteristics of food, and it can be used as a bio-preservative in the food industry.
Another antimicrobial characteristic of D-tryptophan is its antibiofilm activity.A review of studies showed that D-tryptophan alone and specially combined with other D-amino acids have antibiofilm activity even in amounts of 10 Mm.
Finally, few available articles have investigated the antimicrobial activity of D-tryptophan in food models, and since there is a wide diversity of food models, further research in this area is considered necessary.
The results of risk of bias assessment are shown in different colors, such as green (good), yellow (moderate), and red (poor).
D-tryptophan is a safe natural compound, and in vitro studies have shown that D-tryptophan, along with other stresses including osmotic and temperature, has good antimicrobial activity, which makes it an appropriate natural food preservative(Chen et al., 2020).

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I G U R E 2 Effective factors on the antimicrobial activity of D-tryptophan.TA B L E 2 Studies conducted on the antimicrobial effect of D-tryptophan along with osmotic stress on food-borne pathogens.Studies conducted on the antimicrobial effect of D-tryptophan along with temperature stress on food-borne pathogens.investigated the antimicrobial effect of D-tryptophan on food-borne pathogens in different food models.The results shown in Table4indicate the antimicrobial effect of D-tryptophan on L. Monocytogenes were more significant in fresh heavy cream compared to milk, probably due to its lower water activity and higher fat content, which create conditions similar to osmotic stress(Elafify et al., 2020).Elafify et al. (2023) used D-tryptophan for the first time as an anti-spore additive in rice pudding, which could effectively prevent the growth of Bacillus cereus spores.The anti-sporogenesis mechanism of D-tryptophan is due to the autoinhibition phenomenon, which inhibits germination and prevents the growth of spores(Elafify et al., 2023).Chen et al. (2018) investigated the antimicrobial effect of 40 mM D-tryptophan on respiratory active cells of S. baltica and P. fluorescens in marine products at 1%-3% NaCl and 4°C.In this study, the number of cells decreased gradually, and the most significant reduction was observed at 3% NaCl concentration.The importance of respiratory active cells is due to their ability to aerobically spoil marine products because their number determines their spoilage potential.In fact, by preventing the growth of respiratory active cells, D-tryptophan delays the spoilage process in marine products (with a high spoilage rate) (Chen et al., 2018).Chen et al. (2022) also evaluated the effect of D-tryptophan at 3.5% NaCl and 4°C on the amount of total volatile basic nitrogen (TVB-N) in fish fillets.The initial amount of TVB-N in fish fillets was 8.0 mg/100 g.It reached 30 mg N/100 g in the control (without D-tryptophan) on the eighth day, which is considered rotten fish.However, with 40 mM D-tryptophan, the amount of TVB-N reached below 30 mg N/100 g on day 10 of the storage.The amount of TVB-N is considered an indicator of spoilage in marine products, and its reduction by D-tryptophan may be due to the impact of D-tryptophan on reducing the population of spoilage microorganisms or their metabolic activity in the oxidative deamination of non-protein nitrogen compounds(Chen et al., 2022).Elafify et al. (2019) investigated the impact of 40 mM Dtryptophan on the D-value.In this study, the D-value in the control (without D-tryptophan) at50, 55, and 60°C was 25.05, 16, and   10.57min, respectively.With D-tryptophan, the D-value was 12.28,   11.27, and 8.32 min, respectively (Elafify et al., 2019).It was similar to the results obtained byElafify et al. (2020).In this article, the Dvalue in control (without D-tryptophan) at 55, 60, and 65°C was 200, 97, and 86 min, respectively.While with 40 mM of D-tryptophan, it was 136, 75, and 75 min.These studies showed that the presence of D-tryptophan reduced the D-value, which also reduced the negative effect of high temperatures on nutrients(Elafify et al., 2020).

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Mechanism of co-administration of D-tryptophan and temperature stress.In general, when the bacterial cell is exposed to D-tryptophan, it creates physiological defensive responses through changing metabolic activities to protect itself, and temperature is one of these controlling factors; therefore, when the temperature changes, physiological defensive responses are disturbed, and the bacterial cell becomes vulnerable to D-tryptophan.TA B L E 4Studies conducted on the use of D-tryptophan as a natural preservative along with several stressful conditions in food.

Figure 4 .
Figure 4.Although the mechanism of their antibiofilm activity is not well known, it may be related to the ability of D-amino acids to prevent initial adhesion by changing the extracellular matrix.For example, it has been reported that Bacillus subtilis and Staphylococcus aureus bind to other cells through a series of surface proteins, and several D-amino acids prevent the colonization of these proteins on

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Mechanism of antibiofilm activity of D-tryptophan.TA B L E 5 Articles that have investigated the antibiofilm effect of D-tryptophan against food-borne pathogens.