Inhibitory effects of selected plant essential oils on the growth of four pathogenic bacteria: E. coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus and Listeria monocytogenes
Introduction
Foodborne illness resulting from consumption of food contaminated with pathogenic bacteria has been of vital concern to public health. Among the reported outbreaks in the United States during 1993–1997 period for which the etiology was determined, bacterial pathogens caused the largest percentage of outbreaks (75%) and the largest percentage of cases (86%) (Olsen, MacKinnon, Goulding, Bean, & Slutsker, 2000). Salmonella sp., Listeria monocytogenes and E. coli accounted for the largest number of outbreaks, cases, and deaths. In Canada, the cost to treat the foodborne disease due to meat and meat products contamination is estimated to $500 millions per year (Todd, 1989). To reduce health hazards and economic losses due to foodborne microorganisms, the use of natural products as antibacterial compounds (Conner, 1993, Dorman and Deans, 2000) seem to be an interesting way to control the presence of pathogenic bacteria and to extend the shelf life of processed food. These compounds could be added during the food process. Among these products, essential oils from spices, medicinal plants and herbs have been shown to possess antimicrobial activities and could serve as a source of antimicrobial agents against food pathogens (Deans and Ritchie, 1987, Kim et al., 1995). Several references on the antimicrobial and antifungal efficiency of essential oils are available in the literature (Burt and Reinders, 2003, Cox et al., 2000, Delaquis et al., 2002, Mejholm and Dalgaard, 2002, Nielsen and Rios, 2000). More particularly, essential oils and their components are known to be active against a wide variety of microorganisms, including Gram-negative (Helander et al., 1998, Sivropoulou et al., 1996) and Gram-positive bacteria (Kim et al., 1995). Gram-negative bacteria were shown to be generally more resistant than Gram-positive ones to the antagonistic effects of essential oils because of the lipopolysaccharide present in the outer membrane (Russel, 1991) but this was not always true (Karapinar & Aktung, 1987).
Essential oils are the odorous, volatile products of an aromatic plant’s secondary metabolism, normally formed in special cells or groups of cells, found in many leaves and stems. They are commonly concentrated in one particular region such as leaves, bark or fruit, and when they occur in various organs in the same plant, they frequently have different composition profiles. Essential oils have long served as flavoring agents in food and beverages, and due to their versatile content of antimicrobial compounds, they possess potential as natural agents for food preservation (Conner, 1993). The antimicrobial activity of essential oils is assigned to a number of small terpenoids and phenolic compounds (thymol, carvacrol, eugenol), which also in pure form demonstrate high antibacterial activity (Conner, 1993, Didry et al., 1993, Karapinar and Aktung, 1987). The antimicrobial activities of oregano, savory, and thyme were first reported during 1950s and it was established that the inhibitory effect of oregano, savory, and thyme is likely due to their high content of thymol and carvacrol, which are among the most efficient herbal antibacterial agents known (Nevas et al., 2004, Sivropoulou et al., 1996). However, there are often large differences in the reported antimicrobial activity of oils from the same plant. The reasons of this variability can be due to the different geographical sources, the harvesting seasons, the genotype, the climate, the drying procedure and the distilled part of the plant. All of this variability influences the chemical composition and the relative concentration of each constituent in the essential oils (McGimpsey and Douglas, 1994, Rhyu, 1979, Salgueiro et al., 1997, Venskutonis, 1996). A number of essential oils constituents exhibit significant antimicrobial properties when tested separately (Kim et al., 1995, Lambert et al., 2001). However, there is evidence that essential oils are more strongly antimicrobial than is accounted for by the additive effect of their major antimicrobial components; minor components appear, therefore, to play a significant role (Paster, Menasherov, Ravid, & Juven, 1995).
Even if essential oils are considered as safe (GRAS) (Lambert et al., 2001), their use is often limited by organoleptical criteria. For this reason, it will be necessary to determine the minimum concentration necessary to inhibit the growth of pathogenic bacteria without affecting the sensory quality of the food. Also, the majority of the preceding studies carried out with commercial essential oils seldom compared the antimicrobial activity of these oils on the basis of their chemical composition. The objective of the present study was to evaluate the minimum inhibitory concentration (MIC) and the maximal tolerated concentration (MTC) of whole essential oils. The antibacterial activity of twenty-eight oils was evaluated on four bacterial pathogens (Gram+ and Gram−).
Section snippets
Preparation of bacterial strains
Escherichia coli O157:H7 EDL 933 (INRS-Institut Armand Frappier, Laval, QC, Canada), Salmonella Typhimurium SL1344 (INRS-Institut Armand-Frappier, Laval, QC, Canada), Listeria monocytogenes 2812 1/2a (Health Canada, St-Hyacinthe, QC, Canada) and Staphylococcus aureus ATCC 29213 were maintained at −80 ± 1 °C in Brain Heart Infusion broth (BHI, Difco Laboratories, Detroit, MI, USA) containing 10% glycerol. Prior to the experiment, working cultures were prepared by subculturing 100 μl of each stock
Results and discussion
The MICs and MTCs of each essential oil are presented in Table 1. Of the 28 essential oils tested, Corydothymus capitatus, Cinnamomum cassia, C. verum (bark), Satureja montana and Origanum heracleoticum seem to be the most efficient essential oils against the four pathogenic bacteria tested. The MIC and MTC values obtained in the presence of E. coli O157:H7, S. Typhimurium, S. aureus and L. monocytogenes were ⩽ 0.05% and ⩽ 0.025% (vol/vol) for these five essential oils.
Even if all essential oils
Conclusions
Food contamination is still an enormous public health problem, but may be better controlled by the use of natural preservatives. This study showed that many essential oils possess an antibacterial activity against the four bacteria tested. All of them influence bacterial growth at different concentrations. Among the essential oils tested, Corydothymus capitatus, Cinnamomum cassia, C. verum from bark, Satureja montana and Origanum heracleoticum were the most active with a smaller MIC on the
Acknowledgement
This research was supported by the Québec Ministry of Agriculture, Fishery and Food (CORPAQ Program).
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