Bacterial radiosensitization by using radiation processing in combination with essential oil: Mechanism of action
Introduction
New trends in food irradiation technology consist to develop combined treatments to reduce the irradiation doses required to kill pathogenic bacteria and/or reduce overall microbial load (Caillet et al., 2006a, Caillet et al., 2006b; Lafortune et al., 2005). It was demonstrated that the combination of natural products and irradiation had a positive effect in increasing the radiosensitization of bacteria (Caillet et al., 2006a, Caillet et al., 2006b; Chiasson et al., 2004; Mahrour et al., 2003). The use of essential oils from spices as antibacterial compounds seem to be an interesting way to control the presence of pathogenic bacteria and/or to extend the shelf life of processed foods (Oussalah et al., 2004, Oussalah et al., 2007). Among these essential oils, oregano oil has been shown to possess antimicrobial activities and could serve as a source of antimicrobial agent against L. monocytogenes (Oussalah et al., 2007). Essential oil can produce protein denaturation and membrane structure disruption, leading to release of intracellular ATP and cell death (Rhayour et al., 2003). Although the actions of essential oils on the cell membrane and the release of cell components were already studied (Oussalah et al., 2007; Rhayour et al., 2003), their actions and the radiation effects on bacterial murein (peptidoglycan) are less well known. Most bacteria are enveloped with a rigid exoskeleton, called murein sacculus, allowing them to maintain their typical shape in spite of a high osmotic pressure inside the cell. Because the peptidoglycan sacculus is unique to bacteria and important for their survival, it is important to determine the effect of antimicrobial agents or treatments on murein walls and cell shape. The objective of this study was to evaluate the effect of three treatments, oregano essential oil alone, irradiation alone and a combination of oregano essential oil and irradiation, on the peptidoglycans of Escherichia coli 0157:H7 and L. monocytogenes murein. The antimicrobial mechanism was also examined by monitoring the release of ATP.
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Bacterial strain and growth conditions
E. coli O157:H7 EDL 933 (Institut Nacional de la Recherche Scientifique [INRS] Institut Armand-Frappier, and L. monocytogenes 2812 1/2a (Health Canada, St-Hyacinthe, Québec, Canada) were maintained at −80 °C in brain heart infusion broth (Difco, Becton Dickinson, Sparks, Md.) containing 10% glycerol. Before the experiment, a working culture was prepared by subculturing 1 ml of stock culture in BHI broth (9 ml), which was incubated at 35 °C through two successive incubations of 24 h to obtain
Relative radiation sensitivity
Results showed that the presence of Spanish oregano essential oils affected significantly (p⩽0.05) the radiosensitivity of E. coli O157H7 and L. monocytogenes,. A respective radiation D10 of 0.27 and 0.36 was observed without Spanish oregano. In presence of this active compound, the radiation D10 decreased, respectively, to 0.087 and 0.13 kGy. These results showed that the Spanish oregano increased, respectively, the radiation sensitivity by 3.10 and 2.75 times.
Muropeptide composition
Peaks obtained with retention
Acknowledgements
The authors are grateful to MDS Nordion International, Inc., for the irradiation operations. This research was supported by the Natural Sciences and Engineering Research Council of Canadian (NSERC).
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