Comparative assessment of acid, alkali and salt tolerance in Listeria monocytogenes virulent and avirulent strains

Listeria monocytogenes is an opportunistic bacterial pathogen of man and animals that has the capacity to survive under extreme environmental conditions. While our knowledge on L. monocytogenes and its ability to sustain within wide pH and temperature ranges and salt concentrations has been largely built on the virulent strains of this species, relatively little is known about avirulent strains in this regard. In this study, we extend our analysis on avirulent L. monocytogenes strains. By subjecting three virulent (EGD, 874 and ATCC 19196) and three avirulent (ATCC 19114, HCC23 and HCC25) strains to various pH and salt concentrations, it was found that L. monocytogenes recovered well after treatment with 100 mM Tris at pH 12.0, and to a lesser extent at pH 3.0. Interestingly, avirulent L. monocytogenes strains showed a somewhat higher tolerance to alkali than virulent strains. This unique feature of avirulent L. monocytogenes strains may potentially be exploited for the development of a rapid technique for differentiation between avirulent and virulent strains. Furthermore, all L. monocytogenes strains tested were resistant to saturated NaCl (about 7 M, or 40% w/v) for a long period of time (20 h and possibly longer). Together, these results highlight that acid, alkali, and/or salt treatments commonly used in food product processing may not be sufficient to eliminate L. monocytogenes, and therefore stringent quality control measures at the beginning and end of the food manufacturing process is essential to ensure that such food products are free of listerial contamination.