Effect of weak acids on Listeria monocytogenes survival: Evidence for a viable but nonculturable state in response to low pH

Listeria monocytogenes is capable of surviving environmental conditions that are normally fatal to many other bacteria, enabling this pathogen to remain on foods and eventually establish infection after consumption. This study investigated the extent to which L. monocytogenes is killed by several weak acids, with particular emphasis on the commonly used food preservative, potassium sorbate. The effects of potassium sorbate were found to be highly pH dependant, with significant killing only observed at a pH of 4.0. At this pH, the order of effectiveness of the acids tested was Na benzoate > Na propionate > acetic acid > K sorbate. Temperature had a significant impact on sensitivity, with K sorbate being effective at 50 mM and elevated (37 °C) temperature, but failing to affect survival when cells were at 4 °C or 21 °C. Finally, we found that cells grown in the presence of K sorbate at pH 4.0 entered a viable but nonculturable ("VBNC") state, but became nonviable by 24 h. In contrast, cells incubated under these conditions in the absence of K sorbate entered the VBNC state and maintained viability throughout the 24 h study period.