Cold and carbon dioxide used as multi-hurdle preservation do not induce appearance of viable but non-culturable Listeria monocytogenes

摘要

Aims: To study whether the exposure to cold (4 °C) and carbon dioxide which results in the elongation of Listeria cells, induces a viable but nonculturable (VBNC) state. Methods and Results: When cold and CO_2 stressed L. monocytogenes were observed under a fluorescence microscope, using the LIVE/DEAD BacLight~(TM) bacteria viability kit (Molecular Probes, Eugene, OR, USA), the healthy, mildly injured, and the putative VBNC cells accounted for 30.0% of the stressed cell population. By using the selective plate count, 31.4% of the same stressed cell population was found to be healthy and mildly injured (putative VBNC cells not included). If these were VBNC state cells present, we should have observed a significant difference between the above two numbers. In fact, there was no significant difference between the results obtained from those two methods. Conclusions: There were no VBNC state cells observed in the stressed cell population. We conclude that cold and CO_2 do not induce L. monocytogenes to enter a VBNC state. Significance and Impact of the Study: Cold and modified atmospheres are widely used in fresh muscle food and fruit preservation. Whether they would induce L. monocytogenes into a VBNC state is of a great concern for microbial food safety.