Survival and genomic adaptation of Listeria monocytogenes in long-term culture

In natural environments, bacteria encounter adverse conditions that affect their growth. The exposure to stressors causes bacteria to escape unfavourable conditions by genetically adapting to them or by entering dormancy states. In this study, the aim was to evaluate the survival strategies of L. monocytogenes in long-term cultures that simulate real-life conditions that this pathogen may encounter in food-related environments (such as nutrient deprivation and cold storage). At selected time points, L. monocytogenes in the long-term stationary phase was tested for its culturability, for the presence of viable but nonculturable (VBNC) cells and mutations that were possibly involved in the adaptation and survival of this pathogen. Based on our findings, the cultures were able to survive long-term incubation at both 4 °C and 30 °C, but the survival strategies differed between the two temperatures. At 4 °C, the culturability of the cultures was maintained throughout the entire experiment, with lower counts of both mutations and VBNC-state cells. On the other hand, the ability of some cultures to grow in bacteriological media at 30 °C stopped between 180 and 365 days of incubation, and they developed greater numbers of mutations and VBNC cells. Notably, mutations in genes rpoB, clpX, spoT, encoding metabolic regulators, stress response factors, protein control systems, and other important transcriptional regulators could be involved in adaptation. Overall, these results highlight a difference in the survival behaviour of L. monocytogenes cultures based on incubation temperatures, in which refrigeration temperatures increased the stability of the cultures, a finding with direct relevance for food safety.