Listeria monocytogenes responds to cell density as it transitions to the long-term-survival phase

Listeria monocytogenes was recently found to enter a long-term-survival (LTS) phase, which may help explain its persistence in natural environments and within food processing plants. The purpose of this study was to investigate the effects of initial cell density, initial pH and type of broth (fresh vs. spent) on the transition of L. monocytogenes to the LTS phase and model the change in viable population density with time. Initial cell density (~ 106–~ 1010 CFU/ml) and initial pH (5.36–6.85) both significantly affected the transition of L. monocytogenes to the LTS phase (P < 0.001) with initial cell density being the main determining factor. In contrast, type of broth did not significantly affect cell density change during the transition of stationary-phase cells at high initial density to the LTS phase (P > 0.05). After 30-d incubation no significant differences in cell densities were observed between either type of broth or between any of the initial cell density/pH treatment combinations (P > 0.05), where the mean viable cell density was 4.3 ± 1.1 × 108 CFU/ml. L. monocytogenes responded to viable cell density in accordance with the logistic equation during transition to the LTS phase. The Agr quorum-sensing system does not appear to play a role in the transition to the LTS phase. Further research is needed to better understand the control mechanisms utilized by L. monocytogenes as it transitions to a coccoid, resistant and stable density state in the LTS phase.