The influence of pre-cultural conditions on the individual cell lag phase and generation time of Listeria monocytogenes

In this study, we assessed the impact of pre-cultural temperature and pH on the lag phase of individual Listeria monocytogenes cells at 7 degrees C. In a first step, the pre-cultural temperature effect (37, 15, 10, 7, 4 and 2 degrees C) was investigated on a subsequent growth at 7 degrees C and pH 7.4. In a second step, low pre-cultural temperatures (10, 7 and 4 degrees C) were combined with a controlled pH at 7.4 and 5.7 with a subsequent growth at 7 degrees C and different pH values (7.4, 6.0 and 5.5). Growth was monitored by OD measurements at 600 nm. For all temperature-pH combinations, the individual cell lag phase and the subsequent growth rate were determined using a three-phase linear growth model. Around 100 replications were made for each set of conditions. The results were plotted as histograms, and distributions were fitted to these data. It was observed that at low pre-cultural temperatures, a high proportion of L. monocytogenes cells was able to grow with almost no lag phase. Regarding the pH effect, the pH transition from the pre-cultural to a growth media was proportional to the mean values of the lag phases. There was no remarkable effect observed on the generation time. Regarding the statistical distributions, the exponential distribution yielded the best fit in most cases, as the highest probability was observed for very small lag phases and decreased with lag phase increase. To apply the results obtained at an industrial level, a simulation in @RISK to predict the distribution of the lag phase of L. monocytogenes at different transitions of temperature was performed. These findings have important consequences to estimate the possible risks concerned L. monocytogenes for refrigerated food products