Effects of growth phase and temperature on σ^b activity within a Listeria monocytogenes population: Evidence for RsbV-independent activation of σ^b at refrigeration temperatures

The alternative sigma factor σ B of Listeria monocytogenes is responsible for regulating the transcription of many of the genes necessary for adaptation to both food-related stresses and to conditions found within the gastrointestinal tract of the host. The present study sought to investigate the influence of growth phase and temperature on the activation of σ B within populations of L. monocytogenes EGD-e wild-type, ΔsigB, and ΔrsbV throughout growth at both 4°C and 37°C, using a reporter fusion that couples expression of EGFP to the strongly σ B -dependent promoter of lmo2230. A similar σ B activation pattern within the population was observed in wt-egfp at both temperatures, with the highest induction of σ B occurring in the early exponential phase of growth when the fluorescent population rapidly increased, eventually reaching the maximum in early stationary phase. Interestingly, induction of σ B activity was heterogeneous, with only a proportion of the cells in the wt-egfp population being fluorescent above the background autofluorescence level. Moreover, significant RsbV-independent activation of σ B was observed during growth at 4°C. This result suggests that an alternative route to σ B activation exists in the absence of RsbV, a finding that is not explained by the current model for σ B regulation.