The Alternative Sigma Factor σ L of L. monocytogenes Promotes Growth Under Diverse Environmental Stresses

Listeria monocytogenes are important foodborne pathogens that can cause outbreaks of serious human disease. These organisms frequently colonize and proliferate on preserved food products despite exposure to stress conditions induced by low storage temperatures, inclusion of organic acid–based preservatives, and high osmolarity. To assess alternative sigma factor σ^L contributions to such stress resistance of L. monocytogenes, quantitative RT-PCR assays and sigL gene deletion mutagenesis were applied in L. monocytogenes EGDe. Transcription of sigL was significantly induced by growth of EGDe under cold, organic acid, and elevated NaCl salt concentration stress conditions. The growth of a ΔsigL strain exposed to these stress conditions was also found to be significantly impaired in comparison to that of its isogenic wild-type strain. The contribution of σ^L to transcription control of cold and NaCl stress adaptation genes, oppA, cspD, and clpP, was also comparatively assessed in ΔsigL and wild-type EGDe cells. Transcription of the oppA gene, which encodes the OppA protein that also promotes L. monocytogenes cold growth, was significantly reduced in cold stress–grown ΔsigL cells compared to levels of the wild-type EGDe strain. These findings therefore suggest important roles of σ^L regulatory pathways in facilitating resistance of L. monocytogenes organisms against stress conditions associated with low storage temperatures, exposure to organic acid, and elevated NaCl salt concentrations.