Transcriptomic and Phenotypic Analyses of the Sigma B-Dependent Characteristics and the Synergism between Sigma B and Sigma L in Listeria monocytogenes EGD-e

Numerous gene expression and stress adaptation responses in L. monocytogenes are regulated through alternative sigma factors sigma(B) and sigma(L). Stress response phenotypes and transcriptomes were compared between L. monocytogenes EGD-e and its Delta sigB and Delta sigBL mutants. Targeted growth phenotypic analysis revealed that the Delta sigB and Delta sigBL mutants are impaired during growth under cold and organic-acid stress conditions. Phenotypic microarrays revealed increased sensitivity in both mutants to various antimicrobial compounds. Genes de-regulated in these two mutants were identified by genome-wide transcriptome analysis during exponential growth in BHI. The Delta sigB and Delta sigBL strains repressed 198 and 254 genes, respectively, compared to the parent EGD-e strain at 3 degrees C, whereas 86 and 139 genes, respectively, were repressed in these mutants during growth at 37 degrees C. Genes repressed in these mutants are involved in various cellular functions including transcription regulation, energy metabolism and nutrient transport functions, and viral-associated processes. Exposure to cold stress induced a significant increase in sigma(B) and sigma(L) co-dependent genes of L. monocytogenes EGD-e since most (62%) of the down-regulated genes uncovered at 3 degrees C were detected in the Delta sigBL double-deletion mutant but not in Delta sigB or Delta sigL single-deletion mutants. Overall, the current study provides an expanded insight into sigma(B) and sigma(L) phenotypic roles and functional interactions in L. monocytogenes. Besides previously known sigma(B)- and sigma(L)-dependent genes, the transcriptomes defined in Delta sigB and Delta sigBL mutants reveal several new genes that are positively regulated by sigma(B) alone, as well as those co-regulated through sigma(B)- and sigma(L)-dependent mechanisms during L. monocytogenes growth under optimal and cold-stress temperature conditions.