Listeria monocytogenes sB Has a Small Core Regulon and a Conserved Role in Virulence but Makes Differential Contributions to Stress Tolerance across a Diverse Collection of Strains

Listeria monocytogenes strains are classified in at least three distinct phylogenetic lineages. There are correlations between lineage classification and source of bacterial isolation; e.g., human clinical and food isolates usually are classified in either lineage I or II. However, human clinical isolates are overrepresented in lineage I, while food isolates are overrepresented in lineage II. sB, a transcriptional regulator previously demonstrated to contribute to environmental stress responses and virulence in L. monocytogenes lineage II strains, was hypothesized to provide differential abilities for L. monocytogenes survival in various niches (e.g., food and human clinical niches). To determine if the contributions of sB to stress response and virulence differ across diverse L. monocytogenes strains, sigB mutations were created in strains belonging to lineages I, II, IIIA, and IIIB. Paired parent and sigB mutant strains were tested for survival under acid and oxidative stress conditions, Caco-2 cell invasion efficiency, and virulence using the guinea pig listeriosis infection model. Parent and sigB mutant strain transcriptomes were compared using whole-genome expression microarrays. sB contributed to virulence in each strain. However, while sB contributed significantly to survival under acid and oxidative stress conditions and Caco-2 cell invasion in lineage I, II, and IIIB strains, the contributions of sB were not significant for these phenotypes in the lineage IIIA strain. A core set of 63 genes was positively regulated by sB in all four strains; different total numbers of genes were positively regulated by sB in the strains. Our results suggest that sB universally contributes to L. monocytogenes virulence but specific sB-regulated stress response phenotypes vary among strains.