σ B contributes to Listeria monocytogenes invasion by controlling expression of inlA and inlB

Abstract

The ability of Listeria monocytogenes to invade non-phagocytic cells is important for development of a systemic listeriosis infection. The authors previously reported that a L. monocytogenes Δ sigB strain is defective in invasion into human intestinal epithelial cells, in part, due to decreased expression of a major invasion gene, inlA . To characterize additional invasion mechanisms under the control of σ B , mutants were generated carrying combinations of in-frame deletions in inlA , inlB and sigB . Quantitative assessment of bacterial invasion into the human enterocyte Caco-2 and hepatocyte HepG-2 cell lines demonstrated that σ B contributes to both InlA and InlB-mediated invasion of L. monocytogenes . Previous identification of the σ B -dependent P2 prfA promoter upstream of the major virulence gene regulator, positive regulatory factor A (PrfA), suggested that the contributions of σ B to expression of various virulence genes, including inlA , could be at least partially mediated through PrfA. To test this hypothesis, relative invasion capabilities of Δ sigB and Δ prfA strains were compared. Exponential-phase cells of the Δ sigB and Δ prfA strains were similarly defective at invasion; however, stationary-phase Δ sigB cells were significantly less invasive than stationary-phase Δ prfA cells, suggesting that the contributions of σ B to invasion extend beyond those mediated through PrfA in stationary-phase L. monocytogenes . TaqMan quantitative reverse-transcriptase PCRs further demonstrated that expression of inlA and inlB was greatly increased in a σ B -dependent manner in stationary-phase L. monocytogenes . Together, results from this study provide strong biological evidence of a critical role for σ B in L. monocytogenes invasion into non-phagocytic cells, primarily mediated through control of inlA and inlB expression.