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

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Abstract
The ability ofListeria monocytogenesto invade non-phagocytic cells is important for development of a systemic listeriosis infection. The authors previously reported that aL. monocytogenesΔsigBstrain 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 ofσ B, mutants were generated carrying combinations of in-frame deletions ininlA,inlBandsigB. Quantitative assessment of bacterial invasion into the human enterocyte Caco-2 and hepatocyte HepG-2 cell lines demonstrated thatσ Bcontributes to both InlA and InlB-mediated invasion ofL. monocytogenes. Previous identification of theσ B-dependent P2 prfA promoter upstream of the major virulence gene regulator, positive regulatory factor A (PrfA), suggested that the contributions ofσ Bto expression of various virulence genes, includinginlA, could be at least partially mediated through PrfA. To test this hypothesis, relative invasion capabilities of ΔsigBand ΔprfAstrains were compared. Exponential-phase cells of the ΔsigBand ΔprfAstrains were similarly defective at invasion; however, stationary-phase ΔsigBcells were significantly less invasive than stationary-phase ΔprfAcells, suggesting that the contributions ofσ Bto invasion extend beyond those mediated through PrfA in stationary-phaseL. monocytogenes. TaqMan quantitative reverse-transcriptase PCRs further demonstrated that expression ofinlAandinlBwas greatly increased in aσ B-dependent manner in stationary-phaseL. monocytogenes. Together, results from this study provide strong biological evidence of a critical role forσ BinL. monocytogenesinvasion into non-phagocytic cells, primarily mediated through control ofinlAandinlBexpression.