σ B-dependent expression patterns of compatible solute transporter genes opuCA and lmo1421 and the conjugated bile salt hydrolase gene bsh in Listeria monocytogenes

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Abstract
Listeria monocytogenesis a food-borne pathogen that can persist and grow under a wide variety of environmental conditions including low pH and high osmolarity. The alternative sigma factorσBcontributes toL. monocytogenessurvival under extreme conditions. The purpose of this study was to identify and confirm specificσB-dependent genes inL. monocytogenesand to characterize their expression patterns under various stress conditions.opuCA,lmo1421andbshwere identified as putativeσB-dependent genes based on the presence of a predictedσB-dependent promoter sequence upstream of each gene.opuCAandlmo1421encode known and putative compatible solute transporter proteins, respectively, andbshencodes a conjugated bile salt hydrolase (BSH). Reporter fusions and semi-quantitative RT-PCR techniques were used to confirmσB-dependent regulation of these stress-response genes and to determine their expression patterns in response to environmental stresses. RT-PCR demonstrated thatopuCA,lmo1421andbshtranscript levels are reduced in stationary-phaseL. monocytogenesΔsigBcells relative to levels present in wild-type cells. Furthermore, BSH activity is abolished in aL. monocytogenesΔsigBstrain. RT-PCR confirmed growth-phase-dependent expression ofopuCA, with highest levels of expression in stationary-phase cells. TheL. monocytogeneswild-type strain exhibited two- and threefold induction ofopuCAexpression and seven- and fivefold induction oflmo1421expression following 10 and 15 min exposure to 0·5 M KCl, respectively, as determined by RT-PCR, suggesting rapid induction ofσBactivity in exponential-phaseL. monocytogenesupon exposure to salt stress. Single-copy chromosomalopuCA–gusreporter fusions also showed significant induction ofopuCAexpression following exposure of exponential-phase cells to increased salt concentrations (0·5 M NaCl or 0·5 M KCl). In conjunction with recent findings that indicate a role foropuCAandbshinL. monocytogenesvirulence, the data presented here provide further evidence of specificσB-mediated contributions to both environmental stress resistance and intra-host survival inL. monocytogenes.

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