σ B-dependent gene induction and expression in Listeria monocytogenes during osmotic and acid stress conditions simulating the intestinal environment

Abstract

Listeria monocytogenes must overcome a variety of stress conditions in the host digestive tract to cause foodborne infections. The alternative sigma factor sigma(B), encoded by sigB, is responsible for regulating transcription of several L. monocytogenes virulence and stress-response genes, including genes that contribute to establishment of gastrointestinal infections. A quantitative RT-PCR assay was used to measure mRNA transcript accumulation for the virulence genes inlA and bsh, the stress-response genes opuCA and lmo0669 (encoding a carnitine transporter and an oxidoreductase, respectively) and the housekeeping gene rpoB. Assays were conducted on mid-exponential phase L. monocytogenes cells exposed to conditions reflecting osmotic (0.3 M NaCl) or acid (pH 4.5) conditions typical for the human intestinal lumen. In exponential-phase cells, as well as under osmotic and acid stress, inlA, opuCA and bsh showed significantly lower absolute expression levels in a L. monocytogenes DeltasigB null mutant compared to wild-type. A statistical model that normalized target gene expression relative to rpoB showed that accumulation of inlA, opuCA and bsh transcripts was significantly increased in the wild-type strain within 5 min of acid and osmotic stress exposure; lmo0669 transcript accumulation increased significantly only after acid exposure. It was concluded that sigma(B) is essential for rapid induction of the tested stress-response and virulence genes under conditions typically encountered during gastrointestinal passage. As inlA, bsh and opuCA are critical for gastrointestinal infections in animal models, the data also suggest that sigma(B) contributes to the ability of L. monocytogenes to cause foodborne infections.