Contributions of σB and PrfA to Listeria monocytogenes salt stress tinder food relevant conditions

Listeria monocytogenes is well known to survive and grow under several stress conditions, including salt stress, which is important for growth in certain foods as well as for host infection. To characterize the contributions, to salt stress response, of transcriptional regulators important for stress response and virulence (i.e., sigma(B) and PrfA), we analyzed three L monocytogenes parent strains and isogenic mutants (Delta sigB, Delta pifA, and Delta sigR Delta priA), representing different serotypes and lineages, for their ability to grow, at 25 C, in BHI with 1.9 M NaCI. With regard to growth rate, only the lineage IV strain presented a significant difference between the parent strain and both of its respective mutants lacking ptfA (AprfA and AsigBAprfA). Conversely, the lineage land II parent strains showed significantly shorter lag phase in comparison to their respective AsigB mutant strains. Intestinal epithelial cell invasion assay and hemolytic activity assays showed a significant role for oB in the former and for PrfA in the latter. To explore the mechanism that may contribute to the extended lag phase in the AsigB mutant strain and survival and growth of the parent strain upon salt shock, whole genome transcription profiling was performed to compare transcript levels between the lineage I, serotype 1/2b, parent strain and its isogenic AsigB mutant after 30 min of lag phase growth at 25 C in the presence of 1.9 M NaCI (salt shock) without aeration. Microarray data showed significantly higher transcript levels for 173 genes in the parent strain as compared to the AsigB strain. Overall, 102 of the 173 oB up-regulated genes had been identified in previous studies, indicating that 71 genes were newly identified as being up-regulated by oB in this study. We hypothesize that, among these genes newly identified as oB up-regulated, four genes (Imo2174, 1mo0530, 1mo0527 and 1mo0529) may play a major role in response to salt stress. Lmo2174 contains domains that facilitate sensing and producing a transduction signal in the form of cyclic di-GMP, which may activate the enzymes Lmo0527, Lmo0529 and Lmo0530, which encode proteins similar to those responsible for synthesis of exopolysaccharides that may protect the cell by changing the cell wall structure during salt stress. Overall, our data showed that oB, but not PrfA, contributes to growth under salt stress. Moreover, we show that the GB regulon of a L monocytogenes lineage I strain challenged with salt shock includes salt stress-specific as well as previously unidentified oB up-regulated genes. (C) 2014 Elsevier B.V. All rights reserved.