Temporal transcriptomic analysis of the Listeria monocytogenes EGD-e σB regulon

Abstract

The opportunistic food-borne gram-positive pathogen Listeria monocytogenes can exist as a free-living microorganism in the environment and grow in the cytoplasm of vertebrate and invertebrate cells following infection. The general stress response, controlled by the alternative sigma factor, σ^B, has an important role for bacterial survival both in the environment and during infection. We used quantitative real-time PCR analysis and immuno-blot analysis to examine σ^B expression during growth of L. monocytogenes EGD-e. Whole genome-based transcriptional profiling was used to identify σ^B-dependent genes at different growth phases. We detected 105 σ^B-positively regulated genes and 111 genes which appeared to be under negative control of σ^B and validated 36 σ^B-positively regulated genes in vivo using a reporter gene fusion system. Genes comprising the σ^B regulon encode solute transporters, novel cell-wall proteins, universal stress proteins, transcriptional regulators and include those involved in osmoregulation, carbon metabolism, ribosome- and envelope-function, as well as virulence and niche-specific survival genes such as those involved in bile resistance and exclusion. Ten of the σ^B-positively regulated genes of L. monocytogenes are absent in L. innocua. A total of 75 σ^B-positively regulated listerial genes had homologs in B. subtilis, but only 33 have been previously described as being σ^B-regulated in B. subtilis even though both species share a highly conserved σ^B-dependent consensus sequence. A low overlap of genes may reflects adaptation of these bacteria to their respective environmental conditions.