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

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, sigmaB, 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 sigmaB expression during growth of L. monocytogenes EGD-e. Whole genome-based transcriptional profiling was used to identify sigmaB-dependent genes at different growth phases. We detected 105 sigmaB-positively regulated genes and 111 genes which appeared to be under negative control of sigmaB and validated 36 sigmaB-positively regulated genes in vivo using a reporter gene fusion system. Genes comprising the sigmaB 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 sigmaB-positively regulated genes of L. monocytogenes are absent in L. innocua. A total of 75 sigmaB-positively regulated listerial genes had homologs in B. subtilis, but only 33 have been previously described as being sigmaB-regulated in B. subtilis even though both species share a highly conserved sigmaB-dependent consensus sequence. A low overlap of genes may reflects adaptation of these bacteria to their respective environmental conditions.