ΣB-dependent protein induction in Listeria monocytogenes during vancomycin stress

Listeria monocytogenes is a food-borne pathogen that can survive under a wide range of environmental and energy stress conditions. The general stress response controlled by Sigma B largely contributes to stress resistance in L. monocytogenes. Moreover, the bacterial cell wall is the first defense against cellular stress and as such is the target of numerous antibiotics. We therefore hypothesize that Sigma B contributes to monitoring the integrity of cell walls. We evaluated Sigma B activity in wild type and delta sigB mutant L. monocytogenes containing reporter fusions (Sigma B-dependent opuCA promoter and a lacZ reporter gene) during the early exponential growth phase by measuring the specific activity of beta-galactosidase after vancomycin (2 mu g mL-1 final concentration) stress. Sigma B activity is significantly induced only in the wild-type strain by addition of vancomycin. In addition, we identified Sigma B-dependent vancomycin-inducible proteins using LC-ESI-MS/MS analysis. Two independent proteomic analyses confirmed the minimum twofold upregulation of 18 vancomycin-inducible Sigma B-dependent stress response proteins in the wild-type strain compared with the delta sigB mutant. The functions of these proteins are associated with cell wall biogenesis, intracellular transport, general stress response, cell metabolism and virulence. These results suggest that the Sigma B protein may contribute to the monitoring of cell wall integrity.