Role of the alternative sigma factors s and s in survival of Salmonella enterica serovar Typhimurium during starvation, refrigeration and osmotic shock

Received 20 June 2006 Revised 4 September 2006 Accepted 19 September 2006 The ability of Salmonella enterica serovar Typhimurium to survive environmental stress requires specific, coordinated, responses, which induce resistance to the stress condition. This study investigated the relative contribution of s and s, the sigma factors regulating extracytoplasmic and general stress response functions, respectively, to survival at low temperature and also in media of differing osmotic strength, conditions relevant to food preservation. To determine if low-temperature storage is a signal for sand s-mediated survival, the ability of S. Typhimurium rpoE, rpoS and rpoE/rpoS mutants to survive in a saline starvation-survival model at a refrigeration temperature (4.5 6C) was examined. Under these conditions, the rpoE mutant was significantly (P<0.05) compromised compared to the parent and to an rpoS mutant. The double mutant in rpoE and rpoS displayed a cumulative defect in survival. In hyperosmotic environments (low aw) containing 6 % NaCl and at refrigeration temperature, both sigma factors were important for maximum survival but s played the dominant role. Analysis of the metabolic activity of starved populations at 4.5 and 37 6C revealed significantly (P<0.001) elevated electron-transport system activity in mutants in rpoE and rpoS, indicating a role for sand s-regulated genes in maintaining energy homeostasis. Together these data demonstrate that s and s are important for survival of S. Typhimurium in conditions encountered during food processing and that the relative contribution of s and s is critically dependent on the precise nature of the stress.