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

¹ Division of Veterinary Pathology, Infection and Immunity, School of Clinical Veterinary Science, University of Bristol, Langford, Bristol BS40 5DU, UK
² Institute of Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
³ Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK

Correspondence
Mark Roberts
m.roberts{at}vet.gla.ac.uk

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 ^E 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 ^E- and ^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 °C) 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 a_w) 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 °C revealed significantly (P<0.001) elevated electron-transport system activity in mutants in rpoE and rpoS, indicating a role for ^E- and ^S-regulated genes in maintaining energy homeostasis. Together these data demonstrate that ^E and ^S are important for survival of S. Typhimurium in conditions encountered during food processing and that the relative contribution of ^E and ^S is critically dependent on the precise nature of the stress.

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