The starvation-stress response of Salmonella enterica serovar Typhimurium requires σE-, but not CpxR-regulated extracytoplasmic functions

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Abstract
Starvation of Salmonella enterica serovar Typhimurium (S. Typhimurium) for an exogenous source of carbon and energy (C-starvation) induces the starvation-stress response (SSR). The SSR functions to (i) maintain viability during long-term C-starvation and (ii) generate cross-resistance to other environmental stresses. The SSR is, at least partially, under the control of the alternative sigma factor, σS. It is hypothesized that C-starvation causes cell envelope stresses that could induce the σE and/or Cpx regulons, both of which control extracytoplasmic functions and, thus, may play a role in the regulation of the SSR. In support of this hypothesis, Western blot analysis showed that the relative levels of σE increased during C-starvation, peaking after approximately 72 h of C-starvation; in contrast, CpxR levels remained relatively constant from exponential phase up to 72 h of C-starvation. To determine if σE, and thus the regulon it controls, is an essential component of the SSR, several mutant strains were compared for their abilities to survive long-term C-starvation and to develop C-starvation-induced (CSI) cross-resistances. An rpoE mutant strain was significantly impaired in both long-term C-starvation survival (LT-CSS) and in CSI cross-resistance to challenges with 20 mM H2O2 for 40 min, 55 °C for 16 min, pH 3·1 for 60 min and 870·2 USP U polymyxin B ml−1 (PmB) for 60 min, to varying degrees. These results suggest that C-starvation can generate signals that induce the rpoE regulon and that one or more members of the σE regulon are required for maximal SSR function. Furthermore, evidence suggests that the σE and σS regulons function through separate mechanisms in the SSR. In contrast, C-starvation does not appear to generate signals required for Cpx regulon induction which support the findings that it is not required for LT-CSS or cross-resistance to H2O2, pH 3·1 or PmB challenges. However, it was required to achieve maximal cross-resistance to 55 °C. Therefore, σE is a key regulatory component of the SSR and represents an additional σ factor required for the SSR of Salmonella.