Induction of rpoS in Salmonella typhimurium by nutrient-poor and depleted media is slower than that achieved by a competitive microflora.

The presence of a viable competitive microflora, at greater than 10(8) cfu ml-1, is known to advance the induction of RpoS-mediated gene expression in a sub-population of Salmonella typhimurium. As starvation is known to induce RpoS, one action of the competitive microflora could be to cause depletion of essential nutrients. The aim of the current experiments was to determine whether this was the case by examining RpoS induction in Salm. typhimurium in reduced nutrient media. RpoS-mediated gene expression in Salm. typhimurium was not advanced so significantly in 'conditioned' or diluted medium as it was in the presence of competitors, which indicates that nutrient depletion was not the responsible mechanism. The effect of a competitive microflora has implications for models of bacterial survival during food processing, as RpoS ultimately regulates both stress resistance and virulence.