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 in duce RpoS, one action of the competitive microflora could be to cause deple tion of essential nutrients. The aim of the current experiments was to dete rmine whether this was the case by examining RpoS induction in Salm. typhim urium in reduced nutrient media. RpoS-mediated gene expression in Salm. typ himurium was not advanced so significantly in 'conditioned' or diluted medi um as it was in the presence of competitors, which indicates that nutrient depletion was not the responsible mechanism. The effect of a competitive mi croflora has implications for models of bacterial survival during food proc essing, as RpoS ultimately regulates both stress resistance and virulence.