Induction of RpoS-dependent functions in glucose-limited continuous culture: What level of nutrient limitation induces the stationary phase of Escherichia coli?

treA and osmY expression and RpoS protein levels were investigated in glucose-limited continuous culture. The level of induction of these stationary-phase markers became as high during growth at a D of 0.1 to 0.2 h^-1 as in carbon-starved batch cultures but only in rpoS⁺ bacteria. The stress protectant trehalose was actually produced at higher levels at low growth rates than in stationary-phase cultures. The pattern of induction of RpoS-dependent activities could be separated from those regulated by cyclic AMP (cAMP) or endoinduction, and the induction occurred at extreme glucose limitation. Escherichia coli turns to a protective stationary-phase response when nutrient levels fall below approximately 10^-7 M glucose, which is insufficient to saturate scavenger transporters regulated by cAMP plus endoinducers, and this response is optimally expressed at 10^-6 M glucose. The high-level induction of protective functions also explains the maintenance energy requirement of bacterial growth at low dilution rates.