Growth rate related concentration changes of the starvation response regulators σS and ppGpp in glucose-limited fed-batch and continuous cultures of Escherichia coli

The entry of Escherichia coli into glucose starvation is connected to the induction of both the stringent response and the general stress response. The concentration of the corresponding response regulators ppGpp and sigma(S) (RpoS, KatF) was investigated during glucose-limited growth at different growth rates and with different cultivation strategies, including fed-batch fermentation, chemostat, and a continuous-cultivation technique with computer-controlled decrease of the feeding rate (A-stat). This study confirms the model of the in vivo effect of ppGpp on sigma(S) by the direct measurement of ppGpp and sigma(S) levels. The response of both regulators was clearly shown in situations where a nutrient shift occurs. A correlation in the concentration of both regulators was also shown under steady-state conditions. However, during transients with slow continuous changes of the nutrient availability, this concerted reaction of ppGpp and sigma(S) is less apparent, indicating the specific importance of these regulators for the adaptation of the cells to fast changes of environmental parameters.