Crucial role of ppGpp in the resilience of Escherichia coli to growth disruption

Bacteria grow in constantly changing environments that can suddenly become completely deleted in essential nutrients. The stringent response, a rewiring of the cellular metabolism mediated by the alarmone (p)ppGpp, plays a crucial role in adjusting bacterial growth to the severity of the nutritional insult. The ability of (p)ppGpp to trigger a slowdown of cell growth or induce bacterial dormancy has been widely investigated. However, little is known about the role of (p)ppGpp in promoting growth recovery after severe growth inhibition. In this study, we performed a time-resolved analysis of (p)ppGpp metabolism in Escherichia coli as it recovered from a sudden slowdown in growth. Results show that E. coli recovers by itself from the growth disruption provoked by the addition of serine hydroxamate, the serine analogue that we used to induce the stringent response. Growth inhibition was accompanied by a severe disturbance of metabolic activity and more surprisingly, by a transient overflow of valine and alanine. Our data also show that ppGpp is crucial for growth recovery since in the absence of ppGpp, E. coli9s growth recovery was slower. In contrast, an increased concentration of pppGpp was found to have no significant effect on growth recovery. Interestingly, the observed decrease in intracellular ppGpp levels in the recovery phase correlated with bacterial growth and the main effect involved was identified as growth dilution rather than active degradative process. This report thus significantly expands our knowledge of (p)ppGpp metabolism in E. coli physiology.