Hungry bacteria--definition and properties of a nutritional state.

Bacteria are sometimes neither starving nor under nutrient-excess conditions. When growing with suboptimal levels of nutrients, hungry bacteria express appropriate cellular responses. This review discusses approaches to defining the hunger response in both molecular and growth kinetic terms. The gene expression changes unique to hunger conditions are described, using Escherichia coli as the primary example. Metabolite changes with hunger and starvation and the differing role of the stationary phase regulator RpoS also lead to the hypothesis proposed in this review that bacteria undertake distinct approaches to hunger and starvation. Indeed, an understanding of the difference between hunger and starvation and the incompatibility between hunger and starvation responses explains some of the paradoxical mutational adaptations, such as rpoS inactivation, found in natural populations.