Figure 5

Dynamic view of RpoS status and metabolic fitness in natural E. coli populations. It is postulated that the ancestral E. coli strain possesses a functional rpoS allele (RpoS⁺) but cannot grow well on poor nutrient sources (Suc^--). Upon exposure to nutrient limitation, mutants (Suc⁺⁺) exhibiting enhanced metabolic activity can be selected and become dominant among the population. These mutants consist of two groups, RpoS⁺ and RpoS^-. Under stress conditions, however, the proportion of RpoS^- mutants decreases because of the loss of protection by RpoS-controlled functions, and the abundance of strains with functional RpoS increases. Because cells likely are constantly facing selection between nutrient limitation and stress in nature, the population of E. coli isolates is in a dynamic status in terms of RpoS function and metabolic fitness.