Not quite dead enough: on bacterial life, culturability, senescence, and death.

A number of regulatory networks are functionally integrated in starving cells of Escherichia coli to reduce oxidation of target macromolecules and to enhance the cell's ability to withstand environmental insults. However, despite the fact that starving wild-type E. coli cells enhance their capacity to manage oxidative stress, the proteins of these cells become increasingly oxidized and the cells gradually lose their ability to reproduce. Indeed, it has been argued that starved and growth-arrested bacterial cells show the same signs of senescence as aging cells of higher organisms and that free radicals may be involved in the gradual loss of bacterial culturability observed in a stationary phase culture. Another model suggests that the apparent loss of viability of starved cells is a programmed and adaptive response in which the cells enter a reversible non-culturable state; the theory of the formation of viable but non-culturable cells. Recent data concerning the physiology and biochemistry of starved E. coli cells favor the model that starvation-induced loss of culturability is the result of stochastic deterioration rather than a programmed and adaptive phenomenon, and these data will be reviewed here.