Oxidative stress as a trigger for growth phase-specific sigmaE-dependent cell lysis in Escherichia coli

Sigma(E), encoded by rpoE, causes lysis of viable but nonculturable (VBNC) cells at the early stationary phase in Escherichia coli. However, the factors that trigger this lysis phenomenon remain unidentified. In this study, we found that the defective strain of katE, encoding catalase, exhibited a sigma(E)-dependent cell lysis phenotype, showing a decrease in turbidity of cell culture and an increase in accumulation of proteins in the culture medium as well as an increase in rpoE expression. The level of intracellular reactive oxygen species became maximal at the transition period from exponential to stationary phases, and VBNC cells gradually increased after that period. In the wild-type strain, both sodA, encoding superoxide dismutase, and katE plasmid clones almost completely suppressed cell lysis, largely reduced reactive oxygen species, and substantially increased culturable cells. Consistent results were obtained in a defective strain of rseA, encoding an anti-sigma(E), in which sigma(E) is constitutively expressed. These results suggest that oxidative stress is a major factor responsible for VBNC cell formation and cell lysis at the early stationary phase. Therefore, we assume that oxidative stress that has accumulated in the transition period gives rise to VBNC cells, which are in turn lysed by a sigma(E)-dependent process.