Survival of Escherichia coli exposed to visible light in seawater: analysis of rpoS-dependent effects.

We investigated the effect of visible light on Escherichia coli in seawater microcosms. Escherichia coli lost its ability to form colonies in marine environments when exposed to artificial continuous visible light. Survival of illuminated bacteria during the stationary phase was drastically reduced in the absence of the sigma factor (RpoS or KatF) that regulates numerous genes induced in this phase. In the stationary phase, double catalase mutants katE katG and mutants defective in the protein Dps (both catalase and Dps are involved in resistance to hydrogen peroxide (H2O2)), were more sensitive to light. In the exponenital phase, a mutation in oxyR, the regulatory gene of the adaptive response to H2O2, increased sensitivity to light, further suggesting that deleterious effects might be associated with H2O2 production. However, in the stationary phase, the katE katG dps mutant was considerably more resistant to visible light than the rpoS mutant, suggesting rpoS-dependent protection against deleterious effects other than those related to H2O2. The deleterious action of visible light was less important when the salinity decreased. In freshwater, rpoS and katE katG dps mutants did not show a drastic difference in sensitivity to light suggesting that osmolarity sensitizes E. coli to those deleterious effects of visible light that are unrelated to H2O2.