VISIBLE-LIGHT DAMAGE TO ESCHERICHIA-COLI IN SEAWATER - OXIDATIVE STRESS HYPOTHESIS

The effect of visible light on Escherichia coli H10407 in seawater mic rocosms was investigated. Light damage was estimated by loss of colony -forming ability. Illumination of E. coli suspended in oligotrophic se awater with visible light at an intensity of about 40 klux caused a dr astic decrease of culturable bacteria which turned to a viable but non -culturable state. In seawater E. coli exhibited weak metabolic activi ty as estimated by H-3 methyl-thymidine incorporation in the cell. Vis ible light did not significantly alter this metabolic activity and did not involve detectable oxidation of lipid membranes as evaluated by g as chromatography analysis of fatty acids. The involvement of oxygen a nd reactive oxygen species in phototoxicity was studied. A decrease of the toxic effect was observed when E. coli was exposed to visible lig ht under anaerobic conditions. Scavengers of reactive oxygen species e xhibited variable protective effects. beta-Carotene, a singlet oxygen scavenger, and superoxide dismutase were equally ineffective. On the o ther hand, catalase, which eliminates hydrogen peroxide and thiourea, a hydroxyl radical scavenger, showed a net protection. In addition des ferrioxamine B, an iron chelator, was also effective in reducing photo toxicity, probably by preventing hydroxyl radical generation by decomp osition of hydrogen peroxide in the presence of iron (Fenton reaction) . Therefore, hydrogen peroxide and hydroxyl radical seem to be reactiv e intermediates of oxygen-dependent (type II) photosensitized reaction s.