Effect of 3% hydrogen peroxide on the viability of Serratia marcescens. J. Bacteriol. 91:925-929. 1966.-Populations of Serratia marcescens were exposed to 3% H202 at temperatures from 0 to 20 C. The reaction appeared to follow an Arrhenius plot, but variable numbers of diminutive colonies were found after cell numbers started to decrease. Colony numbers varied on different sampling media and increased when additional incubation was imposed. The overall reaction was sensitive to age of culture, and growth capabilities of treated samples varied with time of treatment, especially during times when no loss of viability was noted. Catalase activity per cell did not correlate with changes in sensitivity; iron added to growth medium increased catalase activity and decreased sensitivity, but not in the same manner. Although the fundamental reaction is presumably molecular in nature, present methods of viability assay measure more than single events and are not suitable for these studies. One end product of metabolism in aerobic bacteria is presumed to be hydrogen peroxide (4), which the cell could convert to water by the action of catalase; in high concentrations, however, H202 is bactericidal. The mechanism of disinfectant action is not known, but excess amounts of H202 are believed to oxidize some vital internal cell constituent (6). Cellular catalase could protect cells by destroying external H202. Initially, we sought information on the thermodynamics of the disinfectant reaction. We investigated the influence of temperature, iron in the growth medium, culture age, and catalase activity on the disinfectant action of 3 % hydrogen peroxide. We conclude that the overall reaction does not conform to simple kinetic theory. MATERIALS AND METHODS Serratia marcescens, strain 8UK, was grown at 31 C in a chemically defined medium described by Dimmick (2). A 1-ml amount of a sterile solution of ferric ammonium citrate containing 1 mg/ml of iron was sometimes added per liter of growth medium. Data shown in Fig. 1, 2, and 3 were obtained when "house" distilled water from a block-tin still was used. The water was stored in a stainless-steel tank and was delivered through plastic pipe with stainless-steel valves. Data in the remaining figures were obtained with use of water redistilled in a Corning AG-2 all-glass unit. Unless noted, samples of a single, third-passage 925 Vol. 91, No. 3