Contribution of Mn-cofactored superoxide dismutase (SodA) to the virulenceof Streptococcus agalactiae

Superoxide dismutases convert superoxide anions to molecular oxygen and hyd rogen peroxide, which, in turn, is metabolized by catalases and/or peroxida ses. These enzymes constitute one of the major defense mechanisms of cells against oxidative stress and hence play a role in the pathogenesis of certa in bacteria. We previously demonstrated that group B streptococci (GBS) pos sess a single Mn-cofactored superoxide dismutase (SodA). To analyze the rol e of this enzyme in the pathogenicity of GBS, we constructed a sodA-disrupt ed mutant of Streptococcus agalactiae NEM316 by allelic exchange. This muta nt was subsequently cis complemented by integration into the chromosome of pAT113/Sp harboring the wild-type sodA gene. The SOD specific activity dete cted by gel analysis in cell extracts confirmed that active SODS were prese nt in the parental and complemented strains but absent in the sodA mutant. The growth rates of these strains in standing cultures were comparable, but the sodA mutant was extremely susceptible to the oxidative stress generate d by addition of paraquat or hydrogen peroxide to the culture medium and ex hibited a higher mutation frequency in the presence of rifampin. In mouse b one marrow-derived macrophages, the sodA mutant showed an increased suscept ibility to bacterial killing by macrophages. In a mouse infection model, af ter intravenous injection the survival of the sodA mutant in the blood and the brain was markedly reduced in comparison to that of the parental and co mplemented strains whereas only minor effects on survival in the liver and the spleen were observed. These results suggest that SodA plays a role in G BS pathogenesis.