H2O2 produced by viridans group streptococci may contribute to inhibition of methicillin-resistant Staphylococcus aureus colonization of oral cavities in newborns

In an accompanying report, we showed that viridans group streptococci may prevent methicillin-resistant Staphylococcus aureus (MRSA) colonization of the oral cavities of newborns. In the present study, we investigated the mechanism of prevention in vitro. Most viridans group streptococci had bacteriocin-like activity and killed MRSA, Burkholderia cepacia, Enterobacter aerogenes, and Pseudomonas aeruginosa; however, Escherichia coli, Enterobacter cloacae, and Candida albicans were resistant. The activity was induced only by H2O2-secreting strains and was inhibited by horseradish peroxidase or catalase in a dose-dependent manner. The mean concentration of H2O2 produced by 18 strains of viridans group streptococci (1 x 10(8) cfu in 200 muL of culture medium +/- standard deviation was 1.24 +/- 0.60 mmol. Viridans group streptococci inhibited MRSA growth in saliva as well as in culture media. These results indicate that H2O2 produced by viridans group streptococci may inhibit MRSA colonization of oral cavities in newborns.