Lactoperoxidase and thiocyanate protect bacteria from hydrogen peroxide

Lactoperoxidase and thiocyanate were shown to protect Escherichia coli and three oral streptococcal species from the bactericidal effect of hydrogen peroxide under aerobic conditions. Lactoperoxidase in the absence of thiocyanate was also protective for two of the bacterial species in a dilution medium but potentiated hydrogen peroxide toxicity for the other two under the same conditions. The products of the reaction between hydrogen peroxide and thiocyanate in the presence of lactoperoxidase were not bactericidal except in the case of E. coli, and then only under special conditions. The results demonstrate the effectiveness of lactoperoxidase and thiocyanate in protecting living cells from hydrogen peroxide toxicity. Although the effect on human cells was not examined in this study, extrapolation of these results to the cells of the oral mucosa would suggest an important protective role of lactoperoxidase and thiocyanate against the toxic effects of hydrogen peroxide in the oral cavity.