H2O2 formation by Streptococcus mitis was measured by the catalase-dependent conversion of [14C]formate to 14CO2 ; it was optimal at pH 6.0–6.5 and required glucose. The H2O2 formed by S. mitis could be employed as a component of an antimicrobial system that also included lactoperoxidase (LPO) and either iodide or thiocyanate ions in the concentrations present in saliva. The antimicrobial effect of the LPO-iodide-S. mitis system was measured by the decrease in the viable cell count of the target organisms (Escherichia coli, Staphylococcus aureus, Candida tropicalis). The antimicrobial effect of the LPO-thiocyanate-S. mitis system was measured by the decrease in the rate of growth or the rate of uptake of [14C]valine by the target organisms (E. coli, S. aureus). Mixed or parotid saliva could replace LPO and thiocyanate ions in the S. mitis-dependent inhibition of bacterial growth and valine uptake. The presence in saliva of a peroxidase-mediated, antimicrobial system dependent on microbial metabolism for H2O2 and its role as a natural host defense mechanism are considered.

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