Antibacterial effect of myeloperoxidase against Streptococcus mutans

Myeloperoxidase‐chloride‐H2O2 antimicrobial system is an integral part of leukocyte defense against pathogenic microorganisms. The system is present in gingival crevicular fluid and in whole saliva and, thus, possibly contributes to the non‐antibody defense in the human mouth. In fact, at physiological concentrations myeloperoxidase‐Cl−‐H2O2 system effectively killed Streptococcus mutans both at acid and neutral pH. However, thiocyanate (SCN−) ions at concentrations similar to those in human saliva abolished this bactericidal action indicating that SCN− rather than Cl− is the physiologic substrate of myeloperoxidase in whole saliva. The peroxidation of SCN− (by myeloperoxidase or salivary peroxidase) yields oxidation products which inhibit the growth and metabolism of S. mutans without notable loss of cell viability. Our results suggest that the presence of SCN− in saliva protects human oral tissues (and bacteria) from highly cytotoxic oxidation products of Cl− and, instead, bacteriostatic and non‐toxic oxidation products of SCN− are generated. In the gingival crevices, however, the SCN−/ Cl− ratio may be too small for effective block of Cl− oxidation by myeloperoxidase, and the oxidation products of Cl−, such as hypochlorite or hypochlorous acid, may contribute to the breakdown of periodontal tissues during inflammation.