Inhibition of Candida albicans by the Peroxidase/SCN-/H2O2 system

Effects of the salivary peroxidase (SPO) system on the growth, glucose uptake and metabolic activities of oral bacteria are well documented but the effects on oral fungi are virtually unknown. Therefore, the viability of Candida albicans (ATCC 28366) exposed to the peroxidase/SCN-/H2O2 system was studied in sterilized saliva, in phosphate-buffered saline (PBS) and in potassium chloride. The growth of C. albicans in glucose-supplemented saliva was faster at pH 5.5 than at pH 7. The addition of the complete SPO (or lactoperoxidase) system to either sterilized saliva, KCl (50 microM) or PBS at pH 5.5 inhibited dose-dependently the viability of C. albicans in KCl, but no inhibition was found in PBS or saliva. Maximal inhibition was achieved in 2 h and with > 320 microM of peroxidase-generated HOSCN/OSCN-. However, physiological salivary concentrations of phosphate (> or = 1.0 mM) and PBS blocked the antifungal effect of HOSCN/OSCN-. The relative proportions of SCN- and H2O2 were critical to the antifungal effects. With 0.2 mM KSCN, a complete loss of viability was achieved, though the HOSCN/OSCN- concentrations did not exceed 100 microM. It is concluded that C. albicans is sensitive to HOSCN/OSCN- but salivary concentrations of phosphate block the antifungal effect of the peroxidase systems.