Mixed continuous cultures of Streptococcus mutans with Streptococcus sanguis or with Streptococcus oralis as a model to study the ecological effects of the lactoperoxidase system.

Mixed continuous cultures of Streptococcus species were obtained, using complex carbohydrate (mucin) as a source of nutrients, to study the ecological effects of oxygen and the lactoperoxidase system. S. mutans NCTC 10449 was unable to grow as a pure culture on mucin, but attained a significant population size in the presence of S. oralis and S. sanguis strains. The cell densities of the anaerobic mixed cultures decreased when oxygen was supplied, and S. mutans was more suppressed by oxygen than were S. sanguis and S. oralis. However, the concentrations of hydrogen peroxide (30 mumol/l in the mixed culture of S. mutans with S. sanguis and 640 mumol/l in the culture with S. oralis) indicated a certain resistance of the organisms to hydrogen peroxide. Addition of lactoperoxidase and thiocyanate to the oxygen-supplied cultures had a differential effect on the streptococcal populations. While S. mutans was inhibited, and even disappeared in the culture with S. oralis, the growth of S. sanguis and S. oralis was unaffected. This latter observation was in accordance with the OSCN- reductase activities of these organisms. When hydrogen peroxide was also added together with lactoperoxidase and thiocyanate, a further inhibition of S. mutans in the culture with S. sanguis was observed. Under these conditions, S. oralis was also inhibited, perhaps by the strong accumulation of OSCN-, exceeding the capacity of the OSCN- reductase. The effects of lactoperoxidase on mixed cultures may reflect the situation in the mouth.(ABSTRACT TRUNCATED AT 250 WORDS)