Influence of a model human defensive peroxidase system on oral streptococcal antagonism

Abstract

Streptococcusis a dominant genus in the human oral cavity, making up about 20 % of the more than 800 species of bacteria that have been identified, and about 80 % of the early biofilm colonizers. Oral streptococci include both health-compatible (e.g.Streptococcus gordoniiandStreptococcus sanguinis) and pathogenic strains (e.g. the cariogenicStreptococcus mutans). Because the streptococci have similar metabolic requirements, they have developed defence strategies that lead to antagonism (also known as bacterial interference).S. mutansexpresses bacteriocins that are cytotoxic towardS. gordoniiandS. sanguinis, whereasS. gordoniiandS. sanguinisdifferentially produce H2O2(under aerobic growth conditions), which is relatively toxic towardS. mutans. Superimposed on the inter-bacterial combat are the effects of the host defensive mechanisms. We report here on the multifarious effects of bovine lactoperoxidase (bLPO) on the antagonism betweenS. gordoniiandS. sanguinisversusS. mutans. Some of the effects are apparently counterproductive with respect to maintaining a health-compatible population of streptococci. For example, the bLPO system (comprised of bLPO+SCN−+H2O2) destroys H2O2, thereby abolishing the ability ofS. gordoniiandS. sanguinisto inhibit the growth ofS. mutans. Furthermore, bLPO protein (with or without its substrate) inhibits bacterial growth in a biofilm assay, but sucrose negates the inhibitory effects of the bLPO protein, thereby facilitating adherence ofS. mutansin lieu ofS. gordoniiandS. sanguinis. Our findings may be relevant to environmental pressures that select early supragingival colonizers.