Streptococcus is 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 gordonii and Streptococcus sanguinis) and pathogenic strains (e.g. the cariogenic Streptococcus mutans). Because the streptococci have similar metabolic requirements, they have developed defence strategies that lead to antagonism (also known as bacterial interference). S. mutans expresses bacteriocins that are cytotoxic toward S. gordonii and S. sanguinis, whereas S. gordonii and S. sanguinis differentially produce H 2 O 2 (under aerobic growth conditions), which is relatively toxic toward S. 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 between S. gordonii and S. sanguinis versus S. 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 " +H 2 O 2 ) destroys H 2 O 2 , thereby abolishing the ability of S. gordonii and S. sanguinis to inhibit the growth of S. 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 of S. mutans in lieu of S. gordonii and S. sanguinis. Our findings may be relevant to environmental pressures that select early supragingival colonizers. INTRODUCTIONThe term 'microbial ecosystem' has been defined as an organization of micro-organisms that live in a particular niche, while subject to the environmental pressures of the niche (Raes & Bork, 2008). These pressures may be biotic (e.g. self, other microbial species, plants and animals) and abiotic (e.g. temperature, pH, nutrients, etc.) in origin. Dental plaques are examples of dynamic and exceedingly complex microbial ecosystems that are thought to comprise at least 800 bacterial species (Kroes et al., 1999;Paster et al., 2006;Aas et al., 2005Aas et al., , 2008Preza et al., 2008;Becker et al., 2002;Paster et al., 2001;Dethlefsen et al., 2007). Many of the environmental factors that influence the speciation of oral biofilms have been reviewed in other articles (Marsh, 2005;Overman, 2000;Rosan & Lamont, 2000;Sissons, 1997;Socransky & Haffajee, 2005;ten Cate, 2006), including the effects of pH (Burne & Marquis, 2000), nutrients (Sissons et al., 2007), fluoride (Bowden, 1990) and various antimicrobials (Dashper et al., 2007;Fine et al., 2001;Leung et al., 2005;Yang et al., 2006;Zaura-Arite et al., 2001). The effects of the host immune system on oral biofilms are less well understood. This paper focuses on the influence of defensive peroxidases on interstreptococcal interactions.There are two host-derived defensive peroxidases in the human oral cavity, salivar...