A Strain ofStreptococcus mitisInhibits Biofilm Formation of Caries Pathogens via Abundant Hydrogen Peroxide Production

Abstract

Commensal oral streptococci that colonize supragingival biofilms deploy mechanisms to combat competitors within their niche. Here, we determined thatStreptococcus mitismore effectively inhibited biofilm formation ofStreptococcus mutanswithin a seven species panel. This phenotype was common amongst all assayed isolates ofS. mutans, but was specific to a single strain ofS. mitis, ATCC 49456. The growth inhibitory factor was not effectively carried in spent supernatants ofS. mitis. However, we documented ATCC 49456 to accumulate 4-5 times more hydrogen peroxide (H2O2) than other species tested, and 5-18 times more than otherS. mitisstrains assayed. TheS. mutansbiofilm formation inhibitory phenotype was reduced when grown in media containing catalase or with aS. mitismutant of pyruvate oxidase (spxB;pox), confirming that SpxB-dependent H2O2production was the main antagonistic factor. Addition ofS. mitiswithin hours afterS. mutansinoculation was effective at reducing biofilm biomass, but not for 24 h pre-formed biofilms. Transcriptome analysis revealed responses for bothS. mitisandS. mutans, with severalS. mutansdifferentially expressed genes following a gene expression pattern previously described, while others being unique to the interaction withS. mitis. Finally, we show thatS. mitisalso affected coculture biofilm formation of several other commensal streptococci. Our study shows that strains with abundant H2O2production are effective at inhibiting initial growth of caries pathogens likeS. mutans, but are less effective at disrupting pre-formed biofilms and have the potential to influence the stability of other oral commensal strains.