Function of the Pyruvate Oxidase-Lactate Oxidase Cascade in Interspecies Competition between Streptococcus oligofermentans and Streptococcus mutans

Abstract

Complex interspecies interactions occur constantly between oral commensals and the opportunistic pathogen Streptococcus mutans in dental plaque. Previously, we showed that oral commensal Streptococcus oligofermentans possesses multiple enzymes for H ₂ O ₂ production, especially lactate oxidase (Lox), allowing it to out-compete S. mutans . In this study, through extensive biochemical and genetic studies, we identified a pyruvate oxidase ( pox ) gene in S. oligofermentans . A pox deletion mutant completely lost Pox activity, while ectopically expressed pox restored activity. Pox was determined to produce most of the H ₂ O ₂ in the earlier growth phase and log phase, while Lox mainly contributed to H ₂ O ₂ production in stationary phase. Both pox and lox were expressed throughout the growth phase, while expression of the lox gene increased by about 2.5-fold when cells entered stationary phase. Since lactate accumulation occurred to a large degree in stationary phase, the differential Pox- and Lox-generated H ₂ O ₂ can be attributed to differential gene expression and substrate availability. Interestingly, inactivation of pox causes a dramatic reduction in H ₂ O ₂ production from lactate, suggesting a synergistic action of the two oxidases in converting lactate into H ₂ O ₂ . In an in vitro two-species biofilm experiment, the pox mutant of S. oligofermentans failed to inhibit S. mutans even though lox was active. In summary, S. oligofermentans develops a Pox-Lox synergy strategy to maximize its H ₂ O ₂ formation so as to win the interspecies competition.