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

Complex interspecies interactions occur constantly between oral commensals and the opportunistic pathogenStreptococcus mutans in dental plaque. Previously, we showed that oral commensalStreptococcus oligofermentans possesses multiple enzymes for H2 O2 production, especially lactate oxidase (Lox), allowing it to out-competeS. mutans . In this study, through extensive biochemical and genetic studies, we identified a pyruvate oxidase (pox ) gene inS. oligofermentans . Apox deletion mutant completely lost Pox activity, while ectopically expressedpox restored activity. Pox was determined to produce most of the H2 O2 in the earlier growth phase and log phase, while Lox mainly contributed to H2 O2 production in stationary phase. Bothpox andlox were expressed throughout the growth phase, while expression of thelox 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 H2 O2 can be attributed to differential gene expression and substrate availability. Interestingly, inactivation ofpox causes a dramatic reduction in H2 O2 production from lactate, suggesting a synergistic action of the two oxidases in converting lactate into H2 O2 . In anin vitro two-species biofilm experiment, thepox mutant ofS. oligofermentans failed to inhibitS. mutans even thoughlox was active. In summary,S. oligofermentans develops a Pox-Lox synergy strategy to maximize its H2 O2 formation so as to win the interspecies competition.