Redox signaling modulates nitrite transport in a commensal Streptococcus

Commensal streptococci modulate health and homeostasis in the oral cavity through the production of hydrogen peroxide (H₂O₂), a redox-active molecule with potent antimicrobial activity. Our group has previously shown that Streptococcus parasanguinis antagonizes oral and respiratory pathogens not only using H₂O₂, but also through the reaction of H₂O₂ with readily available environmental nitrite (NO₂), which produces reactive nitrogen species (RNS), such as peroxynitrite (ONOO^-). The production of H₂O₂ by oral streptococci requires the activity of pyruvate oxidase (PoxL), an enzyme that is essential for S. parasanguinis to inhibit neighboring pathogens and modulate homeostasis during polymicrobial infections. However, it remains unknown how H₂O₂ impacts NO₂ acquisition in S. parasanguinis. In this study, we explored the role of PoxL in NO₂ transport and NO₂-dependent fitness and physiology in S. parasanguinis. Loss of PoxL resulted in decreased biofilm development and colonization in a Drosophila melanogaster model compared to wild-type bacteria, however, defective biofilm formation and colonization in the poxL mutant was restored by the addition of exogenous NO₂. Further analysis of the poxL mutant revealed a decrease in intracellular NO₂ compared to wildtype bacteria. In support of these findings, expression of the formate-nitrite transporter (Spaf_1142) was significantly decreased in the poxL mutant compared to wildtype, and also by the addition of catalase. Lastly, we observed decreases in several glycolytic proteins and a decrease in ATP production in the poxL mutant, which was increased by the addition of nitrite. Overall, this work highlights a coordinated regulatory interplay of PoxL activity and H₂O₂ production on NO₂ transport and metabolism.