O₂-inducible H₂O₂-forming NADPH oxidase is responsible for the hyper O₂ sensitivity of Bifidobacterium longum subsp....

Bifidobacteria are beneficial anaerobes, and their O₂ sensitivity levels differ among species as a function of unknown molecular mechanisms. Bifidobacterium longum subspecies infantis (B. infantis), a predominant colonizer of the gastrointestinal tract of infants, showed a hyper O₂-sensitive growth profile with accompanying a production of H₂O₂. In this study, we characterized an NADPH oxidase as a key enzyme responsible for this microbe's hyper O₂ sensitivity. A dominant active elution peak of H₂O₂-forming NADPH oxidase activity was detected in the first step of column chromatography, and the purified NADPH oxidase (NPOX) was identified as a homolog of nitroreductase family proteins. The introduction of the gene encoding B. infantis NPOX (npoxA) into O₂-tolerant Bifidobacterium minimum made the strain O₂ sensitive and allowed it to produce H₂O₂. Knockout of the npoxA gene in B. infantis decreased the production of H₂O₂ and mitigated its B. infantis hyper O₂ sensitivity. A transcript of B. infantis npoxA is induced by O₂, suggesting that the aerobic production of toxic H₂O₂ is functionally conserved in B. infantis.