O(2)-inducible H(2)O(2)-forming NADPH oxidase is responsible for the hyper O(2) sensitivity of Bifidobacterium longum subsp. infantis

Bifidobacteria are beneficial anaerobes, and their O(2) 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 O(2)-sensitive growth profile with accompanying a production of H(2)O(2). In this study, we characterized an NADPH oxidase as a key enzyme responsible for this microbe’s hyper O(2) sensitivity. A dominant active elution peak of H(2)O(2)-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 O(2)-tolerant Bifidobacterium minimum made the strain O(2) sensitive and allowed it to produce H(2)O(2). Knockout of the npoxA gene in B. infantis decreased the production of H(2)O(2) and mitigated its B. infantis hyper O(2) sensitivity. A transcript of B. infantis npoxA is induced by O(2), suggesting that the aerobic production of toxic H(2)O(2) is functionally conserved in B. infantis.