Distribution and activity of nitrate and nitrite reductases in the microbiota of the human intestinal tract

The human intestinal microbiota plays a vital role in health. One of the most protective benefits is the bacterial nitrogen metabolism of gut bacteria, which reduces nitrate (NO 3 − ) and nitrite (NO 2 − ) to ammonia or nitric oxide, preventing the formation of carcinogenic nitrosamines. In this study, we shed light on the gut bacterial NO 2 − /NO 3 − degradation, its efficacy, and the effects on the steady‐state NO 2 − concentration in the human colon. Highly abundant gut bacteria that represent the most prominent phyla were analyzed for their potential to reduce NO 2 − or NO 3 − . Escherichia coli showed the greatest efficiency, which indicates a key role in the detoxification and prevention of nitrosamine formation. Species of the genera Bacteroides and Phocaeicola also contributed to NO 2 − reduction due to their high abundance. The total activity of stool samples was about 620 μmol NO 2 − h −1 , indicating that NO 2 − concentration in the human stool should be very low. We also show that bacterial NO 2 − reduction is necessary to allow NO 2 − ‐sensitive microorganisms to colonize the intestine, preventing a pathological shift in the composition of the intestinal microbiota. The results illustrate that the gut microbiota plays a central role in NO 2 − detoxification, ensuring microbiota integrity and potentially preventing nitrosamine formation and gut‐associated cancers.