Bacterial nitrate and nitrite metabolism in the human gut

The human gastrointestinal tract harbors a diverse population of approximately 10¹⁴ prokaryotic cells, predominantly residing in the colon. This gut microbiota actively interacts with the gut epithelium, influencing intestinal health, immune modulation, and disease development. While there has been extensive research on the nitrogen cycle in the environment, the nitrogen metabolism in the human intestine apart from E. coli is still poorly understood. Nitrogen is a crucial element for life, forming essential biomolecules, and its various oxidation states undergo important transformations. However, some of these transformations may be involved in reactions that lead to the production of carcinogenic nitroso compounds. This study aimed to clarify the enzymatic reactions and kinetic mechanisms of the nitrate and nitrite metabolism by establishing a gut model, featuring ten prevalent core species which represent the major phyla of the gut. This model utilized organisms of the phyla Bacteroidota, Bacillota, Proteobacteria, Actinobacteria, and Verrucomicrobia. During growth experiments, organisms were cultivated at 37°C for 24 hours and key parameters such as optical density and pH were recorded. The bioavailable amounts of nitrate and nitrite during cultivation were adapted to the physiological concentration of 100 μM present in the colon. The concentration of both compounds was quantified using colorimetric assays and HPLC analysis. These experiments revealed in the small intestine a significant nitrite degradation capacity for L. reuteri, S. vestibularis and V. atypica while in the large intestine, a reduction was predominantly observed in E. coli, B. cellulosilyticus, B. xylanisolvens and P. dorei. In contrast, nitrate degradation in the small intestine was only observed through V. atypica and in the large intestine through E. coli. This study provides insights into the diverse nitrate and nitrite reduction capabilities in prevalent gut bacteria, emphasizing the significance of E. coli in the intestinal metabolic landscape, as it exhibits the highest degradation rates.