The Biomass and Composition of the Gut Microbiota Modify Anaerobic Metabolism

The lumen of the gastrointestinal tract is host to a large mass of microbiota, and this environment is largely anaerobic. Previous studies have estimated that bacteria contribute 1–3% (~1–3 kg) of an adult human's body mass and that bacteria typically exhibit an ~7 kcal/kg/hr metabolic rate, which implies a total contribution of 150–450 kcal/d to the host's apparent energy expenditure. Surprisingly, however, few studies have attempted to directly quantify the metabolic rate of the gut microbiota in the in vivo setting. We hypothesized that the biomass and composition of the bacterial community would both contribute to the metabolic rate of the gut microbiota, and that the energy flux through this biomass would be anaerobic in nature. Total resting metabolic rate (tRMR) was assessed using direct calorimetry; aerobic (aRMR) was determined using respirometry, and anaerobic RMR (anRMR) was determined as the difference in results between the two methods. Wildtype C57BL/6J mice were anesthetized with ketamine/xylazine to measure baseline tRMR, and tRMR after surgical removal of the cecum. A subset of animals was also pre-treated with risperidone.