Amino acid metabolism by pig intestinal bacteria and consequences for intestinal health and environment: current knowledge and future directions.

Amino acids available for bacteria within the small intestine of pigs are originating mainly from the dietary and endogenous proteins. These amino acids, in addition to those synthesized by the intestinal bacteria, are used in different metabolic pathways. The amino acids that cannot be synthesized by the intestinal bacteria can be considered as indispensable for them and must thus be obtained from the surrounding luminal fluid. In the pig large intestine, the amino acids used by bacteria originate presumably mainly from proteins not fully digested in the small intestine. In this narrative review, we present the current knowledge concerning the metabolism of amino acids by bacteria found in the pig gut and the effects of the different amino acid-derived bacterial metabolites on the metabolism and physiology of the intestinal microbes. The effects of some among these compounds, either beneficial or deleterious, on the intestinal epithelial cells are then recapitulated. Next, the adverse impact of the emission of two bacterial metabolites derived from amino acids, namely ammonia and hydrogen sulfide, on the environment is described. Future directions in the field of the control of the metabolism of amino acids by the intestinal bacteria for the maintenance of pig intestinal health, as well as for the reduction of the emission of polluting substances are proposed.