Recycling of Undigested Proteins Provided by the Host to the Large Intestine Microbiota: Implication for Intestinal Bacterial Anabolism, Growth, and Physiology

Although the digestion of dietary and endogenous proteins by the exocrine pancreatic proteases and peptidases in the small intestine luminal fluid is highly efficient for most proteins, it has been approximated that between 3 and 11 g of proteins and peptides are transferred from the small to the large intestine in humans. Here, the proteins and peptides are degraded by the bacterial proteases and peptidases, releasing amino acids in free form. These amino acids are utilized by the abundant population of bacteria, notably amino acids that the bacteria are unable to synthesize, and which can thus be considered as indispensable for these microorganisms. The metabolism of amino acids by colonic bacteria is related to the synthesis of proteins while some specific amino acids are used for DNA and RNA synthesis and in catabolic pathways allowing ATP synthesis and leading to the production of metabolites with documented roles in the metabolism and physiology of both commensal and pathogenic intestinal microorganisms. In the present review, we examine the recycling of undigested host’s protein by the large intestine microbiota and the metabolism of amino acids by the intestinal bacteria. We describe in addition how these metabolic pathways are involved in bacterial growth and communication, as well as in bacterial physiology in terms of virulence, resistance to detrimental environmental conditions, and capacity to form biofilms.