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 roughly approximated that between 3 and 11 g of alimentary proteins and peptides are moving from the small intestine to the large intestine in humans. Here, this nitrogenous material is degraded by the bacterial protease and peptidase activities, releasing amino acids. 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 anabolism of amino acids by colonic bacteria is related to the synthesis of proteins while some specific amino acids are used for the synthesis of the purine and pyrimidine rings in DNA and RNA. Catabolism of specific amino acids allows for ATP synthesis and results in the production of metabolites with documented roles in the metabolism and physiology of commensal and pathogenic microorganisms among the intestinal microbiota. In the present narrative review, we examine the recycling of the undigested host's proteins by large intestine bacteria and the metabolism of released amino acids. In addition, we describe 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.