Functional Comparison of Bacteria from the Human Gut and Closely Related Non-Gut Bacteria Reveals the Importance of Conjugation and a Paucity of Motility and Chemotaxis Functions in the Gut Environment

The human GI tract is a complex and still poorly understood environment, inhabited by one of the densest microbial communities on earth. The gut microbiota is shaped by millennia of evolution to co-exist with the host in commensal or symbiotic relationships. Members of the gut microbiota perform specific molecular functions important in the human gut environment. This can be illustrated by the presence of a highly expanded repertoire of proteins involved in carbohydrate metabolism, in phase with the large diversity of polysaccharides originating from the diet or from the host itself that can be encountered in this environment. In order to identify other bacterial functions that are important in the human gut environment, we investigated the distribution of functional groups of proteins in a group of human gut bacteria and their close non-gut relatives. Complementary to earlier global comparisons between different ecosystems, this approach should allow a closer focus on a group of functions directly related to the gut environment while avoiding functions related to taxonomically divergent microbiota composition, which may or may not be relevant for gut homeostasis. We identified several functions that are overrepresented in the human gut bacteria which had not been recognized in a global approach. The observed under-representation of certain other functions may be equally important for gut homeostasis. Together, these analyses provide us with new information about this environment so critical to our health and well-being.