Commensal Bacteria in Health and Disease

Abstract

Commensal gut bacteria have received a small fraction of the research effort that has been accorded to pathogens. The importance of commensal bacteria for normal gut function and health is increasingly being recognized. These bacteria play crucial roles in the metabolism of dietary components and the supply of energy to the gut mucosa and other host tissues, and influence the gut environment. They provide protection against pathogens through effective competition for growth requirements and binding sites while also participating in genetic exchange. Analyses of 16S rRNA genes amplified from gut and fecal samples have demonstrated remarkable diversity within the microbial communities of the gastrointestinal tract. Lactate is produced by a wide range of commensal gut bacteria but is not normally detected at concentrations above 2 mM in the feces of healthy individuals, although it is detected at much higher concentrations in patients with ulcerative colitis. Several low- G+C-content gram-positive species, including Ruminococcus hydrogenotrophicus, have been shown to be acetogenic and can both consume hydrogen and supply acetate when in coculture with the butyrate producer Roseburia intestinalis. Bacterial attachment must play a key role in the colonization of insoluble substrates. The impact of diet on susceptibility to pathogen infection may therefore be exerted partly through the effects of diet on the balance of commensal species and metabolic activity, as discussed in this chapter. In addition, poor diet is considered to contribute to long-term gut disorders, including inflammatory bowel disease, irritable bowel syndrome, and colorectal cancer.