A colon without commensals (bottom) cannot repair damage.
Medzhitov/Elsevier
The benefits of commensal, or nonpathogenic gut bacteria, are well known. For example, these bacteria digest tricky carbohydrates and supply certain micronutrients. But inappropriately enthusiastic responses to the bacteria are characteristic of inflammatory bowel diseases (IBD) such as Crohn's disease. So Medzhitov suspected that mice less able to respond to bacterial products might end up better off.
Instead, mice deficient for Toll-like receptor (TLR) responses to bacterial products such as lipopolysaccharide (LPS) were more susceptible than wild-type mice to a colon-damaging chemical. The increased damage and death was not the result of bacterial invasion, as lymphocyte infiltration was low, and epithelial damage and bleeding...
