Host responses to the human microbiome.

The human gut is home to vast numbers of bacteria (gut microbiota), which outnumber the cells in the human body by an order of magnitude. The gut microbiota has coevolved with humans and can be considered an organ of similar size as the liver, containing more than 1,000 cell types (bacterial species) and encoding 150-fold more genes than are present in the human genome. Accordingly, the gut microbiota may have profound effects on various host responses, either directly or indirectly, by modifying food components or endogenously produced molecules into signaling molecules. Recent findings suggest that an altered gut microbial composition is associated with inflammatory bowel disease and obesity, indicating that the gut microbiota should be considered a contributing factor in several common diseases.