Symbiotic microorganisms, both beneficial and potentially pathogenic, reside in the gastrointestinal tract of humans. Imbalances in the bacterial composition, known as dysbiosis, are thought to be a major determinant in inflammatory bowel diseases such as Crohn's disease in humans. It has long been known that intestinal microorganisms are important for the development of intestinal tissues. A recent study has demonstrated that a human symbiotic microorganism called Bacteroides fragilis protects animals from experimental colitis induced by Helicobacter hepaticus. Polysaccharide A (PSA) from this bacteria singlehandedly protects humans from the inflammatory bowel diseases.