Several genetically engineered 'knock out' and transgenic animal models of IBD have been reported in which the role of the enteric flora has been directly demonstrated by comparing animals raised in germ free versus conventional facilities. For example, the gastrointestinal Crohn's-like inflammation that occurs in interleukin-10 (IL-10) deficient mice is attenuated when the animals are kept in a germ free environment and becomes more pronounced and widespread when they are switched to conventional facilities. (Kuhn R et al. Cell 1993;75:263-274).