The indigenous gastrointestinal (GI) tract microflora has profound effects on the anatomical, physiological and immunological development of the host. The indigenous microflora stimulates the host immune system to respond more quickly to pathogen challenge and, through bacterial antagonism, inhibits colonization of the GI tract by overt exogenous pathogens. Indigenous GI bacteria are also opportunistic pathogens and can translocate across the mucosal barrier to cause systemic infection in debilitated hosts.