Escherichia are common bacteria that colonize the human large intestine.
Most are opportunistic normal flora but some are potent pathogens. Transmission of diarrheal disease is generally person to person, usually related to hygiene, food processing and sanitation. Escherichia are Gram-negative bacilli that ferment lactose. Most are motile by peritrichious flagella. Approximately 170 different O antigens have been delineated and some of these are cross-reactive with Shigella , Salmonella and Klebsiella .