Culture of Escherichia coli on MacConkey agar. E. coli are gram-negative rod-shaped, facultative anaerobic bacteria that normally inhabit the human and mammal intestine and support digestion. Though harmless in their natural environment they can cause opportunistic infections when they arrive from intestine to other body regions, most commonly the urinary-tract. There also exist some pathogenic Escherichia coli species such as enterotoxigenic (ETEC) and enterohaemorrhagic Escherichia coli (EHEC), enteroinvasive (EIEC) and enteroaggregative Escherichia coli (EAEC) which are associated with contaminated water and food and cause diarrhoeal diseases.