Escherichia coli is predominant in the normal facultative anaerobic microflora of the human intestine. These commensal strains of E. coli usually remain harmlessly confined to the intestinal lumen and have been shown to play an important role in maintaining intestinal physiology. However, given the right opportunities, any E. coli strain can probably cause invasive disease and E. coli has therefore aptly been called an opportunistic pathogen. On the other hand, several highly adapted E. coli clones have evolved the ability to cause a broad spectrum of human diseases. Infections due to these pathogenic E. coli may be limited to the mucosal surfaces or can disseminate throughout the body. Three general clinical syndromes result from infection with inherently pathogenic E. coli strains: (i) enteric/diarrhoeal disease, (ii) urinary tract infection and (iii) sepsis/meningitis. Among the intestinal pathogens, six categories have been defined, based primarily on pathogenic mechanisms (Kaper et al., 2004): enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enterotoxigenic E. coli (ETEC), enterohaemorrhagic E. coli (EHEC), enteroaggregative E. coli (EAEC or EAggEC) and diffusely adherent E. coli (DAEC).