Enteroinvasive, enteropathogenic, and enterohaemorrhagic E. coli were significantly more acid tolerant than nonpathogenic strains such as E. coli K12 (Gorden and Small 1993). For the high level of acid tolerance of many O157: H7 strains, various mechanisms may be involved depending on the growth phase of the bacteria undergoing the acid stress. The development of acid tolerance can be pH-dependent, pH-independent or a combination of both types (Lin and others 1995).