Enterohemorrhagic Escherichia coli

Escherichia coli is one of the most studied bacteria. It is part of the normal bacterial flora resident in the intestines of many animals, including humans, and is commonly used as a nonpathogenic indicator of recent fecal contamination and of fecally associated pathogenic organisms such as salmonella [33]. However, numerous strains of E. coli exist which are not commensal. Pathogenic E. coli produces toxins of various types and toxicities that cause various diseases. These toxins have been described previously [34]. Diar-rheagenic E. coli are subdivided into six classes based on the symptoms they produce and virulence factors they possess [35]. Of these groups, the enterohemorrhagic (EHEC) class is of most concern, due to its low infectious dose and its association with hemorrhagic colitis (HC), hemolytic uremic syndrome (HUS), and thrombotic thrombocytopenic purpura (TTP). HUS occurs primarily in children under 10 years of age and has a mortality of 3 to 5% [35]. Children's susceptibility to HUS led the FDA to issue a warning in November 2001 to the public concerning the health risk of consuming untreated juices by children [36].

Although there are several serotypes of EHEC known, the most common serotype, particularly in the U.S., Canada, Great Britain, and parts of Europe, is E. coli O157:H7 [35]. In the years from 1998 to 2000 the Center for Disease Control (CDC) recorded 86 outbreaks attributed to E. coli. Of these, 68 were identified as outbreaks caused by E. coli O157:H7 [37]. The great majority of these outbreaks were either from meat products or had an unknown source. Despite the fact that the majority of EHEC outbreaks are not associated with fresh fruit or juice made from fresh fruit, outbreaks associated with fresh fruit or fresh juice are of concern, since these products are associated with a healthy lifestyle and are generally consumed raw.

EHEC strains of E. coli are not normal endogenous microflora of fresh juice or of the fruit used to produce fresh juice. Their presence on fruit and in fruit juice is believed to be the consequence of some form of fecal contamination prior to consumption. Cattle have been implicated as a major reservoir of this organism [38-41]. Wild animals such as deer may be an additional source of the organism [42]. Wild birds have also been implicated as vectors for contamination, particularly those living near landfills [43]. Presumably, birds become infected at landfills and then may carry infection to farm fields and/or cattle. In addition to birds, transfer of E. coli O157:H7 by fruit flies has been demonstrated [44].

From epidemiological data it is clear that E. coli O157:H7 can survive well enough in low pH juice to result in serious illness. Of particular note was the Western states outbreak during October 1996 from contaminated apple cider that resulted in 66 cases of illness and one death [45,46]. Although the pH of most apple and orange juice is low enough to either significantly slow or inhibit growth of E. coli, EHEC strains have tolerance to high levels of acid allowing for extended survival time [3,47]. Tolerance to high acid levels is a complex induced response involving three distinct mechanisms and is enhanced in stationary phase cells [3,7,8,47].

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