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E. coli O157

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Synonyms: E. coli O157:H7, Escherichia coli , enterohemorrhagic E. coli (EHEC), shiga toxin-producing E. coli (STEC), verocytotoxic E. coli (VTEC)
Contaminant Type: Microbial

Escherichia coli (E. coli) O157:H7, a serotype of the bacterium E. coli, can produce Shiga-like toxins which cause severe illnesses in humans [855]. E. coli is found in feces; illnesses may be associated with exposure to contaminated water supplies [857].

E. coli inhabits the intestines of animals and humans. When cultured aerobically, E. coli is the dominant species found in feces. In 1995, more than 60 E. coli serotypes had been identified that produced Shiga-like toxins; the serotype O157:H7 was the most predominant and most frequently associated with human infections worldwide [856]. E. coli O157:H7 has a low infectious dose; studies have estimated exposure between 20 and 700 organisms can cause illness.

The majority of outbreaks have been linked with the consumption of contaminated food. However, water-related exposures have been reported from recreational uses of surface waters and consumption of water from private drinking wells [857]. Drinking water sources may become contaminated with E coli O157:H7 when human or animal feces containing the bacteria are introduced into drinking water supplies via sewage overflows or agricultural runoff.

The survival of E. coli O157:H7 in surface waters depends on a number of biological, physical, and chemical factors. These include but are not limited to nutrient availability, ultraviolet (UV) radiation, temperature, and predation. E. coli O157:H7 were reported to survive for two months in samples collected from a lake, a fecal contaminated puddle, a river, and an animal trough [857]. In comparison, another study reported detections of E. coli O157:H7 after 17 days within a sterile distilled municipal water sample at ambient conditions. The same municipal source water maintained in the lab at 15°C had a positive detection for E. coli O157:H7 after 31 days [858].

Shiga toxins cause severe illness by affecting the lining of the intestine [855]. Once exposed, incubation periods range between one and eight days [855].

Public water systems are required to test their drinking water supplies for total coliforms (indicator bacteria for E. coli) under the USEPA Total Coliform Rule. If a water analysis yields positive results for total coliforms, the public water system must resample for either fecal coliforms or E. coli. Currently, there is not a maximum contaminant level (MCL) for E. coli or for the specific serotype E. coli O157:H7. However, E.coli O157 was included on the  USEPA Contaminant Contaminant List 3 (CCL3) published on September 21, 2009.  The CCL3 includes contaminants that are currently unregulated, are known or anticipated to occur in public water supplies, and which may require regulation under the Safe Drinking Water Act [859].

The largest waterborne outbreak of E. coli O157:H7 was documented in 1989 in Missouri, where more than 200 people became infected and four died. The pathway of exposure could not be identified, but potential explanations include contamination by backflow during a water main break [856].

Date of Literature Search: August 2009

855 Virginia Bioinformatics Institute ; 2009; Pathogen Information (PathInfo); http://pathport.vbi.vt.edu/pathinfo/pathogens/E.coli_O157H7_Info.shtml; As posted on September 1, 2009. Virginia Bioinformatics Institute Blacksburg, VA 24061
856 Peter Feng; 1995; Escherichia coli serotype O157:H7 novel vehicles of infection and emergence of phenotypic variants; Emerging Infectious Diseases; 1:2:47
857 Avery L., Williams A., Killham K., Jones D.; 2008; Survival of Escherichia coli O157:H7 in waters from lakes, rivers, puddles and animal-drinking troughs; Science of the Total Environment; 389:2-3:378
858 McGee P., Bolton D., Sheridan J., Earley B., Kelly G., Leonard N.; 2002; Survival of Escherichia coli O157:H7 in farm water: its role as a vector in the transmission of the organism within herds; J. Applied Microbiology; 93:4:706
859 USEPA; 2009; USEPA Contaminant Candidate List 3 (CCL 3); http://www.epa.gov/safewater/ccl/ccl3.html#ccl3; As posted December 16, 2009.