1. General Principles
The presence of Escherichia coli in water has been of concern for more than 100 years. Escherichia coli is the type species, or reference species, of the genus Escherichia, which also includes Escherichia albertii, E. fergusonii, E. hermannii, and E. vulneris. Among microorganisms, E. coli is probably the most studied species. Although E. coli is a normal inhabitant of the human intestinal tract, some E. coli strains cause intestinal infections or attach to intestinal cells and produce enterotoxins.1–3 The usual result is diarrhea. In this section, these gut pathogens are referred to as diarrheagenic E. coli. This term is more specific4 than pathogenic E. coli because it distinguishes the enteric pathogens from those strains that cause extraintestinal infections such as meningitis, bacteremia, and wound and urinary-tract infections.4 Specific procedures for the isolation and identification of E. coli strains that cause extraintestinal infections can be found in manuals that deal with clinical microbiology.4
Diarrheagenic E. coli are important causes of intestinal infections worldwide, but particularly in developing countries.2,3 Many cases of E. coli diarrhea result from foodborne and person-to-person transmission, but waterborne outbreaks also occur.1,2,5–9 From 1972 through 1994, E. coli accounted for only 2 of 291 (0.7%) drinking water-related outbreaks in the United States that were reported to CDC, one caused by an enterotoxin-producing E. coli strain and the other by a strain of E. coli O157. However, E. coli accounted for 4 of 30 outbreaks associated with recreational water for the years 2001–2002,9 with 3 of these attributed to E. coli O157:H7.
Four groups of E. coli are well established enteric pathogens: Shiga toxin-producing E. coli (STEC) [of which entero-hemorrhagic E. coli (EHEC) is a subgroup], enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and enteropathogenic E. coli (EPEC).2,3 The term E. coli O157 is used throughout this section to refer to diarrheagenic members of this group, with or without the H7 antigen. E. coli O157 is a member4 of the STEC group, and is an important pathogen in the United States and many other countries. Because E. coli O157 causes life-threatening infections, methods and commercial reagents for its isolation and identification in human clinical specimens and food are available. These procedures are applicable to water analysis and are covered below.