Escherichia coli is the predominant, facultative anaerobe of the human intestine. Beneficial strains of E. coli typically colonize the infant gastrointestinal tract within a few hours after birth. The presence of this bacterial population in the intestine suppresses the growth of harmful bacteria and is important for synthesizing appreciable amounts of B vitamins. E. coli usually remains harmless when confined to the intestinal lumen. However, in debilitated or immuno-suppressed humans, or when gastrointestinal barriers are violated, even normal, “non-pathogenic” strains of E. coli can cause infection (2, 5, 9, 10).

It is also known that some E. coli strains have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even very healthy people. Strains of E. coli that cause diarrhea include strains that cause traveler’s diarrhea (enterotoxigenic E. coli), persistent diarrhea (enteroaggregative E. coli), watery diarrhea of infants (enteropathogenic E. coli), hemorrhagic colitis (bloody diarrhea), and hemolytic uremic syndrome (enterohemorrhagic E. coli) (2, 5, 8, 10).

Enterohemorrhagic E. coli
E. coli O157:H7 is a strain of enterohemorrhagic E. coli [EHEC]. The growth of this strain in the human intestine produces a large quantity of toxin(s) that cause severe damage to the lining of the intestine and other organs of the body. These toxins [referred to as verotoxin (VT) or shiga-like toxin (SLT)] are very similar, if not identical, to the toxins produced by Shigella dystenteria (5).

The incubation period for EHEC diarrhea is usually 3 to 4 days, although incubation times can be as long as 5 to 8 days or as short as 1 to 2 days. Usually the first complaint of illness is non-bloody diarrhea, which is preceded by crampy abdominal pain and fever of short duration in many patients. Vomiting occurs in about half of the patients during the period of non-bloody diarrhea and/or other times in the illness. Within 1 or 2 days, the diarrhea becomes bloody, and the patient experiences increased abdominal pain. This state usually lasts between 4 to 10 days. In severe cases, fecal specimens are described as “all blood and no stool.” In most patients, the bloody diarrhea resolves with no long-term impairment. However, in about 10% of patients younger than 10 years and in many elderly patients, the illness will progress to hemolytic uremic syndrome (HUS) and other complications (8).

HUS is a leading cause of kidney failure in children, which often requires dialysis and may ultimately be fatal. Other systemic manifestations of illness due to E. coli O157:H7 include a central nervous system involvement, hypertension, myocarditis, and other cardiovascular complications that may result in death or severe disability. In some cases, the illness is indicative of some forms of heart disease and has been responsible for strokes in small children. These complications are attributed to direct or indirect actions of verotoxins absorbed from the intestinal tract (6, 8).

There is evidence of transmission of this pathogen through direct contact between people. E. coli O157:H7 can be shed in feces for a median period of 21 days with a range of 5 to 124 days, according to studies reported by Nataro and Kaper (8).

Infective Dose
According to the FDA (5), the infectious dose for E. coli O157:H7 is unknown. However, a compilation of outbreak data indicates that it may be as low as 10 organisms. In a review, Paton and Paton (10) cite references for infectious dose as ranging from 1 to 100 colony-forming units. These data show that it takes a very low number of microorganisms to cause illness in young children, the elderly and immune-compromised people.

The first notable incidents of E. coli O157:H7 in the United States were reported in 1982, when two outbreaks of hemorrhagic colitis occurred, one in Oregon and one in Michigan. Both outbreaks were traced to fast-food restaurants of the same chain. Infected individuals had eaten hamburgers that were contaminated. Frozen ground beef patties had not been heated sufficiently to inactivate these pathogenic bacteria (3, 9).

Since 1982, a continuing number of outbreaks of infection due to E. coli O157:H7 have been reported in the U.S., Canada, and the United Kingdom (8, 9, 10). These incidents have occurred in homes, child-day care centers, swimming pools, schools, nursing homes, institutions for the mentally retarded, and in fast-food and other restaurant operations. Vehicles of transmission have been traced back to ground meat (particularly ground beef), fermented sausages, raw milk, unpasteurized apple cider and apple juice, raw vegetables (e.g., lettuce, alfalfa sprouts), mayonnaise, and unchlorinated water. A few of the most notable incidents are described in the following paragraphs.

In the fall of 1988, an incident occurred at a junior high school in Minnesota that led to illness in 30 students. Four were hospitalized. There were no fatalities. Frozen, partially cooked beef patties were incriminated in this incident. The patties had not been heated sufficiently by the processor to inactivate E. coli in the center. The beef patties were reheated before they were served to students, but some, again, were not reheated adequately to inactivate E. coli O157:H7 (7).

From November 15, 1992 through February 28, 1993 more than 500 laboratory-confirmed infections with E. coli O157:H7 and 4 associated deaths occurred in 4 states (Washington, Idaho, California, and Nevada). Many young children were involved. The outbreaks were traced to a fast-food chain and restaurants in these states serving regular hamburgers and jumbo hamburgers. A meat traceback by the Centers for Disease Control team identified 5 slaughtering plants in the United States and 1 in Canada as the likely sources of carcasses used in the contaminated lots of meat. The animals slaughtered in these slaughtering operations were traced to cattle auctions in 6 western states (1, 9).

In the fall of 1996, unpasteurized apple juice from a processing facility in California was determined to be responsible for an outbreak of E. coli O157:H7. In August 1997, Hudson Foods recalled 25 million pounds of ground beef after an E. coli outbreak was traced to its plant in Columbus, Nebraska.

The FDA 1997 Food Code (4) recommends that all ground beef products, as well as other raw ground meat products, be cooked until all parts of the product reach a temperature of 155°F (68.3°C) for 15 seconds. At this temperature, the interior of the ground beef is only slightly pink. Note that a thermometer must be used to check meat temperatures, since the color of meat and juices are not reliable indicators of the “doneness” of meat. [The FDA also lists the following acceptable food temperatures and times for the preparation of raw ground meat: 150° F (66° C) for 1 minute, or 145° F (63° C) for 3 minutes.]

Non-O157:H7 Serotypes
Most reported outbreaks of enterohemorrhagic E. coli infection have been caused by O157:H7 strains. This suggests that this serotype is more virulent or more transmissible than other serotypes. However, other serotypes of enterohemorrhagic E. coli have been implicated in outbreaks, and the incidence of disease due to non-O157:H7 serotypes seems to be rising (8). More than 50 of these serotypes have been associated with bloody diarrhea or HUS in humans. The Table 1, adapted from Johnson et al. (6), lists some of these serotypes.

The most common non-O157:H7 serotypes associated with human disease include: O26:H11, O103:H2, O111:NM, and O113:H21. At least 10 of these outbreaks due to these organisms have been reported in Japan, Germany, Italy, Australia, the Czech Republic, and the United States. These outbreaks have involved 5 to 234 people, and for most of them, the source of infection could not be determined (6). In many countries such as Chile, Argentina, and Australia, non-O157:H7 serotypes have been found to be responsible for the majority of HUS cases. Non-bloody diarrhea has also been associated with some of these non-O157:H7 serotypes.