E. coli O157:H7 is one of thousands of serotypes of Escherichia coli. E. coli O157:H7 was first recognized as a pathogen in 1982 during an investigation into an outbreak of hemorrhagic colitis associated with consumption of hamburgers from a fast-food chain restaurant (McDonalds). For many years, most recognized STEC outbreaks were associated with STEC O157. Despite the dominance of STEC O157, at least 150 non-O157 strains of E. coli are known to cause human illness and have been associated with outbreaks.
Although foods of bovine origin are the most common cause of both outbreaks and sporadic cases of E. coli O157:H7 infections, outbreaks of illnesses have been linked to a wide variety of food items. For example, produce has been the source of substantial numbers of outbreak-related E. coli O157:H7 infections since at least 1991. Outbreaks have been linked to alfalfa, clover and radish sprouts, lettuce, and spinach. Other vehicles for outbreaks include unpasteurized juices, yogurt, dried salami, mayonnaise, raw milk, game meats, hazelnuts, and raw cookie dough.
E. coli O157:H7 bacteria and other pathogenic E. coli mostly live in the intestines of cattle, but E. coli bacteria have also been found in the intestines of chickens, deer, sheep, and pigs. A 2003 study on the prevalence of E. coli O157:H7 in livestock at 29 county and three large state agricultural fairs in the United States found that E. coli O157:H7 could be isolated from 13.8% of beef cattle, 5.9% of dairy cattle, 3.6% of pigs, 5.2% of sheep, and 2.8% of goats. Over 7% of pest fly pools also tested positive for E. coli O157:H7. Shiga toxin-producing E. coli does not make the animals that carry it ill. The animals are merely the reservoir for the bacteria.
What makes E. coli O157:H7 and non-O157 STEC remarkably dangerous is its very low infectious dose, and how relatively difficult it is to kill these bacteria. “E. coli O157:H7 in ground beef that is only slightly undercooked can result in infection.” As few as 20 organisms may be sufficient to infect a person and, as a result, possibly kill them. And unlike generic E. coli, the O157:H7 serotype multiplies at temperatures up to 44 Fahrenheit, survives freezing and thawing, is heat-resistant, grows at temperatures up to 111 F, resists drying, and can survive exposure to acidic environments. And, finally, to make it even more of a threat, E. coli O157:H7 bacteria are easily transmitted by person-to-person contact.
Since there is no fail-safe food safety program, consumers need to “drive defensively” as they navigate from the market to the table. It is no longer enough to take precautions only with ground beef and hamburgers; anything ingested by family members can be a vehicle for infection. Shiga toxin-producing E. coli are so widely disseminated that a wide variety of foods can be contaminated. Direct animal-to-person and person-to-person transmission is not uncommon.