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E. coli (Escherichia coli) is a common bacterial inhabitant of the intestines of all animals, including humans. It is, in fact, the dominant species of bacteria found in feces, and is known to have hundreds of strains virtually all of which are harmless, or, in some cases, helpful. Normally, E. coli serves a useful function in the body by suppressing the growth of harmful bacterial species and synthesizing appreciable amounts of vitamins.
E. coli serotype O157:H7 is a rare variety of E. coli that produces toxins which are capable of inflicting damage to the lining of the intestine. These toxins are closely related or identical to the toxin produced by Shigella dysenteriae and are referred to as Shiga toxins. In very rare instances, the infection can progress to hemolytic uremic syndrome ("HUS") and kidney failure. E. coli O157:H7 can survive at low temperatures as well as under acidic conditions, and the infectious dose is relatively small.
Undercooked or raw hamburger (ground beef) has been implicated in many outbreaks of E. coli O157:H7 but, as previously mentioned, raw or undercooked beef is only one of many potential sources. Historically, outbreaks have implicated water, spinach, alfalfa sprouts, unpasteurized fruit juices, dry-cured salami, lettuce, game meat, and cheese curds. E. coli O157:H7 has also been found in sheep, pigs, goats, poultry, and deer. Wild and domestic animals that live around cattle farms may also harbor this species. Some other risk factors include drinking or swimming in contaminated water, handling animal feces and eating fruit and vegetables fertilized or irrigated with animal manure. Poor personal hygiene (e.g., not washing hands properly) and unsafe food preparation practices also increase the risk of contracting or spreading E. coli.
Infection from E. coli O157:H7 is characterized by mild to severe cramping (abdominal pain) and diarrhea which is initially watery but can become bloody. Some individuals exhibit watery diarrhea only. Occasionally, vomiting occurs. Fever is typically either low-grade or absent. The illness is usually self-limited, and lasts for only a few days. As noted, in very rare cases, illness may be complicated by HUS or thrombotic thrombocytopenic purpura (TTP). Some individuals, the very young in particular, are more suseptible to serious complications. On the opposite end of the spectrum, some infected individuals remain asymptomatic.
Symptoms usually manifest within 2 to 4 days. Most people recover without specific treatment within days after onset. There is no evidence that antibiotics improve the course of disease. In fact, it is believed that treatment with some antibiotics may precipitate kidney complications. Anti-diarrheal agents, such as loperamide (Imodium), should be avoided.
There are several microbiological methods that can be used to isolate E. coli O157:H7. Unlike typical E. coli, O157:H7 isolates do not ferment sorbitol and are negative with the MUG assay; therefore, these criteria are commonly used for selective isolation. Sorbitol-MacConkey agar has been used extensively to isolate this organism from clinical specimens. Hemorrhagic colitis agar, a selective and differential medium, is used in a direct plating method to isolate O157:H7 from foods. A third procedure uses a Sorbitol-MacConkey medium containing potassium tellurite and Cefixime. Faster testing methods using a variety of technologies, including recombinant DNA methods, are being developed.
As is the case with all foodborne pathogens, safe handling and preventive measures can eliminate the risk of becoming ill from E. coli O157:H7. Some key preventative measures include:
In the home:
- Cook ground beef to at least 165 degrees Fahrenheit or until the meat is brown and the juices run clear;
- Drink only milk products and fruit juices that have been pasteurized;
- Wash all fruit and vegetables adequately and carefully;
- Wash hands with soap after using the restroom;
- Take special care when handling and disposing of diapers;
- Wash hands before food preparation;
- Wash hands immediately after handling raw meat;
- Take care to clean all kitchen utensils and surfaces after handling raw meat to avoid cross-contamination; and
- Make sure all drinking water has been properly treated.
On the farm:
- Use potable quality water for washing fruit and vegetables;
- Manage fecal waste so as not to contaminate water supplies;
- Avoid spreading fecal material via clothes and shoes and tools; and
- Wash hands with soap after petting or handling animals.
Schneider, Keith and Renee M. Goodrich, and Melissa A. Kirby. “Preventing Foodborne Illness: E. coli 0157: H7”. Food Science and Human Nutrition Department, Florida Cooperative Extension Service, IFAS, University of Florida. January, 2003.
Mead, P. S. and P. M. Griffin, 1998: Escherichia coli O157:H7. Lancet 352, 1207-1212.