E. coli O157:H7
E. coli O157:H7 is a foodborne pathogen that causes food poisoning.
E. coli O157:H7 is the most commonly identified and the most notorious Shiga toxin-producing E. coli (STEC) serotype in the United States. This serotype is a common cause of bloody diarrhea and the most common cause of hemolytic uremic syndrome (HUS), a severe complication that involves kidney failure and is fatal in approximately 5-6% of cases. E. coli O157:H7 causes an estimated 63,153 illnesses, 2,138 hospitalizations, and 20 deaths in the United States each year due to foodborne transmission alone. It is estimated that foodborne E. coli O157:H7 infections cost $255 million annually in this country. However, foodborne transmission is thought to account for only 68% of all E. coli O157:H7 infections; therefore, estimated case numbers and associated costs should be increased by 32% to account for other sources (e.g., water, animal contact, or person-to-person contact).
E. coli O157:H7 was first recognized as a cause of disease in 1982 during investigations of two outbreaks of hemorrhagic colitis, one in Oregon and one in Michigan, associated with consumption of hamburgers from the same popular national fast food restaurant chain. This was a truly new strain of E. coli that had evolved recently, as examination of more than 3,000 E. coli cultures obtained between 1973 and 1982 found that only one (from 1975) was serotype O157:H7. Subsequent evolutionary studies indicated that E. coli O157:H7 evolved from enteropathogenic E. coli serotype O55:H7, a cause of non-bloody diarrhea, through the acquisition of phage-encoded Shiga toxin 2 and other virulence factors.
In the 10 years that followed the 1982 outbreaks, there were approximately 30 additional E. coli O157:H7 outbreaks recorded in the United States. These outbreaks garnered modest public health attention, but E. coli O157:H7 was still largely unknown to the public.
That all changed in 1993, when an enormous, severe E. coli O157:H7 outbreak associated with consumption of hamburgers served by the Jack in the Box restaurant chain was identified in the Pacific Northwest. There were 501 cases of illness, 151 hospitalizations, 45 cases of HUS, and 3 deaths in Washington state alone, with additional cases in California, Nevada, and Idaho. The investigation revealed that restaurant outlets were serving contaminated beef and were not cooking the hamburgers thoroughly. The enormity of this outbreak, which included the deaths of young children, triggered one of the largest food safety regulatory, public health, and industry responses in history, the effects of which are ongoing today.
Beginning in 1993, E. coli O157:H7 outbreaks began to be recognized with increasing frequency, and to this day, E. coli O157:H7 remains one of the most important foodborne pathogens in the United States. Beef is the most common vehicle of foodborne E. coli O157:H7 outbreaks, accounting for almost half of outbreaks. Most beef outbreaks are associated with ground beef, but several outbreaks due to mechanically tenderized (a.k.a. needle- or blade-tenderized) steaks and a variety of other beef products have also occurred. Leafy greens (e.g., romaine lettuce, iceberg lettuce, spinach) are the second most common cause of E. coli O157:H7 outbreaks; this has been an important and difficult food safety problem for more than two decades. The problem has only grown in recent years, with multiple large, widely publicized outbreaks. Raw dairy products (unpasteurized milk, and to a lesser extent raw milk cheeses, butter, and ice cream) are also a relatively frequent cause of E. coli O157:H7 outbreaks. Beef, leafy greens, and raw dairy products combine to account for almost 90% of foodborne E. coli O157:H7 outbreaks. Over 70% of all E. coli O157 illnesses are thought to be acquired from beef and vegetable row crops (primarily leafy greens). Sprouts (alfalfa, clover, radish) and unpasteurized apple juice/cider are also frequent causes of E. coli O157:H7 outbreaks. Outbreaks have also been linked to a wide variety of other food items, including game meats (e.g., venison), lamb, bison, dried salami, strawberries, cucumbers, hazelnuts, raw cookie dough, and raw flour (an increasingly recognized vehicle in recent years).
Foodborne transmission accounts for about two-thirds of all E. coli O157:H7 outbreaks and illnesses. Importantly, the other third are caused by contact with animals (primarily cattle, goats, and sheep) or their environments, person-to-person transmission (primarily among children in daycare settings), and drinking water or recreational water (e.g., swimming in freshwater lakes or ponds, or in pools with inadequate chlorination). E. coli O157:H7 infections occur year-round, but peak markedly during the summer and early fall.
How does E. coli O157:H7 cause disease?
E. coli O157:H7 has a very low infectious dose. In other words, ingestion of a small number of bacterial cells, less than 100, can be enough to cause infection and disease. E. coli O157:H7’s ability to cause disease in humans is a result of its ability to produce numerous virulence factors, most notably Shiga toxin (Stx). The primary target of Stx is endothelial cells, which line the interior surface of blood vessels.
In addition to Stx, E. coli O157:H7 produces numerous other virulence factors, including proteins which aid in the attachment to the intestinal wall and colonization of the bacteria in the intestine, and others that can destroy red blood cells.
After ingestion, E. coli O157:H7 bacteria rapidly multiply in the large intestine and attach to cells in the intestinal lining. This attachment facilitates transfer of Stx into the small blood vessels within the intestinal wall. In the process, O157 bacteria cause an “attaching and effacing lesion,” resulting in erosions and ulcerations in the colon surface. This causes bloody diarrhea, and occasionally very severe local effects such as perforation of the colon. Also, Stx gains access to the blood stream and is carried to the kidneys and elsewhere, and this process is what can lead to HUS, a severe complication that involves kidney failure.