The Spectrum of Eschericia coli Infections

Abstract

Different classes of E coli have been associated with a number of distinctive diarrheal illnesses. Among these are the enterotoxigenic E coli (ETEC), enteroinvasive E coli (EIEC), and enterohemorrhagic E coli (EHEC). Of the latter, E coli O157:H7 is the prototypic strain. Each class of E coli has distinct somatic (O) and flagellar (H)antigens and specific virulence characteristics.Distinguishing pathogenic E colifrom strains that are part of the normal stool flora is not possible by usual clinical microbiologic procedures. E coli O157:H7 is an exception; it can be screened by using MacConkey agar with sorbitol substituted for lactose. Most E colirapidly ferment sorbitol, but O157:H7 strains do not. The Centers for Disease Control and Prevention(CDC) recommend that clinical microbiology laboratories perform this test, but CDC survey data suggest that as of 1996, only about 50% of laboratories were doing so.Transmission of E coli that cause diarrhea occurs by the fecal-oral route and frequently is mediated by contaminated food or water. E coliO157:H7 can spread rapidly in this fashion and also has a bovine reservoir. As a result, transmission has been reported from undercooked beef, unpasteurized milk, and products contaminated with cow feces(eg, apple cider).ETEC strains have been associated with self-limited gastrointestinal illness characterized by abdominal cramping and watery stools. They can cause illness in individuals of all ages, and in developing countries they are an important cause of diarrhea in infants. These strains are also the major cause of travelers’diarrhea, with infection typically resulting from ingestion of contaminated food or water.EIEC strains resemble Shigellabiochemically and invade intestinal epithelial cells to produce disease. Not surprisingly, the illness they cause is similar to the dysentery caused by Shigella, with diarrhea,fever, crampy abdominal pain, and tenesmus.EHEC produce toxins similar to those found in Shigella dysenterriaetype 1 (called shiga-like or verotoxins). E coli O157:H7 is the best studied of these strains, although a number of other toxin-producing E coli have been described recently. EHEC strains can cause diarrhea,hemorrhagic colitis, and severe abdominal pain. They also are associated with the hemolytic-uremic syndrome (HUS), which can affect patients of all ages, and with post-diarrheal thrombotic thrombocytopenic purpura, which is seen only in adults. In epidemic outbreaks of E coli O157:H7, 8% to 10% of infected individuals subsequently may develop HUS, which is characterized by hemolytic anemia,thrombocytopenia, and acute renal failure.Treatment of E coli-associated diarrhea is primarily supportive,with particular attention paid to the status of hydration and electrolyte balance. Antimotility drugs should not be administered to children who have inflammatory or bloody diarrhea. ETEC diarrhea is usually self-limited, but if it is prolonged,antibiotic therapy may shorten the illness. Although fluoroquinolones, such as ciprofloxacin, are effective, they are not indicated for routine pediatric use. For children,trimethoprim-sulfamethoxazole (TMP-SMX) is considered the drug of choice, as it is for the Shigella-like dysentery caused by EIEC. Any child, most commonly an infant, suspected of having systemic infection with E coli should receive intravenous antibiotic treatment, with the choice of agent guided by the specific sensitivities of the organism isolated from culture. An aminoglycoside or a cephalosporin is appropriate as initial therapy, pending sensitivity data. Antimicrobial therapy does not appear to be beneficial in EHEC-associated infection and may, in fact, enhance the risk for progression to HUS.HUS has become an important cause of renal failure among children in North America,and E coli O157:H7 is responsible for the majority of cases. Infection may occur either sporadically or in outbreaks, the largest of which affected more than 700 people in the western United States. Most commonly, the organism is spread by undercooked ground beef. E colifrom the intestines of apparently healthy cattle can contaminate meat when the animals are slaughtered. During grinding, the beef from many animals is mixed, so even a single infected steer can adulterate large quantities of meat. If cooking is not adequate to kill even organisms living at the center of a hamburger patty, surviving E coli reach the human intestine to release their toxin and cause disease.Of additional concern to pediatricians, E coli O157:H7 spreads quite easily from child to child by the fecal-oral route. Not surprisingly,there has been a report from Minnesota of high secondary attack rates among children in child care settings. Because prevention makes so much more sense than treatment,particularly for an illness as serious as HUS, we need to be alerting families to the importance of being sure that any ground beef their children eat is thoroughly cooked. So much for steak tartare!