Escherichia coli

Background Escherichia coli are a remarkable group of organisms with a wide range of infections, including meningitis, septicaemia, and urinary infections. They are often also nonpathogenic. Those that cause GE also have a wide range of pathogenic mechanisms and are divided into various fairly distinct groups: enteropathogenic (EPEC), enterohe-morrhagic (EHEC), enteroinvasive (EIEC), and enterotoxigenic (ETEC) are the main ones, although some groups—diffusely adherent (DAEC) and enter-oaggregative (EAEC)—have recently been described. Some EHEC strains produce a shiga (or verocyto-) toxin, STEC, which includes E. coli O157:H7 as well as other strains. However, because the O157 strains are much more common, STEC strains are classified as O157 and non-O157. Shiga toxin is produced by other bacteria also, including S. dysen-teriae type 1. Only these verocytotoxin-producing strains are considered in detail here because they are commonly foodborne and can cause serious illness and death.

Escherichia coli GE is not a notifiable infection, so there are few if any data on its impact on communities. Moreover, ETEC is not identified routinely by the stool culture methods commonly used. Infections with ETEC strains are common worldwide at all ages. These strains are the most common known cause of travellers' diarrhea but can also be spread by food. This toxigenic group includes strains that produce heat-labile and heat-stable enterotoxins. Heat-labile enterotoxin is closely related to cholera toxin and causes profuse watery diarrhea. EPEC strains are common infections in neonates and infants, tend to spread from person to person, and are not commonly known to be associated with food. EIEC and the two newer strains are rare. EIEC outbreaks related to food have been occasionally described, including one caused by French cheese exported to the United States.

In the United States, E. coli O157:H7 is estimated to cause 20 000 cases and 250 deaths annually, 67% of outbreaks are foodborne, 8% waterborne, and 22% transmitted case to case. Swimming in contaminated water can also transmit the infection. Escherichia coli O157:H7 was recognized as a cause of FP only in 1982. Some strains with the verocytotoxin (VT) gene produce a toxin that causes the hemolytic-uraemic syndrome (HUS). Other E. coli strains also produce VT.

Escherichia coli, including E. coli O157:H7, is a normal inhabitant of the intestines of many mammals, including cattle, sheep, and goats. Contamination can occur directly from the intestines to carcass to meat or via the faeces of these animals to raw vegetables and other foods.

Growth and survival The organism can survive for a considerable length of time on contaminated meat and vegetables. In two outbreaks caused by apple juice, the organism was present on the surface of apples that had fallen to the ground. The orchard was frequented by deer that were subsequently found to be carrying the organism. Manure was also used. The apples were not processed adequately to kill most organisms, and waxing may have sealed the organisms onto the surface of the fruit. The cider was not pasteurized. The organism is more resistant than salmonella to acid: E. coli O157:H7 has been shown to survive for 21 days in cider at a pH of 3.7-3.9 at 4 °C, with only approximately a 5% kill-off. It can grow very successfully over several weeks in manure slurries. The infectious dose is thought to be small, so case-to-case infection may occur. Like most vegetative organisms, it is destroyed by heat.

Characteristic sequence of events In a town in North Cumbria, England, 61 patients had diarrhea, many with blood, over three weeks. A total of 114 people were found to be infected, ranging in age from 3 months to 85 years. Investigations implicated a farm supplying pasteurized milk. Nine days before the first case, a problem had occurred in the heat-exchanger plates of the pasteurization unit. No tests were undertaken after new plates were fitted, and temperature monitoring was inadequate. The unit was one that a few months before had been the subject of a food hazard warning. Escherichia coli O157 was isolated from 66 environmental and animal feces samples on the farm but not from the milk or the pasteurization plant.

In an outbreak in the United States, 501 patients became ill after eating inadequately processed hamburgers from a restaurant chain. HUS developed in 45 cases, and 3 died.

Undercooked hamburgers and ground beef are a common cause of E. coli O157:H7 infection. The process of grinding beef can spread the organism from the surface of the meat to the inside. Other vehicles of infection include raw milk, unchlorinated water, apple juice, unwashed fruits and vegetables including alfalfa sprouts and radish tops, or swimming in unchlorinated pools.

Clinical features The infectious dose for E. coli O157:H7 is thought to be fewer than 700 organisms. The incubation period of 3-5 days is long compared with that of most other FP bacteria. As with most GI infections with a long incubation, symptoms are mostly associated with the lower GI tract. Vomiting is uncommon, and abdominal pain and diarrhea, often bloody, are the main symptoms. Fever is usual. The illness may last a few days, and the antibiotic ciprofloxacin is now the treatment of choice for severe or prolonged illnesses. HUS is characterized by hemolytic anemia, thrombocytope-nic purpura, renal failure, and a death rate of 3-5%.

Diagnosis The usual method of diagnosis is to isolate the organism from stools or food, which is straightforward. However, because most of the E. coli in the intestine is part of the normal flora and nonpathogenic, it is necessary to demonstrate virulence by further tests or assigning it to a serotype, which normally requires more sophisticated techniques in specialist laboratories. Serotyping is performed on the somatic cell wall antigens (O antigen) and the flagellar antigen (H). On the basis of the serotyping of the O antigens, the organisms can be classified as EPEC, ETEC, etc. DNA tests are increasingly being used. Thus, E. coli O111 is an EPEC strain, O115 with an H antigen is an ETEC strain, O115 without an H antigen is an EIEC strain, and O157:H7 is an EHEC strain. Toxins are now tested for using enzyme-linked immunosor-bent assay or DNA probes. For EIEC strains, the conjunctival sac of a guinea pig is used. Serology tests are also used, but they are not reliable indicators of recent infection.

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