Generic E. coli is found in the intestinal track of both man and animals. Thus, the finding of generic E. coli in ready-to-eat foods is an indication of fecal contamination. Fecal contamination in turn indicates that other harmful organisms, whether they be bacterial (Salmonella, Shigella, Campylobacter), viral (hepatitis A, norovirus, rotovirus), helminthic or protozoal parasites (Taenia, Toxoplasma, Cryptosporidium, Giardia), could be present.
In addition, the test for generic E. coli does not rule out and actually includes a few types of E. coli that are dangerous. These pathogenic strains of E. coli have the ability to cause anything from simple self-limiting diarrhea to systemic disease resulting in multi-organ failure and death. It is for these reasons that the confirmation of generic E. coli in ready-to-eat food is followed by an automatic recommendation for a thorough review of the constituent ingredients, as well as finished product re-testing and task-oriented training of those individuals involved in the preparation of those specific ready-to-eat food items (i.e. deli foods or protein salads).
The following is a description of the specific types of pathogenic E. coli, most of which would be included in the test for generic E. coli.
Escherichi coli, commonly referred to as E. coli, is a member of the normal microflora of the intestinal tract of warm-blooded animals including humans. These bacteria have a symbiotic relationship with the host and serve a useful function in the body by suppressing the growth of harmful bacterial species and by synthesizing appreciable amounts of vitamins.
However, several strains have developed specific virulent traits that enable them to cause a variety of diseases, ranging from diarrhea to hemolytic uremic syndrome and death. The four major groups of pathogenic E. coli are enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enterotoxigenic E. coli (ETEC), and enterohemorrhagic E. coli (EHEC). EHEC is also known as a Shiga-toxin producing E. coli while the other three are considered by many to be generic or non-Shiga toxin producing E. coli.
Enteropathogenic E. coli is the oldest recognized category of pathogenic E. coli. Pathogenesis of EPEC involves localized adherence of the bacteria to intestinal cells. This adherence causes an inflammatory response and a physical alteration of the integrity of the intestine. Symptoms of EPEC infection include profuse watery, and sometimes bloody, diarrhea. The only known reservoir for this organism is humans.
EPEC are highly infectious for infants and are a leading cause of infantile diarrhea in developing countries. In the sporadic documented cases of adult disease the infectious dose is presumably much higher, at approximately 106 organisms. Outbreaks have been linked to the consumption of contaminated drinking water and some meat products.
The pathogenic mechanism and disease produced by enteroinvasive E. coli closely resembles that of Shigella. EIEC penetrate and multiply within intestinal cells causing widespread cell destruction and illness. Symptoms include severe abdominal cramps, malaise, watery stools, tenesmus, and fever. Less than 10% of cases develop further illness including the passage of multiple fluid stools containing blood and mucus (dysentery).
The only known reservoir for this organism is humans. Evidence suggests that illness is transmitted by consumption of contaminated food. EIEC infections are endemic in developing countries. The infectious dose of EIEC is thought to be as few as 10 organisms, similar to Shigella, but studies have shown that at least 106 organisms are required to cause illness in healthy adults.
Enterotoxigenic E. coli is an important cause of traveler's diarrhea in people from industrialized countries who visit developing countries. It is also an important cause of dehydrating diarrhea in infants and children in developing countries. ETEC are capable of producing a heat-labile enterotoxin (LT - destroyed or altered by heat), a heat-stabile enterotoxin (ST - heat resistant), or both (LT/ST). These organisms invade the small intestine where they proliferate and produce these toxins which induce fluid secretion.
Profuse watery diarrhea without blood or mucus, similar to that of a Vibrio cholera infection, usually occurs. Additional symptoms include abdominal cramps, low-grade fever, nausea, vomiting and malaise. Humans are the major reservoir for ETEC and infection occurs through consumption of contaminated food and water. The infectious dose of ETEC for healthy adults has been estimated to be at least 108 cells. The young, the elderly, and others that are immunocompromised may be susceptible to lower numbers.
Enterohemorrhagic E. coli is a rare variety of E. coli that is considered moderately invasive and produces large quantities of one or more related, potent toxins. These toxins are called verotoxins and Shiga toxins due to their similarity to the toxins produced by Shigella dysenteriae. They cause severe damage to the lining of the intestine making EHEC the primary cause of hemorrhagic colitis.
The diarrhea associated with infection may range from mild and non-bloody to stools that are virtually all blood. Fever is either low-grade or absent and occasional vomiting may occur. Approximately 8% of cases progress to hemolytic uremic syndrome (HUS), which is the most common cause of acute renal failure in children. E. coli O157:H7 is the most well known serotype in the EHEC group; however, there are several other serotypes of importance including O26, O111, O103, O45, and O121.
Cattle are the major reservoir for EHEC, but humans may also serve as a reservoir for person-to-person transmission. Infection usually occurs through ingestion of food contaminated with ruminant feces. Outbreaks has been linked to beef (usually inadequately cooked ground beef), produce (including melons, lettuce, and alfalfa sprouts), and unpasteurized dairy products (milk). Waterborne transmission has also been documented. The infectious dose is very low, probably similar to that of Shigella.