E.coli O157:H7 infection

Escherichia coli , commonly shortened to E. coli, is a Gramnegative bacterium that lives in the intestinal tract of humans and other warm-blooded animals. There are many sub-types, or strains of the organism. One strain is designated as O157:H7, based on two antigens that are present on the surface of the bacterium and of the locomotive appendage called the flagella.

In contrast to many of the other strains, E. coli O157:H7 is not a normal resident of the humans intestinal tract. When present in the intestinal tract, via the ingestion of contaminated food or water, O157:H7 causes a severe, even life-threatening malady known as hemorrhagic colitis.

E. coli O157:H7 is a strain of enterohemorrhagic E. coli that was initially isolated in Argentina in 1977. The strain is thought to have arisen from a genetic recombination between another E. coli strain and a toxin-producing strain of Shigella dysenterae in the intestinal tract of someone. The resulting genetically altered E. coli now carried the genetic information for the toxins.

Strain O157:H7 was recognized as a cause of illness in 1982. Then, an outbreak of severe diarrhea was microbiologically traced to a batch of undercooked hamburgers. Most cases are stilled associated with improperly cooked contaminated meat. For this reason, the infection has acquired the cache of "hamburger disease." However, numerous other foods can deliver the bacteria , including alfalfa sprouts, unpasteurized fruits juices such as apple juice, lettuce and cheese curds, and raw milk. Contamination of vegetables can occur when they are sprayed in the filed with sewage-containing water and then inadequately washed prior to eating. For example, organically grown produce might not be adequately washed, given the perception that the absence of antibiotics negates the need for washing.

Meat can become contaminated with fecal material during slaughter. The bacteria are subsequently distributed throughout the meat when the meat is ground. Thorough cooking is necessary to kill the bacteria buried in the ground meat. Although not clear yet, indications are that the ingestion of as few as 10 surviving bacteria can be sufficient to trigger the infection.

E. coli O157:H7 is also passed onto humans via water that has been contaminated with fecal material, typically from cattle who are a reservoir of the bacterium. For example, the contamination of the water supply of Walkerton, Ontario, Canada, by run-off from a neighbouring cattle farm in the summer of 2000 caused thousands of illnesses and killed seven people.

The damage of the infection results from two potent toxins produced by the bacteria. The toxins are known as verotoxin and shiga-like toxin. These toxins are very similar in structure and action as those produced by another bacteria of health concern, Shigella dysenteriae, the agent of bacterial dysentery . The toxins exert their effect by both physically damaging the host epithelial cell and by preventing repair of the damage, because of the shutdown of the host cell's ability to manufacture new protein. The toxins bind to a specific receptor called Gb3, which is found on the surface of epithelial cells in blood vessels, smooth muscle cells, kidney cells, and red blood cells. The bound toxins inhibit protein synthesis , thus killing the cells.

The toxic damage occurs following the tight association of the bacteria with the surface of the intestinal epithelial cells. Research has proven that this association relies on the manufacture and extrusion of a specific protein by the bacteria that acts as an anchor to which the bacteria bind. As binding occurs, the host cells change their configuration, becoming socalled pedestals on which each bacterium sits. At this point the binding of the bacteria with the host cells is tenacious and the infection is established.

Hemorrhagic colitis begins as a severe abdominal pain accompanied by watery diarrhea. As damage to the epithelial cells lining the intestinal tract occurs, the diarrhea becomes bloody. Vomiting can also occur. These symptoms, as severe and debilitating as they are, usually last only between one and two weeks, and terminate naturally as the body's immune defenses successfully cope with the infection. Usually no permanent damage results from the infection. However in those who are immunocompromised and in children, the disease can become more disseminated. Damage to the kidney can be so devastating that complete loss of kidney function occurs. If not treated the death rate from hemolytic anemia is high. Even with rapid diagnoses and treatment that includes antibiotics, blood transfusions and kidney dialysis, the death rate is still three to five percent.

Approximately ten to fifteen per cent of those infected with strain O157:H7 develop hemolytic anemia. The syndrome is the leading cause of sudden-onset kidney failure in children in the world. As well, the elderly can develop a condition known as thrombocytopenic purpura, which consists of a fever and nerve damage. In the elderly, this malady can kill almost half of those who become infected.

The chances of infection from E. coli O157:H7 are greatly lessened by proper food preparation, washing of food surfaces that have been exposed to raw ground meat, and proper personal hygiene , especially hand washing. Also, since the bacterium is very sensitive to heat, boiling suspect water prior to drinking the water is a sure way to eliminate the risk of infection from the bacteria and to inactivate the toxins.

Also, a vaccine for cattle is in the final testing stages prior to being approved for sale. Approval is expected in 2002. The vaccine blocks the binding and pedestal formation by the bacteria in cattle. The bacteria remain free in the intestinal tract and so are washed out of the cow. Eliminating the reservoir of the organism lessens the spread of O157:H7 infection to humans.

See also Anti-adhesion mechanisms; Escherichia coli ; Food safety; Vaccination