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E.E.coliO157:H7 infectionCOLI O157:H7 INFECTION
Escherichia coli, commonly shortened to E. coli, is a Gram-
negative 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 sur-
face of the bacterium and of the locomotive appendage called
the flagella.
In contrast to many of the other strains, E. coliO157: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. coliO157: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 recombinationbetween
another E. colistrain and a toxin-producing strain of Shigella
dysenteraein the intestinal tract of someone. The resulting
genetically altered E. colinow carried the genetic information
for the toxins.
Strain O157:H7 was recognized as a cause of illness in- Then, an outbreak of severe diarrhea was microbiolog-
ically traced to a batch of undercooked hamburgers. Most
cases are stilled associated with improperly cooked contami-
nated 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. Contaminationof vegetables can
occur when they are sprayed in the filed with sewage-con-
taining water and then inadequately washed prior to eating.
For example, organically grown produce might not be ade-
quately washed, given the perception that the absence of
antibioticsnegates the need for washing.
Meat can become contaminated with fecal material dur-
ing slaughter. The bacteria are subsequently distributed
throughout the meat when the meat is ground. Thorough cook-
ing 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. coliO157: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 vero-
toxin 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 epithe-
lial cells in blood vessels, smooth muscle cells, kidney cells,
and red blood cells. The bound toxins inhibit protein synthe-
sis, 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 manu-
facture 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 so-
called 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 andwomi_E 5/6/03 2:11 PM Page 171