Issue 5, November 2002
Looks can be deceiving: the case of Escherichia coli
Medicine, University of Toronto
coli O157:H7 may seem like a complicated name for a simple organism.
This rod-shaped, flagellated bacterium may appear innocent; however,
it can be toxic and become a serious public health concern.
such concern turned into a serious outbreak in Walkerton, a small
rural town in southwestern Ontario with a population of about 5,000.
In May 2000, many of Walkerton's citizens began to fall ill, experiencing
stomach cramps, diarrhea, fever, nausea, and vomiting.
of aerobic and anaerobic metabolism
at high temperatures
people eventually died and about 2,300 others contracted the ailment,
which turned out to be an E. coli O157:H7 infection.
The municipal water system had inadvertently dispensed a toxic brew
of the bacteria to Walkerton residents.
Without wanting to, Walkerton turned into an example of the importance
of understanding the biology behind E. coli so that scientists
can prevent future outbreaks.
What is E.coli?
coli is a single-celled organism belonging to the large bacterial
family Enterobacteriaceae, the enteric bacteria. Its genus name,
Escherichia, is derived from the name of its discoverer,
German physician Theodor Escherich, who in 1885 first isolated and
characterized the bacterium.
'Enterobacteriaceae' originates from the Greek word enterikos,
meaning "that which pertains to the intestine." Medically, the Enterobacteriaceae
are one of the most important families of bacteria because its members
are the most frequent cause of urinary tract infections and are
a serious wound pathogen. A number of genera within the family include
Salmonella, Shigella and Yersinia, which are human
intestinal pathogens. Several other Escherichia species are
normally found in the human gastrointestinal (GI) tract.
coli is the predominant facultative organism in the human GI tract,
which means that E. coli can grow and metabolize glucose in
both the presence of oxygen (aerobic conditions) and the absence of
oxygen (anaerobic conditions). The bacterium protects the intestinal
tract from potentially harmful bacterial infections, aids in digestion,
and helps in the absorption of many necessary vitamins, like vitamins
B12 and K. Since E. coli is regularly present in
human intestines and feces, the bacterium has been tracked in nature
as an indicator of fecal pollution in soil and water.
E. coli cells are rod-shaped and flagellated.
Right: E. coli colonies grow readily on an agur plate.
(Courtesy of University of Wisconsin-Madison Department
of Bacteriology, 2002,http://www.bact.wisc.edu/Bact330/lectureecoli)
All enteric bacteria can ferment glucose to produce acid and gas.
E. coli, specifically, is physiologically versatile. Under
anaerobic conditions it can grow by fermentation or anaerobic respiration.
This allows E. coli to adapt to both the anaerobic environment
inside the intestine and external aerobic environments.
E. coli is a model organism serving as an experimental tool
for understanding other organisms. As such, it is widely used by microbiologists
and molecular biologists to study prokaryotic life and is therefore
said to be the most thoroughly studied life form on the planet.
Not surprisingly then, the entire sequence of the E. coli genome
has been known since 1997. The E. coli Genome Project at the
University of Wisconsin-Madison has sequenced several strains of the
bacterium: E.coli 042, E.coli CFT073, E.coli
E2348/69, E.coli H10407, E. coli K-12, and E.coli
In genetic engineering, E. coli is the microorganism preferred
for use as a host for the gene-splicing techniques used to clone genes.
Why is E. coli toxic?
are more than 700 serotypes, or antigenic types, of E. coli
based on antigens O, H, and K. Serotyping is an important method
used to distinguish the small number of disease-causing strains
of an organism. Antigens O, H, and K are proteins the body recognizes
as foreign and will evoke an immune response to in order to rid
the body of the foreign organism.
E. coli causes three types of infections in humans: urinary
tract infections, neonatal meningitis, and intestinal diseases,
including gastroenteritis. E. coli is most well known for
the intestinal diseases it causes, including the ones that devastated
Walkerton, Ontario, in 2000.
There are five classes of E. coli capable of causing intestinal
diseases, each of which has distinctive features in pathogenesis.
culprit of the Walkerton outbreak was EHEC.
consists of a single strain, serotype O157:H7. E. coli
O157:H7 was first identified in 1982 as the cause of an outbreak
of severe bloody diarrhea traced to undercooked, contaminated hamburgers.
The combination of letters and numbers in the serotype's name refers
to the distinguishing antigen markers found on the bacterial cell's
surface. E. coli O157:H7 causes a diarrheal syndrome distinguished
from those caused by EAggEC, ETEC, EIEC, and EPEC by the presence
of copious bloody discharge and the absence of fever. E. coli
O157:H7 is considered to be "moderately invasive," meaning it
does not readily spread throughout the body.
coli infections are self-limiting,
and will end when the toxins have all been excreted from the
Unfortunately, not much is known about the way E. coli O157:H7
colonizes the intestine or the nature of the adhesive parts of the
bacterium allowing it to attach to other cells. The bacterium does
not readily invade the mucous-secreting cells that line the GI tract.
Instead, E. coli O157:H7 produces a toxin not easily destroyed
or altered by heat, virtually identical to Shiga toxin. The Shiga
toxins are a family of toxins produced by organisms including Shigella
dysenteriae type I, which is also pathogenic upon ingestion.
In an April 2000 American Family Physician article, Dr. Richard
Sadovsky of the American Academy of Family Physicians said, "these
toxins have a cytotoxic effect on intestinal epithelial cells, creating
an intense inflammatory response that causes the characteristic
bloody diarrhea. Systemic spread of Shiga toxin causes renal endothelial
cell toxicity and may be responsible for hemolytic uremic syndrome
The toxin is a protein that causes severe damage to the outermost
layer of intestinal cells - the epithelial cells - resulting in
blood vessel damage, water and salt loss, and profuse bleeding.
In some cases, E-coli O157:H7 is associated with HUS, resulting
in kidney failure and loss of red blood cells. Some organs, like
the kidney, pancreas, and the brain, appear more susceptible than
others to the damage caused by Shiga toxins, possibly due to the
presence of increased numbers of toxin-receptors on these organs.
What are the symptoms of an E. coli infection?
coli O157:H7 infection is characterized by abdominal pain and
severe cramping. Other symptoms may include watery diarrhea with trace
blood levels not discernable to the eye, nausea, and vomiting, accompanied
by what many might consider an unusual lack of fever. Sometimes the
infection may simply cause non-bloody diarrhea and no other symptoms.
The time from ingestion of the pathogen to the presentation of symptoms
is usually three to nine days. When treating an E. coli infection,
it is important to note that antibiotics do not improve the course
of the infection, but may instead precipitate kidney complications;
patients should also avoid over the counter antidiarrheal agents like
loperamide, because they prevent the toxins from being excreted.
sources of E. coli infection
sources (e.g., undercooked ground beef, unpasteurized
apple cider and milk, sandwich meats, raw vegetables,
cheese, and contaminated water).
by hand to mouth contact from someone who has consumed
contaminated food or water.
or inadequate hand-washing and improper food handling.
coli cannot survive when exposed to air,
or on surfaces like tables or counters. It cannot
be spread by normal interactions with friends and
neighbors (e.g. coughing, kissing, touching).
The Ontario Ministry of Health
an E. coli infection
ground beef thoroughly to an internal temperature
pasteurized apple cider and milk.
all fruits and vegetables before eating.
hands thoroughly with soap and water after using the
toilet, handling, pets, livestock, or before preparing
and sanitize counter tops and utensils after contact
with raw meats and poultry.
raw and cooked foods on separate work surfaces using
cold foods at 4°C/40F or lower. Keep hot foods
at a temperature of 70°C/158F or higher.
water from a supply intended for human consumption
and not from open streams or lakes.
E. coli contamination is suspected, water should
be brought to a rapid rolling boil and boiled for
The Ontario Ministry of Health
What are the treatments and outcomes of an E.coli
good supportive care, including hydration and nutrition, is needed,
most patients recover on their own. E. coli infections are
self-limiting, and will end when the toxins have all been excreted
from the body. Patients usually recover completely within five to
10 days and do not suffer any long-term effects.
However, the Centers for Disease Control and Prevention (CDC) report
approximately 2-7% of E. coli infections will lead to HUS.
Children less than five years of age and the elderly are the most
susceptible to this life-threatening condition. Patients suffering
from HUS experience kidney failure and often require dialysis and
transfusions. The CDC reports E. coli-associated HUS infections
lead to abnormal kidney function requiring long-term dialysis in about
one-third of patients; some patients will experience neurological
impairment, including seizures or strokes; others will undergo surgery
to have sections of their bowels removed; and about 8% will develop
other long-term complications, including high blood pressure, blindness,
and paralysis. About 3-5% of HUS patients die from the disease, which
occurs in about 2-7% of patients with E. coli infections.
How is the infection diagnosed?
coli O157:H7 infections are diagnosed by testing for the bacterium
in the stool. The stool specimen is grown on sorbitol-MacConkey
agar, which selects for EHEC. The test is not routine; however,
anyone experiencing a sudden onset of bloody diarrhea should request
it. EHEC can also be isolated from food, soil, and water samples.
The case of Walkerton, Ontario: How did E. coli get into
the municipal drinking water?
contributing factors led to the Walkerton tragedy: unsafe wells,
mismanagement at the public utilities commission, and political
Walkerton's municipal water supply is obtained from a number of
wells in the town's outlying area. Ideally, well water should originate
from rainfall that infiltrates the ground. This water should filter
through an aquifer, defined as a "geologic formation of permeable
layers of underground rock or sand that hold or transmit groundwater
below the water table that will yield water to a well in sufficient
quantities." The water should filter through the sediment layers
over a number of years. The Walkerton aquifer, made up of only 2.4
meters of sand, gravel, and rock, was determined to not be thick
enough to filter E.coli from the well. The source of the
unfiltered E. coli that led to seven deaths in Walkerton
in May 2000 is believed to have been cow manure spread across a
nearby cattle farm. Days of heavy rain preceded the Walkerton E.
coli outbreak and runoff from the cattle farm naturally flows
over the land surrounding the well. The well was shallow, not sealed
around the shaft, and situated downhill from the cattle farm. Coupled
with an aquifer not thick enough to filter the E. coli present
in the cow manure, Walkerton's drinking water became a lethal poison.
Interestingly, Walkerton's water had been contaminated with E.
coli in the past. Fecal bacteria had been detected in the well
as early as 1978, and between 1995 and 1998, E. coli was
found in the well water an average of five to seven times each year.
In 2000, however, the well still remained one of the town's primary
Mismanagement at the public utilities commission
The public utilities commission (PUC) is responsible for the purification
of the town's water supply. This includes controlling for contaminants
like bacteria, chemicals, and particles. The Walkerton PUC used
chlorination to purify the water; however, the PUC staff admitted
to using less disinfectant than recommended by the province and
falsifying chlorine residue levels, logs, lab submission reports,
and annual reports to the ministry. The town and the health officials
were not notified about these problems.
Political expenditure cutbacks
mid 1990s, a trend of budget cuts to get the debt under control was
common throughout the Western hemisphere. Ontario was no different.
The budget cuts and unmonitored privatization occurring prior to May
2000 were blamed in part for the Walkerton tragedy. When the Progressive
Conservatives, the party leading the province when the Walkerton tragedy
occurred, took office in 1995, $100 million was cut from the Ministry
of Environment's budget and more than 900 workers in the field of
enforcement and inspection were laid off.
The PUC staff admitted to using less disinfectant than recommended
by the province and falsifying chlorine residue levels, logs,
lab submission reports, and annual reports to the ministry.
This represented approximately 40% of the ministry's budget and one
third of the staff, and the cutback had an effect on services. For
example, under the previous government's control, from 1993-1994,
75% of water treatment plants were inspected; under the new government,
only 24% were inspected from 1998 to 1999 and only 29% were inspected
from 1999 to 2000. Walkerton's water system was inspected only once
between 1996 to May 2000.
In 1998, a Ministry of Environment inspector found deficiencies in
chlorination and wrote to the Walkerton PUC and the county public
health unit to make specific improvements to the municipal water system;
however, no further inspection occurred to verify the repairs.
the news of the Walkerton outbreak, an independent commission, the
Walkerton Inquiry, was set up under the Ontario Public Inquiries Act
to examine the contamination of the water supply and to make recommendations
to ensure the safety of drinking water. The two-part report, released
in January and May of 2002, states that proper chlorination of drinking
water could have prevented this tragedy. The PUC and the Ontario government
cutbacks also were named as contributors to this event. The report
includes hundreds of findings and recommendations, and can be viewed
at the official Walkerton
Inquiry Web site.
Walkerton is now cited as an example of how any town in North America
could be struck by E.coli infection. No one would have expected
Canada, a prosperous country well endowed with fresh water bodies,
to have an E. coli outbreak in the municipal water system.
Although further research needs to be done to better understand E.
coli infection, preventative measures are also a step in the right
Alberts et al. Essential Cell Biology: An Introduction to the Molecular Biology
of the Cell. Garland Publishing, Inc. New York: 1998.
Brown, John C. What the Heck is an E. coli?. Sept. 16, 1997. University
Oct. 11, 2002.
Canadian Environment Defense Fund. Justice at Walkerton. 2001. 1 June.
Oct. 11, 2002
Centers for Disease Control and Prevention. 20 June 2001 Escherichia
coli O157:H7. http://www.cdc.gov/ncidod/dbmd/diseaseinfo/escherichiacoli_g.htm
Oct. 11, 2002
Eckhardt, Gregg A. The Edwards Aquifer Homepage. 1995 - 2002 http://www.edwardsaquifer.net/glossary.html#artaquifer
Oct. 11, 2002.
E-coli Genome Project. University of Wisconsin-Madison. 2002. http://www.genome.wisc.edu/
Oct. 11, 2002.
Ontario Ministry of Health. E. Coli Bacteria. Nov.23, 2001. http://www.gov.on.ca/health/english/pub/disease/ecoli.html
O'Conner, Dennis. The Walkerton Inquiry. 2001. http://www.walkertoninquiry.com/
Sadovsky, Richard. Foodborne Disease and Shiga Toxin-Producing E.
coli. April 1, 2000. American Family Physician. http://www.aafp.org/afp/20000401/tips/11.html
Todar, Kenneth. "Bacteriology 330 Lecture Topics: Pathogenic E. coli".
1997. University of Wisconsin. http://www.bact.wisc.edu/Bact330/lectureecoli
Oct 11, 2002
Journal of Young
Investigators. 2002. Volume Six.
Copyright © 2002 by Amanda Hu and JYI. All rights reserved.