Looks can be deceiving: the case of Escherichia coli

Author: Hu Amanda
Institution: Biology and Economics
Date: September 2005

Escherichia 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.

One 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.

Seven 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?

E. 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.

Left: 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

E. 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.

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 O157:H7.

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?

There 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.

*enterotoxigenic (ETEC),

*enteroinvasive (EIEC),

*enterohemorrhagic (EHEC),

*enteropathogenic (EPEC),

*and enteroaggregative (EAggEC)

The culprit of the Walkerton outbreak was EHEC. 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.

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 (HUS)."

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?

An E. 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.

What are the treatments and outcomes of an E.coli infection?

Although 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?

E. 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?

Three contributing factors led to the Walkerton tragedy: unsafe wells, mismanagement at the public utilities commission, and political expenditure.

Unsafe Wells

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 water sources.

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

In the 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.

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.

Conclusion

After 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 direction.