Escherichia Coli

Forensic investigations of a food- or water-related outbreak of disease will often focus on select bacteria. One bacterium often associated with contaminated food is Escherichia coli.

Escherichia coli (E. coli) is one of the most well-known and intensively studied bacteria. It normally inhabits the intestinal tract of humans and other warm-blooded mammals. E. coli constitutes approximately 0.1% of the total bacteria in the adult intestinal tract.

Despite its intestinal habitat, some types (strains) of E. coli cause diarrhea and gastroenteritis (an inflammation of the intestinal tract) in infants. If these more infectious types are present in water or food that is ingested, then an infection can result. The vast majority of the many types of E. coli are harmless to humans.

When E. coli is excreted from the intestinal tract, the bacteria are able to survive only a few hours. This characteristic of rapid death was recognized at the beginning of the twentieth century, when the bacterium began to be used as an indicator of fecal pollution of water. The presence of large numbers of E. coli in water is a strong indicator of recent fecal pollution, and so the possible presence of other intestinal bacteria that cause serious disease (i.e., Vibrio, Salmonella, Shigella). Even today, E. coli remains one of the important tests of water quality.

In 1975, the United States Centers for Disease Control and Prevention identified a new strain of E. coli that was designated O157:H7. Strain O157:H7 was first linked to human disease in 1983, when it was shown to have caused two outbreaks of a severe gastrointestinal illness in the Unites States. This strain is capable of causing severe, even lethal, infection. Those who recover sometimes have permanent kidney damage.

The origin of O157:H7 is not known for certain. The consensus among researchers, however, is that O157:H7 arose when a strain of E. coli living in the intestine and which was not disease causing became infected by a virus. The virus carried the gene coding for a powerful toxin called Shiga-like toxin. Thus, the E. coli acquired the ability to produce the toxin.

The toxin can destroy the cells that line the intestinal tract and can enter the bloodstream and migrate to the kidneys and liver. Severe damage to these organs can occur. The intestinal damage causes severe bleeding, which can be lethal in children and elderly people. During the summer of 2000, E. coli O157:H7 contaminated the drinking water of the town of Walkerton, Ontario, Canada. Over 2,000 people became ill and seven people died. The source of the strain was the intestinal tract of cattle, a known natural habitat of O157:H7.