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Escherichia coli O157:H7

General characteristics

E. coli are Gram-negative, rod-shaped bacteria belonging the family Enterobacteriaceae

E. coli is a normal inhabitant of the intestines of all animals, including humans. When aerobic culture methods are used, E. coli is the dominant species found in faeces. Normally E. coli serves a useful function in the body by suppressing the growth of harmful bacterial species and by synthesizing appreciable amounts of vitamins. A minority of E. coli strains are capable of causing human illness by several different mechanisms. E. coli serotype O157:H7 is a rare variety of E. coli that produces large quantities of one or more related, potent toxins that cause severe damage to the lining of the intestine. These toxins [verotoxin (VT), shiga-like toxin] are closely related or identical to the toxin produced by Shigella dysenteriae .

Disease symptoms

Hemorrhagic colitis is the name of the acute disease caused by E. coli O157:H7.

The illness is characterized by severe cramping (abdominal pain) and diarrhoea which is initially watery but becomes grossly bloody. Occasionally vomiting occurs. Fever is either low-grade or absent. The illness is usually self-limited and lasts for an average of 8 days. Some individuals exhibit watery diarrhoea only.

Infective dose -- Unknown, but from a compilation of outbreak data, including the organism's ability to be passed person-to-person in the day-care setting and nursing homes, the dose may be similar to that of Shigella spp. (as few as 10 organisms).


Hemorrhagic colitis is diagnosed by isolation of E. coli of serotype O157:H7 or other verotoxin-producing E. coli from diarrhoeal stools. Alternatively, the stools can be tested directly for the presence of verotoxin. Confirmation can be obtained by isolation of E. coli of the same serotype from the incriminated food.

Associated foods

Undercooked or raw hamburger (ground beef) has been implicated in many of the documented outbreaks, however E. coli O157:H7 outbreaks have implicated alfalfa sprouts, non-pateurised fruit juices, dry-cured salami, lettuce, game meat, and cheese curds. Raw milk was the vehicle in a school outbreak in Canada.


Enterobacteria (incl. E.coli ) are heat-sensitive and will be killed by thorough heating (over 70 °C). Raw or undercooked foods and cross-contamination, when cooked material comes into contact with raw produce or contaminated materials (cutting boards), are the main causes of infection. Proper

Risk populations

All people are believed to be susceptible to hemorrhagic colitis, but young children and the elderly appear to progress to more serious symptoms more frequently.


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