Escherichia coli (E. coli) is a bacterium that is commonly found in the gut of humans and warm-blooded animals. Most E. coli are normal flora or commensals found in the intestinal tract. It is a gram-negative short rod, a member of the family Enterobacteriaceae.
Pathogenic strains of E. coli are distinguished from normal flora by their possession of virulence factors such as exotoxins. Strains of E. coli recognized as causing diarrhea disease include;
- Enterotoxigenic E. coli (ETEC),
- Enteropathogenic E. coli (EPEC),
- Enteroinvasive E. coli (EIEC) and
- Verotoxigenic E. coli (VTEC), also referred to as Enterohaemorrhagic E. coli (EHEC).
EHEC are verotoxigenic E. coli (as they produce one or both verocytoxins vt1 and vt2) or Shiga like toxins.
Carriers/Reservoirs of infections:
The reservoirs for EHEC O157:H7 are ruminants, particularly cattle and sheep, which are infected asymptomatically and shed the organism in feces. Other animals such as rabbits and pigs can also carry this organism.
Mode of transmission: Humans acquire EHEC O157:H7 by direct contact with animal carriers, their feces, and contaminated soil or water, or via the ingestion of contaminated food (such as raw or undercooked meat products, unpasteurized milk & dairy products) and contaminated raw vegetables).
Infectious dose: Infectious dose is very low, which increases the risk of disease.
EHEC can cause severe foodborne disease; diarrhea or hemorrhagic colitis (bloody diarrhea without pus cells) in humans. Hemorrhagic colitis occasionally progresses to hemolytic uremic syndrome (HUS), an important cause of acute renal failure in children and morbidity and mortality in adults.
When infection with EHEC O157:H7 is suspected (haemorrhagic colitis), a presumptive diagnosis can be made by isolating sorbitol non-fermenting E. coli on sorbitol MacConkey agar. The colonies can be identified as E. coli O157 by testing with specific O157 antiserum, available as a latex agglutination test.