E coli O157:H7 is a serotype of E. coli bacteria that form the predominant members of one group of EEC. This EEC group is termed enterohemorrhagic E. coli or EHEC. Unfortunately, other terms in the medical literature describe this group (VTEC or Vero toxin-producing E. coli and STEC or Shiga toxin-producing E. coli). Research suggests that only a small number of E. coli 0157:H7 are needed to cause infection (ingestion of about 10–100 organisms) instead of the thousands to millions needed for infections by other E. coli serotypes. Infection is aided by adhesive receptors (pili or fimbriae) that attach the bacteria to human intestinal cells. The most problems caused by the bacteria are due to two Shiga toxins, termed Stx 1 and Stx 2 and also termed Vero toxins. (Toxins are chemicals that are produced by the bacteria and that damage human cell.) These toxins are almost identical to toxins produced by another related bacterium, Shigella spp that can damage and kill intestinal cells and occasionally cause anemia, damage to platelets, and death of cells in other organs, especially the kidneys.

E. coli 0157:H7 is a major health problem. It is estimated to cause infection in more than 70,000 individuals a year in the United States, and the U. S. Centers for Disease Control and Prevention (CDC) suggests E. coli 0157:H7 is responsible for the majority of "E. coli" outbreaks in the U. S. It has been reported to cause both large outbreaks as well as outbreaks in small numbers of individuals.

This diarrheal illness was first recognized when the CDC personnel isolated E. coli O157:H7 from patients in two separate outbreaks in Oregon and Michigan. The illnesses were associated with eating hamburgers at the restaurants of a national chain; some patients experienced hemorrhagic colitis (inflammation and bleeding of the colon). Thus, hemorrhagic colitis due to E. coli 0157:H7 is commonly referred to as hamburger disease. Since that time, E. coli 0157:H7 also has been associated with contaminated water, foods, and unpasteurized or incorrectly pasteurized (heat treated) dairy products.

The most recent outbreak (October/November of 2010) occurred in five states (California, Arizona, Colorado, New Mexico and Nevada). The CDC linked the outbreak to Gouda cheese sold and given away as free samples at Costco stores.

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