Escherichia coli 0157:H7
Encounter and history:
Escherichia coli are gram negative, rod shaped bacteria that are often used in laboratory studies and are usually harmless, residing in the human digestive tract as part of the normal flora. E. coli 0157:H7 is a pathogenic strain of E. coli that was first identified as a cause of disease in the United States in 1982, during an investigation into an outbreak of gastrointestinal illness. The organism is also called enterohemorrhagic E. coli (EHEC) because it causes abdominal pain with diarrhea that may contain blood. It lives in the intestines of healthy cattle and can contaminate meat during slaughter. It is believed that the widespread use of antibiotics by American farmers has promoted the horizontal gene transfer of antibiotic resistance genes and pathogenicity islands that has led to the creation of new pathogenic strains such as E. coli 0157:H7. The verotoxins that cause the disease are also called shiga toxins because they were acquired from Shigella strains. The illness predominantly occurs in the U.S., Canada, Japan and other industrialized nations of Europe. The average incidence in most countries ranges from 1-30 per 100,000 and the number of confirmed cases is increasing. In the U.S, about 73,000 people are infected anually, with an average of 61 deaths per year.
Entry, Spread and Multiplication:
After food contaminated with E. coli 0157:H7 is ingested, it travels through the digestive system and attaches to the mucosal epithelial cells of the large intestine and forms a pedestal, which leads to destruction of the microvili. The bacteria will then multipy and produce toxins, which, in addition to diarrhea cause vomiting in 50% of cases and fever in 30%. The average incubation period is 3-4 days. The innoculum size is small(as little as 50 cells can cause illness) because E. coli 0157:H7 are acid tolerant and can easily pass through the stomach. The can also grow from 10-50C with an optimal temperature of 37C. Up to 10% of patients will develop hemolytic uremic syndrome, the most important complication from E. coli 0157:H7. This disease, charictarized by acute renal failure, almost exclusively affects children and the elderly and is fatal in as many as 5% of cases with intensive care. E.coli 0157:H7 is responsible for about 90% of HUS cases and is the leading cause of kidney failure in children. It is caused by the spread of the bacteria and the toxins throughout the bloodstream. Any organ may potentially be affected but certain renal cells are the most vulnerable.
The genome of E. coli 0157:H7 was recently sequenced and contains 1,387 new genes absent in non pathogenic strains. Analysis of the genome suggests that there are a variety of potential virulence genes that have yet to be explored, including fimbrial and other adhesions, secretion systems, and toxins. Many of the virulence genes are similar to those associated with pathogenesis in...