An Introduction to E. coli, a Foodborne Pathogen
What is E. coli and how does it cause food poisoning?
Escherichia coli (or E. coli) is the most prevalent infecting organism in the family of gram-negative bacteria known as enterobacteriaceae. E. coli bacteria were discovered in the human colon in 1885 by German bacteriologist Theodor Escherich.  Dr. Escherich also showed that certain strains of the bacterium were responsible for infant diarrhea and gastroenteritis, an important public health discovery. Although E. coli bacteria were initially called Bacterium coli, the name was later changed to Escherichia coli to honor its discoverer. 
E. coli is often referred to as the best or most-studied free-living organism. [1, 3] More than 700 serotypes of E. coli have been identified. [1,4] The “O” and “H” antigens on the bacteria and their flagella distinguish the different serotypes.  It is important to remember that most kinds of E. coli bacteria do not cause disease in humans. [1, 2] Indeed, some E. coli are beneficial, while some cause infections other than gastrointestinal infections, such as urinary tract infections. 
The E. coli that are responsible for the numerous reports of contaminated foods and beverages are those that produce Shiga toxin, so called because the toxin is virtually identical to that produced by Shigella dysenteria type 1.  The best-known and also most notorious E. coli bacteria that produce Shiga toxin is E. coli O157:H7. [1, 4] Shiga toxin–producing E. coli (STEC) cause approximately 100,000 illnesses, 3,000 hospitalizations, and 90 deaths annually in the United States. [39, 54] Most reported STEC infections in the United States are caused by E. coli O157:H7, with an estimated 73,000 cases occurring each year.  A study published in 2005 estimated the annual cost of E. coli O157:H7 illnesses to be $405 million (in 2003 dollars), which included $370 million for premature deaths, $30 million for medical care, and $5 million for lost productivity.