Escherichia coli (commonly E. coli), is a bacterium that is commonly found in the lower intestine of warm-blooded animals. Most E. coli strains are harmless, but some, such as serotype O157:H7, can cause serious food poisoning in humans, and are occasionally responsible for costly product recalls. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K2, or by preventing the establishment of pathogenic bacteria within the intestine.

E. coli are not always confined to the intestine, and their ability to survive for brief periods outside the body makes them an ideal indicator organism to test environmental samples for fecal contamination. The bacteria can also be grown easily and its genetics are comparatively simple and easily-manipulated, making it one of the best-studied prokaryotic model organisms, and an important species in biotechnology. E. coli was discovered by German pediatrician and bacteriologist Theodor Escherich in 1885, and is now classified as part of the Enterobacteriaceae family of gamma-proteobacteria.

Virulent strains of E. coli can cause gastroenteritis, urinary tract infections, and neonatal meningitis. In rarer cases, virulent strains are also responsible for hæmolytic-uremic syndrome (HUS), peritonitis, mastitis, septicemia and Gram-negative pneumonia. Recently it is thought that E. coli and certain other foodborne illnesses can sometimes trigger serious health problems months or years after patients survived that initial bout.

Bacterial infections are usually treated with antibiotics. However, the antibiotic sensitivities of different strains of E. coli vary widely. As Gram-negative organisms, E. coli are resistant to many antibiotics that are effective against Gram-positive organisms. Antibiotics which may be used to treat E. coli infection include amoxicillin as well as other semi-synthetic penicillins, many cephalosporins, carbapenems, aztreonam, trimethoprim-sulfamethoxazole, ciprofloxacin, nitrofurantoin and the aminoglycosides.

Antibiotic resistance is a growing problem. Some of this is due to overuse of antibiotics in humans, but some of it is probably due to the use of antibiotics as growth promoters in food of animals. A study published in the journal Science in August 2007 found that the rate of adaptative mutations in E. coli is "on the order of 10–5 per genome per generation, which is 1,000 times as high as previous estimates," a finding which may have significance for the study and management of bacterial antibiotic resistance.

Antibiotic-resistant E. coli may also pass on the genes responsible for antibiotic resistance to other species of bacteria, such as Staphylococcus aureus. E. coli often carry multidrug resistant plasmids and under stress readily transfer those plasmids to other species. Indeed, E. coli is a frequent member of biofilms, where many species of bacteria exist in close proximity to each other. This mixing of species allows E. coli strains that are piliated to accept and transfer plasmids from and to other bacteria. Thus E. coli and the other enterobacteria are important reservoirs of transferable antibiotic resistance.

Phage therapy—viruses that specifically target pathogenic bacteria—has been developed over the last 80 years, primarily in the former Soviet Union, where it was used to prevent diarrhea caused by E. coli. Presently, phage therapy for humans is available only at the Phage Therapy Center in the Republic of Georgia and in Poland. However, on January 2 2007, the United States FDA gave Omnilytics approval to apply its E. coli O157:H7 killing phage in a mist, spray or wash on live animals that will be slaughtered for human consumption.

Researchers have actively been working to develop safe, effective vaccines to lower the worldwide incidence of E. coli infection. In March 2006, a vaccine eliciting an immune response against the E. coli O157:H7 O-specific polysaccharide conjugated to recombinant exotoxin A of Pseudomonas aeruginosa (O157-rEPA) was reported to be safe in children two to five years old. Previous work had already indicated that it was safe for adults. A phase III clinical trial to verify the large-scale efficacy of the treatment is planned.

Because of its long history of laboratory culture and ease of manipulation, E. coli also plays an important role in modern biological engineering and industrial microbiology. The work of Stanley Norman Cohen and Herbert Boyer in E. coli, using plasmids and restriction enzymes to create recombinant DNA, became a foundation of biotechnology.

E. coli is frequently used as a model organism in microbiology studies. Cultivated strains (e.g. E. coli K12) are well-adapted to the laboratory environment, and, unlike wild type strains, have lost their ability to thrive in the intestine. Many lab strains lose their ability to form biofilms. These features protect wild type strains from antibodies and other chemical attacks, but require a large expenditure of energy and material resources.

Escherichia coli O157:H7 is an enterohemorrhagic strain of the bacterium Escherichia coli and a cause of foodborne illness. Based on a 1999 estimate, there are 73,000 cases of infection and about 60 deaths caused by E.coli O157:H7 each year in the United States. Infection often leads to bloody diarrhea, and occasionally to kidney failure, especially in young children and elderly people. Most illness has been associated with eating undercooked, contaminated ground beef, drinking unpasteurized milk, swimming in or drinking contaminated water, and eating contaminated vegetables.

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