Certain strains of E. coli, such as O157:H7, O121 and O104:H21, produce toxins. Food poisoning caused by E. coli are usually associated with eating unwashed vegetables and meat contaminated post-slaughter. O157:H7 is further notorious for causing serious and even life-threatening complications like Hemolytic Uremic Syndrome (HUS). This particular strain is linked to the 2006 United States E. coli outbreak of fresh spinach. Severity of the illness varies considerably; it can be fatal, particularly to young children, the elderly or the immune compromised, but is more often mild. E. coli can harbor both heat-stable and heat-labile enterotoxins. The latter, termed LT, contains one 'A' subunit and five 'B' subunits arranged into one holotoxin, and is highly similar in structure and function to Cholera toxins. The B subunits assist in adherence and entry of the toxin into host intestinal cells, while the A subunit is cleaved and prevents cells from absorbing water, causing diarrhea. LT is secreted by the Type 2 secretion pathway.
If E. coli bacteria escape the intestinal tract through a perforation (for example from an ulcer, a ruptured appendix, or a surgical error) and enter the abdomen, they usually cause peritonitis that can be fatal without prompt treatment. However, E. coli are extremely sensitive to such antibiotics as streptomycin or gentamycin, so treatment with antibiotics is usually effective. This could change since, as noted below, E. coli quickly acquires drug resistance.
Intestinal mucosa-associated E. coli are observed in increased numbers in the inflammatory bowel diseases, Crohn's disease and ulcerative colitis. Invasive strains of E. coli exist in high numbers in the inflamed tissue, and the number of bacteria in the inflamed regions correlates to the severity of the bowel inflammation. 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.