What makes E. coli pathogenic?
In the chapter on pathogenesis, we introduced the concept of the normal microbial flora, organisms that we can find on the outside and on inside surfaces in the absence of disease. These organisms use the human body as a source of nutrition, and we obtain some measure of benefit from many of them due to the property of microbial antagonism. One of the most prevalent prokaryotes in the human digestive tract is the coliform, Esherichia coli. This organism is relatively benign when found in the colon, however it is a very common cause of nosocomial infections, and accounts for approximately 25% of hospital acquired infections. In these cases, the organism which is causing the complicating infection is acquired from the patient himself, a visitor to the hospital, or one of the patient’s caregivers. This drives home the critical point that this organism is innocuous when it is found where it belongs, and potentially lethal when it is found someplace else!
E. coli infections are also one of the most significant causes of food- or water-borne gastroenteritis. Enterotoxigenic E coli (ETEC,) enteroinvasive E coli (EIEC,) and enterohemorrhagic E coli (EHEC,) are all forms of infection by more virulent strains of E. coli than the ones normally found in the human colon. They possess virulence traits that are not found in the more benign isolates, meaning that they can readily cause illness in pretty much anyone who is unfortunate to ingest them. A new report from Nature Immunology, and summarized in Science Daily describes some of the mechanisms by which E. coli O157:H7 can cause disease so much more easily that other strains of E. coli.
When a microorganism comes into contact with the human immune system, a response occurs against that organism. In the case of a commensal organism, the response may not be particularly powerful, so that the organism maintains a status quo with the host. E. coli O157-H7 strains produce a number of toxins that allow them to cause tissue damage and initiate disease, but the strain has also recently been shown to secrete a protein called NleH1 that affects the balance between host and pathogen. This protein causes the host immune response to be further diminished, allowing the bacterium to not be eliminated by the immune system. This enables the organisms to continue to proliferate in an infected host, and allows it to be passed and spread to uninfected individuals. Scientists are examining this interaction to see exactly what aspects of the immune system are blocked. Initial research suggests that several Gram negative bacteria that cause gastroenteritis share this virulence mechanism, indicating that further elucidation of it’s protective mechanisms might offer protection for several food-acquired diseases.
BONUS: using the power of Google, name in the comment thread one (1) additional virulence factor of E. coli, and how it can damage the body.