Escherichia coli are very common bacteria that, depending upon the circumstances, the specific strain and the type of exposure, may or may not be pathogenic. They're used frequently in biochemical and microbiological research because of their ubiquity, and as there are many different strains, they can also affect many different body systems.
Because there are several different strains of E. coli and because different strains can tolerate different conditions, body systems may or may not be affected depending upon the route of E. coli entry and whether the bacteria can tolerate the system into which they are introduced. For instance, because most strains of E. coli do well at neutral acidity and human body temperature, they typically don't survive oral ingestion, says Dr. Lauralee Sherwood in her book "Human Physiology," because the stomach is so acidic. Acid-tolerant strains, however, do survive, and can produce severe gastroenteritis.
Nonpathogenic E. coli colonize the digestive tract of humans. The bacteria do not cause infection, and in fact, live in symbiosis with their human hosts. The bacteria get nutrition from indigestible food matter that passes through the human intestinal tract, and, in return, the bacteria synthesize vitamin K, which is important to the human ability to clot blood. This effect of E. coli on a body system, writes Dr. Jacquelyn Black in her book "Microbiology," is a positive one.
E. coli, even non-virulent strains, can cause infection if they get from the digestive tract to elsewhere in the body, however. Dr. Black notes the close proximity of the female urethra to the anus makes the urethra vulnerable to E. coli infection if fecal matter is unintentionally transferred to the urethra. E. coli can then ascend the urethra and colonize the bladder, resulting in urinary tract infection. These infections can be painful and, if untreated, may become severe and ascend to the kidneys.
Because E. coli are generally associated with the gut, it's common to assume they don't cause serious infection elsewhere. However, if contaminated material gets into the lungs, E. coli can colonize the lungs and lead to bacterial pneumonia, Dr. Black writes. Unlike viral pneumonia, bacterial lung infection can be treated with antibiotics. It's relatively rare for E. coli to colonize the lungs, however, since they're not bacteria normally contained in aerosolized droplets that are easily inhaled.
Like all bacteria, E. coli adapt very readily to their environments and reproduce quickly. This means that not only can a colony grow rapidly, but also that the colony can adapt to its environment through responding to selective pressure, Drs. Thomas Pollard and William Earnshaw write in their book "Cell Biology." As such, E. coli introduced into a body system can begin to adapt to that system quickly -- and E. coli introduced to antibiotics can develop resistance if the antibiotics are used only for a few days and not all the bacteria are killed.