Escherichia coli (usually abbreviated to E. coli) is one of the main species of bacteria that live in the lower intestines of warm-blooded animals (including birds and mammals) and are necessary for the proper digestion of food, and its presence in groundwater is a common indicator of fecal contamination. ("Enteric" is the adjective that describes organisms that live in the intestines. "Fecal" is the adjective for organisms that live in feces, so it is often a synonym for "enteric.") The name comes from its discoverer, Theodor Escherich. It belongs among the Enterobacteriaceae, and is commonly used as a model organism for the bacteria in general.
The number of individual E. coli bacteria in the feces that one human passes in one day averages 1011 (= one with eleven zeroes after it) to 1013. All the different kinds of fecal coli bacteria and all the very similar bacteria that live in the ground (in soil or decaying plants, of which the most common is Aerobacter aerogenes) are grouped together under the name "coliform" (meaning "like coli") bacteria. Technically, the "coliform group" is defined to be all the aerobic and facultative anaerobic, non-spore-forming, Gram-negative, rod-shaped bacteria that ferment lactose with the production of gas within 48 hours at 35°C.
In the fields of water purification and sewage treatment, E. coli was chosen very early in the development of the technology as an "indicator" of the pollution level of water, meaning the amount of human fecal matter in it. The main reasons for using E. coli are that there are a lot more coliforms in human feces than there are pathogens (such as Salmonella typhosa, which causes typhoid), and E. coli is harmless, so it can't "get loose" in the lab and hurt anyone.
There are, however, three situations where the "harmless" E. coli can cause illness:
- When the bacteria get out of the intestinal tract and into the urinary tract, they can cause an infection often referred to as "honeymoon cystitis" that occurs more often in females and is one of the very few infections it might actually be possible to catch from a dirty toilet seat if you did it just right, although it's usually the result of what's termed "poor toilet habits" in wiping the toilet paper towards the vagina instead of towards the spine after a bowel movement.
- When the bacteria gets out of the intestinal tract through a perforation (= a hole or tear, which could be caused by an ulcer, for example) and into the abdomen, they usually cause an infection called "peritonitis" that can be fatal, although E. coli are extremely sensitive to streptomycin, so treatment with antibiotics is usually effective.
- When some strains of E. coli get into the intestinal tract (usually through the mouth), they can cause dysentery in people whose immune systems are weak, which includes very young children because their systems are not mature yet.