|MadSci Network: Microbiology|
The name "coliform" apparently comes from the resemblance of bacteria in this group to Escherichia coli, a bacterium that was discovered by Escherich in feces. Because feces come from the large intestine (colon), the species was named E. coli (of the large intestine). E. coli is found in the intestines and feces of all warm-blooded animals.
The coliform ("like E. coli") group includes species from several genera. They all are Gram-negative, short rods that cannot form spores but do produce acid and gas from lactose within 48 hours at 35°C. Included are E. coli, Enterobacter aerogenes, Klebsiella pneumoniae, Citrobacter species, and others. Not all of these organisms are from the intestines or feces — some of them live free in the environment. They have "sanitary significance," in that their presence at detectable levels in finished drinking water or at higher than minimal levels in pasteurized milk is considered cause for alarm. Most are not capable of causing disease in humans.
There is a more restricted group of coliforms, called "fecal coliforms," that grow at 44.5°C (above body temperature). This group comprises largely E. coli and K. pneumoniae, the latter of which is not necessarily of fecal origin; for this reason, some prefer that the group be called "thermotolerant coliforms."
A common property of all coliforms is ability to ferment (produce acid from) lactose, the principal sugar of milk. The first step in this process is the separation of lactose into two simpler sugars, glucose and galactose. The enzyme that accomplishes this, beta-galactosidase, is the basis of a number of rapid tests for the presence of coliforms developed in recent years. A chemical that develops a color when acted on by beta-galactosidase is included in the culture medium, and development of the characteristic color indicates the presence of the enzyme, and therefore of coliforms, in the sample.
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