What is Coliform?


“Total Coliforms include species that may inhabit the intestines of warm-blooded animals or occur naturally in soil, vegetation, and water. They are usually found in fecal-polluted water and are often associated with disease outbreaks. Although they are not usually pathogenic themselves, their presence in drinking water indicates the possible presence of pathogens. E. Coli, one species of the coliform group, is always found in feces and is, therefore, a more direct indicator of fecal contamination and the possible presence of enteric pathogens. In addition, some strains of E. Coli are pathogenic.” (1)

“One might assume that the objective in the routine analysis of water would be to search for pathogenic microorganisms. This is not true, however, for the following reasons:

  1. Pathogens are likely to gain entrance into water sporadically, and they do not survive for long periods of time; consequently, they could be missed in a sample submitted to the laboratory.
  2. If they are present in very small numbers, pathogens are likely to escape detection by laboratory procedures.
  3. It takes 24 h or longer to obtain results from a laboratory examination. If pathogens were found to be present, many people would be exposed to infection.

It is known that the pathogens that gain entrance into bodies of water arrive there via intestinal discharges of humans or other animals. Furthermore, certain bacterial species, particularly Escherichia Coli and related organisms designated as coliforms, fecal streptococci (e.g., Streptococcus Faecalis), and Clostridium Perfringens, are normal inhabitants of the large intestine of humans and other animals and are consequently present in feces. Thus, the presence of any of these bacterial species in water is evidence of fecal pollution of human or animal origin. If these organisms are present in water, the way is also open for intestinal pathogens to gain entrance, since they too occur in feces. Since the laboratory examination of water for pathogens is beset with the disadvantages already enumerated, techniques have been developed for the demonstration of bacterial species of known excreta origin, particularly organisms of the coliform group. This approach, which has proved satisfactory in practice, has the following advantages:

  1. Coliform organisms, particularly E. coli, are constantly present in the human intestine in large numbers. It is estimated that billions of these organisms are excreted by an average person in one day.
  2. These organisms live longer in water than intestinal pathogens do.
  3. A healthy person would not, of course, normally excrete pathogenic organisms, but should an intestinal-tract infection develop, the pathogen is likely to appear in the feces. Thus, the presence of coliforms in water is regarded as a warning signal, the water is subject to potentially dangerous pollution.

The coliform group of bacteria includes all the aerobic and facultatively anaerobic, gram-negative, nonsporulating bacilli that produce acid and gas from the fermentation of lactose. The classical species of this group are Escherichia Coli and Enterobacter Aerogenes. The relationship of these organisms to others of the enteric group-Salmonella, Shigella, Proteus, Pseudomonas, and Alcaligenes, all of which are gram-negative, nonsporulating bacilli.

E. Coli is a normal inhabitant of the intestinal tract of man and other animals.
Enterobacter Aerogenes is most frequently found on grains and plants but may occur in the feces of man and other animals. These species bear a very close resemblance to each other in their morphological and cultural characteristics. Consequently, it is necessary to resort to biochemical tests for differentiation. Tests with the following four characteristics are especially important for this purpose:

  1. Ability to produce indole. E. Coli does, and Enterobacter Aerogenes does not.
  2. Amount of acidity produced in special glucose-broth medium and detected by the indicator methyl red. Both organisms produce acid from glucose. However, E. Coli produces a lower pH, which turns the indicator red, whereas Enterobacter Aaerogenes cultures do not produce this color change.
  3. Ability to produce the compound acetylmethylcarbinol in a glucose-peptone medium. This chemical is detected by the Voges-Proskauer test procedure. E. coli does not produce acetylmethylcarbinol, but Enterobacter Aerogenes does.
  4. Utilization of sodium citrate. Enterobacter Aerogenes is capable of utilizing sodium citrate as its sole source of carbon; i.e., it will grow in a chemically defined medium in which sodium citrate is the only carbon compound. E. coli does not grow under the same circumstances.”(2)


(1)  Federal Register. 1985. National primary drinking water regulations; synthetic organic chemicals, inorganic chemicals and microorganisms; proposed rule. Fed. Regist. 50:46936-47022.

(2)  Pelczar, Reid and Chan: Microbiology, McGraw-Hill Co. 4th ed., pp784-5.  Federal Register. 1985. National primary drinking water regulations; synthetic organic chemicals, inorganic chemicals and microorganisms; proposed rule. Fed. Regist. 50:46936-47022.