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Bacteriological Safety

Coliform Bacteria
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"Coliform" bacteria are found in the intestines and wastes of humans and some animals and, therefor, their presence in water indicates that the water has been contaminated with such wastes. The coliform bacteria are not pathogens, but serve as "indicators" to show that disease bacteria could be present and that treatment or corrective measures should be taken. The tests for coliform bacteria have become the basis for the evaluation of Bacteriological Safety throughout North America.

Two different procedures may be used for testing water for coliform bacteria. In the older procedure, a small test tube is dropped upside down into a larger tube, a special liquid media is added, and the entire set is sterilized. In the usual approach, measured volumes of water are added to five such tubes, and the tubes are placed in an incubator for 24 to 48 hours. If coliform bacteria are present in the water, they will cause fermentation of the media and the release of some gas. Some of that gas will be trapped in the small, upside-down inner test tube and be visible as a small bubble.

From the number of tubes which show the gas bubbles, a table can be used to estimate the number of coliform bacteria present. This is not a direct count, but is the "Most Probable Number" (or "MPN") of coliform bacteria per 100 mL (about 4 ounces) of water, based on statistical analysis. The following is an example of the table used when 10 mi volumes of water are added to 5 tubes of the media (see right):

Some laboratories give the MPN in their reports, while others simply indicate the number of negative (no gas) and positive (show gas) tubes with a series of plus and minus signs, +++---, for example, would indicate 2 positive and 3 negative tubes, and from the table, an MPN of 5.1 per 100 mL.

A new procedure for coliform organism tests involves the filtration of a measured volume of water through a sterilized membrane which has pores so small that any bacteria present are retained on the membrane. The membrane is then placed in a small covered dish which contains an absorbent pad saturated with a liquid media. After incubation for 20 hours, each coliform organism originally present will have developed into a "colony," a clearly visible spot on the membrane which has a characteristic shape and color. By simply counting the colonies, a direct reading of the number of coliform organisms in the volume of water filtered is obtained. The advantage of this method is speed, but high concentrations of iron or suspended matter in the water may interfere with the test. As with the multiple tube method, the results are usually reported as the number of coliform organisms per 100 mL.

Consult your local health department for information about coliform test and sampling procedures.

Coliform & E.coli Bacteria

  • The presence of coliform and/or E.coli bacteria indicates that human and/or animal wastes have entered the water supply
  • Chlorination, along with proper retention is the most common disinfectant method in the world to kill coliform and E.coli bacteria
  • Walkerton, Ontario - 7 people died and over 2,000 became ill after consuming water contaminated with E.coli bacteria

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