The Coliform Test
Although a water supply may have been used for a long time without any side effects, its safety cannot be guaranteed without regular testing. Residents of a community may develop a tolerance for certain bacteria to which they are regularly exposed, but strangers often become ill from drinking the same water. For this reason it is important that drinking water be tested regularly for bacteriological quality.
The standard bacteriological method for judging the suitability of water for domestic use is the coliform test. This method of analysis detects the presence of coliform bacteria, which are found in the natural environment (soils and plants) and in the intestines of humans and other warm-blooded animals. They are discharged in the bowel movement; hence, any food or water sample in which this group of bacteria is found is to be suspected of having come into contact with domestic sewage, animal manure, or with soil or plant materials. It follows that such a water supply may contain pathogenic bacteria and viruses which cause such serious human illnesses as typhoid fever, dysentery, hepatitis, etc.
Membrane filters
The present regulations require water systems to take a minimum number of microbiological samples each month based upon the number of persons being served. The larger the population served, the more microbiological samples required per month. The two standard methods for determining the numbers of coliform bacteria in a water sample are the multiple tube fermentation technique and the membrane filter technique. In the multiple tube fermentation technique, a series of fermentation tubes containing special nutrients is inoculated with appropriate quantities of water to be tested and incubated. After 24 hours, the presence or absence of gas formation in the tubes is noted. This is considered a presumptive test for the presence of coliform organisms. A confirming test performed for drinking water samples involves a similar technique using the culture from the positive presumptive test in a different nutrient medium.
In the membrane filter technique, which is less time consuming, an appropriate quantity of water to be tested is filtered through a specially designed membrane filter which traps bacteria. The filter is removed and placed in a special dish with nutrients and incubated for 24 hours. The typical coliform colony has a metallic surface sheen. The results are usually expressed as number of coliform colonies per 100 mi of water sample.