Guideline value for drinking water is: September 2007

Ans. C: Zero coliforms per 100 ml Coliform bacteria are a group of Gram-negative rod-shaped bacteria that are found in the veebrate gastrointestinal tract; their presence in water is an indicator of fecal pollution. The biological quality of drinking water is determined by tests for coliform group bacteria. These organisms are found in the intestinal tract of warm-blooded animals and in soil. Their presence in water indicates pathogenic contamination, but they are not considered to be pathogens. The standard for coliform bacteria in drinking water is "No E.coli or thermotolerant coliform bacteria must be detectable in any 100 ml sample. Coliform presence in a water sample does not necessarily mean that the water is hazardous to drink. The test is a screening technique, and a positive result (more than 1 colony per 100 ml water sample) means the water should be retested. The retested sample should be analyzed for fecal coliform organisms. A high positive test result, however, indicates substantial contamination requiring prompt action. Such water should not be consumed until the source of contamination is determined and the water purified.