This is the most basic test for bacterial contamination of a water supply. Total coliform counts give a general indication of the sanitary condition of a water supply. Therefore, testing for coliform bacteria can be a reasonable indication of whether other pathogenic bacteria are present. E. coli is also an indicator of drinking water quality. E. coli is a sub-group of fecal coliform. Most E. coli bacteria are harmless and have been found in great quantities in the intestines of people and warm blooded animals. However, some strains can cause illness.