Water that comes from a properly protected ground water supply should have no coliform present. The presence of any coliform indicates that the water supply has a connection with some unsanitary source. This may be as simple as insects getting into the well or may be a more serious connection with human or animal waste. In either case, the water should not be consumed without proper treatment. Boiling water for one minute at a rolling boil kills coliform and other disease producing organisms.
Various purifiers can also kill harmful organisms if properly installed.
Protecting yourself and your well from contact with human or animal waste is always prudent. Private wells should be tested at least once a year. Every nine months is a better plan, as it results in the water being tested in various seasons.