A bacteria test checks for the presence of total coliform bacteria and fecal coliform bacteria. These bacteria are not normally present in deeper groundwater sources. They are associated with warm-blooded animals, so they are normally found in surface water and in shallow groundwater (less than 20 to 40 feet in depth).
These bacteria are not necessarily pathogenic bacteria that make people sick but are used as standard indicators of the safety of the water. If these bacteria are present, then the conditions are favorable for harmful bacteria to exist. While some people may be unaffected from drinking the water, many have symptoms of stomach and intestinal illness, diarrhea, etc., especially the young, the old and immuno-compromised individuals.