While the vast majority of bacteria in groundwater are not pathogens, some bacteria, mostly originating with humans and their livestock, are of interest in public health for both animals and people. Among these, most coliforms are not pathogens, but they serve as indicators of the microbial quality of water. An indicator is easily cultured, often present when pathogens are present, but in far greater numbers than the pathogen. Pathogens—the bacteria, protozoa, and viruses that make people sick—can be rare and difficult to detect even if they are present in the water. Total coliforms are indicators and are more common and easy to grow. Testing for them provides a margin of safety.
Pathogens may not be present if coliforms are, but some strains of E. coli (a species in the total coliform group) have been lethal, so their presence should be taken very seriously. Other health effects from pathogenic bacteria can include diarrhea, cramps, nausea, headaches, or other symptoms. They may pose a special health risk for infants, young children, and people with severely compromised immune systems. Even without pathogens present, large numbers of coliform bacteria indicate connection with soil and poorly filtered water, and possibly breaks in sanitary seals protecting water systems.
People become accustomed to the natural bacteria in their water while guests may experience some discomfort or diarrhea.
Bacteria that cause biological fouling of water systems typically do not cause disease themselves, although some, such as species and strains of Pseudomonas, can cause infections and pneumonia. However, they can harbor coliform bacteria and pathogens, impair well and water system function, and cause unsightly discoloration that is an indicator that well and water system cleaning should be performed.