Microbial pollutants include bacteria, viruses, and protozoans. These are living organisms that are visible in water only with the help of a high-powered microscope. Many different kinds of bacteria, some disease causing but many not, may be present in a water supply. The tests discussed below are specific bacteria tests that ar3e used to determine whether disease-causing bacteria might be present in the water. Protozoans are less common in water than bacteria, but a few can be problems. Viruses will not be discussed because they rarely occur in Pennsylvania drinking water; however, viruses such as hepatitis are carried by water and can cause serious illness.

Coliform bacteria are a large group of bacteria that occur throughout the environment. They are used as an indicator organism to indicate the potential for disease-causing bacteria to be present in water. In other words, if coliform bacteria are present, it is presumed that a contamination pathway exists between the bacteria source and the water supply and that disease-causing bacteria may use this pathway to enter the water supply. Coliform bacteria occur frequently in private water systems, usually from contamination by surface runoff or from human or animal wastes.

Most coliform bacteria do not cause disease, but the greater their number the greater the likelihood that disease-causing bacteria may be present. Since coliforms persist in water longer than most disease-causing organisms, the absence of coliform bacteria leads to the assumption that the water supply is microbiologically safe to drink. Consuming water with coliform bacteria present may cause gastrointestinal illnesses, fever, and other flu-like symptoms. Therefore, the drinking water standard requires that no coliform bacteria be present in public drinking water supplies.

Results from coliform bacteria tests are normally expressed as the number of bacteria colonies present per 100 milliliters (ml) of water. Some laboratories may simply express coliform bacteria results as "Present" (P,) or "Absent" (A). In this case, "Present" indicates only that at least one bacterium was present in each 100 ml of water. Occasionally, bacteria results will be expressed as "MPN", which stands for Most Probable Number. This simply means that a statistical relationship was used to estimate the number of bacteria in your sample. Finally, bacteria results also may be expressed as "TNTC," or "Too Numerous To Count", meaning the bacteria concentration was too high to quantify.

Fecal coliform bacteria are a smaller group of bacteria within the coliform bacteria group. Water may be tested for fecal coliform bacteria if the total coliform test is positive. Fecal coliform bacteria are specific to the intestinal tracts of warm-blooded animals are thus a more specific test for sewage or animal waste contamination. The ration of fecal coliform bacteria to fecal streptococcus bacteria is often used to estimate the source of bacterial contamination (see discussion below). Fecal coliform bacteria levels are expressed as the number of colonies per 100 ml of water. No fecal coliform bacteria are permitted in public drinking water supplies.

Fecal streptococcus bacteria are another smaller group of bacteria within the coliform bacteria group that are especially numerous in animal waste (as opposed to human waste). The ratio of fecal coliform to fecal streptococcus bacteria is usually much higher in humans than it is in animals. As a rule of thumb, a fecal coliform to fecal streptococcus ratio greater than 4.0 is indicative of a human source of bacteria such as a septic system. A ratio of less than 1.0 is indicative of an animal source of bacteria such as runoff from a feedlot. Ratios between 1.0 and 4.0 are inconclusive about the source of the bacteria. Fecal streptococcus bacteria are expressed as the number of colonies per 100 ml of water. No fecal streptococcus bacteria are permitted in drinking water.

An even more specific bacteria test is the test for E. coli (short for Escherichia coli). This is a type of fecal coliform bacteria commonly found in the intestines of animals and humans. A positive E. coli result is a strong indication that human sewage or animal waste has contaminated the water.

Hundreds of strains of E. coli exist. Although most are harmless and live in the intestines of healthy humans and animals, a few can produce a powerful toxin that causes severe illness and even death. Infection often causes severe bloody diarrhea and abdominal cramps; sometimes the infection causes non-bloody diarrhea. Frequently, no fever is present. It should be noted that these symptoms are common to a variety of diseases and may be caused by sources other than contaminated drinking water.

E. coli tests are reported as the number of bacteria per 100 ml of water. The presence of any E. coli in a water sample is unacceptable; thus, the primary drinking water standard for E. coli is 0 per 100 ml of water.

The Standard Plate Count (SPC) or Heterotrophic Plate Count (HPC) is a more general indicator of bacterial contamination. On some test reports, this also may be referred to as the "Total Bacteria Count." It measures all of the bacteria, including coliform and many other groups, in a water sample. The SPC is usually reported as the number of bacteria per milliliter of sample. There are no drinking water standards for SPC, but if more than 500 bacteria are counted in one milliliter of sample, further testing for total coliform or fecal coliform bacteria is suggested.

Iron bacteria are a type of bacteria that feed on small amounts of iron in water. Iron bacteria do not constitute a health threat, but they are a nuisance in private water systems because they form gelatinous strands, masses, or thin films that plug pipes, toilets, and plumbing fixtures and reduce flow from wells. Their appearance can vary from orange or brown to clear. Iron bacteria can colonize and entire water system from the well itself through the plumbing, or they may be present only in parts of the plumbing system.

There are no drinking water standards for iron bacteria. Rather, their presence is normally aesthetically degrading enough to require treatment. Water testing is rarely available to determine if iron bacteria are present. Confirmation is usually based upon the visual symptoms in the water.

Giardia lamblia and Cryptosporidium parvum are small microscopic animals known as protozoa. They both can live in the intestinal tract of mammals, including humans. While there, they multiply by producing oocysts. Infected animals and humans can excrete the oocycts, which can then contaminate water sources. Once ingested, the organism emerges from the protective oocysst and infects the lining of the intestine. Both giardiasis and cryptosproidiosis cause severe diarrhea, nausea, fever, headache, vomiting, and loss of appetite. Both illnesses can be life-threatening to people with depressed immune systems.

Many private water system owners are familiar with Giardia and Cryptosporidium as a result of publicity following outbreaks of illnesses in public water supplies. Most of these outbreaks have occurred in communities that use surface water supplies (streams, rivers, lakes) where the oocysts can commonly be found. Giardia and Cryptosporidium are rarely a concern for private water systems using deeper groundwater sources, because the oocysts are efficiently filtered as water passes through soil and rock. Shallow springs or poorly constructed wells that become contaminated with surface water would be most likely to contain Giardia and Cryptosporidium oocysts. This is one reason that roadside springs are not a good alternative source of drinking water.

Both Giardia and Cryptosporidium are measured in water by filtering large volumes of the water through a small filter and examining the filter under a microscope for oocysts. Oocysts should be totally absent from water for it to be safe to drink.