Microorganisms are extremely small organisms, and include bacteria, protozoa and viruses. Some of these are so small that they cannot be seen under a conventional microscope.
Bacteria cells generally range in size from less than 1 to 10 microns long, and from 0.2 to 1 micron wide. While small, there is a huge quantity and variety of them the world over, and they greatly outnumber humans.
The lower forms of bacteria can be categorized as either helpful or harmful to humans. The harmful bacteria we know as those that cause disease. The helpful bacteria speed up the process of decomposing organic waste, and thus aid in purifying water.
Coliform bacteria are a type of bacteria which are mostly harmless, but the group does contain E. coli (Escherichia coli). E. coli grows as part of the normal microbe population in our own digestive tract, as well as in other warm-blooded animals, and serve as a good indicator of sewage contamination in a drinking water source.
The total number of coliform bacteria that may enter a source of drinking water is reduced by three major factors: 1) these bacteria die in large numbers because they generally cannot survive very well in either sewage or cleaner water; 2) they are removed during the water purification process; and 3) they are destroyed during sewage treatment.
Protozoa are single-celled organisms that live mainly in water. Many protozoa we know as parasites, but like bacteria, they can be classified as helpful or harmful. At times, drinking water can become infested with certain protozoa which are not disease causing, but will give the water a fish taste and odor.
One important group of protozoa are those which form cysts, a protective wall these organisms form around themselves when in unfavorable surroundings. Once inside the body of a warm-blooded animal where the conditions are more favorable, the cyst abandons the protective wall and lives in the animal's blood stream. Protozoa are often found in groundwater that has being influenced by surface water sources.
Among the more common of these cysts is the one which carries the waterborne disease, amoebic dysentery. In addition, Giardia lamblia and cryptosporidium are a cause of acute gastrointestinal illness, the most frequently diagnosed waterborne illness. Cryptosporidium is a major cause of severe diarrhea in children. Giardiasis is usually linked to unfiltered surface water that has not been disinfected sufficiently. Other protozoa found in water supplies are microsporidium, toxoplasma and cylclospora.
Not all available home water filters on the market remove protozoa. The good news is that these cysts are usually 2 to 50 microns in diameter, much larger than bacteria, and can be removed from water by Culligan reverse osmosis filtration, the best protection in home filtration.
Viruses are the smallest of the infectious microorganisms. They cannot be seen under a microscope, as they are only 0.004 to 0.1 micron in diameter, and thus can pass through porcelain filters capable of screening out bacteria.
Viruses are a unique kind of parasite that are incapable of growth unless they are in the presence of living cells. They can survive both freezing and drying.
While viruses are more difficult to directly detect in a water source, they can be carried in feces of both animals and humans. This is why fecal coliform bacteria is a good indicator of sewage contamination. The presence of coliform bacteria in sewage may mean that the water source itself is contaminated with other parasites. It is important to note, however, that the absence of coliform bacteria in a water source does not necessarily indicate the absence of parasites in that water source.
(Source: Water Treatment Dealer, July/August 2002, "What are bacteria, protozoa and viruses?" by Lou Smith, Water Quality Consultant, Canadian Water Quality Association)