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.