|MadSci Network: Microbiology|
You haven’t said where the water you drink comes from, so it is hard to rule out possibilities. If your water is from a city supply, it will have been treated according to the U.S. Environmental Protection Agency (EPA) requirements. Water from a surface source (lake, river, or stream) is supposed to receive “complete treatment,” which includes adding a flocculant (makes suspended matter stick together), settling (lets the flocculated suspended matter sink to the bottom of a tank), filtration (sand or some other filter material takes out more suspended material), and disinfection (chlorine, ultraviolet light, or something kills bacteria, viruses, and maybe some other things). This process is pretty much guaranteed to get rid of anything that could cause illness, but it does not sterilize the water — there will still be live bacteria present. Cities that use well water usually just disinfect it.
If your home has its own water source (usually a well), it may be taking water from deep in the ground that does not need all this treatment. However, the water often has “coliform bacteria” in it, whereby at least disinfection is recommended. Small-scale home disinfection units often take large tablets of solid chlorine compound that gradually dissolve as the water passes over them, providing and effective level of chlorine.
What I’m leading up to is that there are several groups of bacteria in water, including “water bacteria,” disease bacteria (and maybe viruses and other disease agents), “indicator bacteria,” and any others that may have washed off the land or out of the air with rain. Bacteriological methods for water are spelled out in Standard Methods for the Examination of Water and Wastewater, published by the American Public Health Association in Washington, DC.
Water bacteria are almost always present. They come in various shapes, but mostly rods that look like cocktail wieners. They are adapted to live in water because they can survive (and grow slowly) at very low levels of “nutrients” — the stuff that bacteria need to grow. They do well at most temperatures above freezing. They are mostly killed by chlorine or ultraviolet disinfection, but a few always survive. They don’t generally make people sick, but they can attach to the inner surfaces of water mains and pipes, causing problems in pushing the water through to the user. One special group of water bacteria are the cyanobacteria (formerly called blue-green algae); these only grow if they get a good supply of light, and some produce poisonous substances that can make people sick.
Disease bacteria usually come from the intestines of animals or people. We flush a toilet and (often) we have contaminated a certain amount of water with human feces. This wastewater, or sewage, must be properly treated in a sewage treatment plant or a septic tank before it gets out into water that someone else may use for drinking. Sometimes sewer pipes leak, and the sewage gets places where it can cause human or animal illnesses. Most of these bacteria, too, are rod-shaped. They may swim rapidly in water, but it is almost impossible to see this with a microscope. These are the target of water disinfection, in that if we drink water that has been disinfected, we won’t get infected. They aren’t in water most of the time. They can be detected by testing, but each type of disease agent has its own test procedure, so we can’t really rely on testing to make sure our drinking water is safe. The tests are expensive, and if we test for the wrong disease agent, we may miss one that could be present and cause illness. This is why we rely on disinfection, in most cases.
So, if the disease bacteria are seldom present in water, how do we know that the “disinfection” is doing any good? This is where the “indicator bacteria” come in. The EPA drinking water standards relate to a test called “total coliforms,” which can be run on samples from anywhere. The coliform bacteria are small, rod-shaped, stain pink rather than purple by the Gram method of staining, and do not form spores like some of the soil bacteria. Originally, the group was defined as producing acid and gas from lactose (the sugar from milk) within 48 hours at 35 degrees Celsius (95 degrees Fahrenheit). Nowadays, simpler and more rapid coliform tests are most often used. If there are coliform bacteria in the water that is going to be treated for drinking, this is a reason to say that the water should be disinfected before people drink it. If detectable levels of these coliform bacteria are found in water that is being distributed to people’s homes, EPA requires that a “boil water” order be issued by the water company, which says that you should only drink the water if you boil if first. So, the coliform test serves two purposes: to show where water disinfection is needed, and to show when disinfection has failed (or maybe some bad water leaked into the pipes). There are some other groups of “indicator bacteria,” but the coliforms are most widely used.
After these groups (water, disease, and indicator bacteria), almost anything may show up in surface water. Even if people are careful not to put pet poop into the storm drains, rain washes the streets, the countryside, and the air and carries bacteria from these sources into water. Some look like strings of beads, others are rods, and still others are corkscrew-shaped. Many such bacteria don’t last long because they are not well adapted to live in the water environment. If rain water infiltrates the soil (called “percolation”) and travels deep under the surface, it is usually purified as it goes down to where water accumulates (called the “water table” or an “aquifer”). Well water often has a lot of dissolved minerals (called “hard water”) but very few bacteria. There are exceptions, however: if there is cracked rock or coarse gravel near the surface, the water may not be purified on its way to the water table and to someone’s well. This is why well water is tested for coliform bacteria; if coliform bacteria are not in the water, it shows that the natural soil purification is probably effective.
Americans are relatively lucky in the matter of drinking water safety. We don’t need to have water that is “sterile” (no bacteria), but some of the disease bacteria that occur in drinking water in the poorer countries of the world cause deadly diseases like cholera and dysentery. Waterborne disease is relatively rare in the U.S., and these illnesses are more likely to be caused by protozoa ( Giardia lamblia or Cryptosporidium parvum, for example) or viruses. Since your question was about bacteria, I won’t go into the sources and problems of these other bad guys.
Try the links in the MadSci Library for more information on Microbiology.