|MadSci Network: Microbiology|
The water that comes out of the faucet when we turn it on is from either a public system or maybe a private well. Most public systems, and a few private wells, use disinfectant (usually chlorine, which is pretty much like the chlorine bleach you buy at the grocery store) to try to kill disease agents like viruses and certain bacteria. This usually succeeds, though it doesn't take care of Cryptosporidium, a protozoan parasite that got into Milwaukee's water in 1993 and made over 400,000 people ill. Even if all the disease agents are killed by disinfection, the water is not sterile — there are still bacteria present. These are species that live in the water or on the inside surfaces of water pipes, where some of them build up slimy "biofilms." So, when you turn on the tap, you are getting water with some (hopefully, harmless) bacteria in it. The water may still have some active chlorine in it, but levels are usually very low.
What happens next depends on what kind of sink the faucet runs into. If it's a kitchen sink and you're washing food, the water may dislodge bacteria that were on the raw food. These could be plant and soil bacteria, if you're washing veggies, or animal bacteria like Salmonella if you're washing a raw chicken. Sometimes vegetables have animal bacteria on them because animal manure was used as fertilizer without having been held or treated so the bacteria would die. Many foods are cooked — and disease bacteria killed — before we eat them, but the bacteria we washed off into the sink may still be there. We can use scouring powder or a number of other household products to kill most of these bacteria, but most of the time just soap (or detergent) and more water will make enough of them go away that we are not going to get ill.
If the sink is in the bathroom, people will use it to wash their hands after going to the toilet. In this case, there will probably be some bacteria in the sink that originally came from our intestines. Bacteria from our mouths, noses, and throats get into the sink when brush our teeth and rinse our mouths. Again, these can mostly be killed by various household products, but our health doesn't depend on disinfecting the sink every day. We need to keep stuck-on material to minimum on sink surfaces and to avoid water that stands around on the surface or in the basin. Bacteria won't multiply on a dry surface. The hardest part of the sink to disinfect is down in the drain trap; but if the sink is operating properly, the stuff down there isn't coming back to bother us and can largely be ignored.
Finally, some homes have sinks that receive laundry water, and schools and other public buildings have janitors' sinks. Laundry water has bacteria that come from our clothing, probably including diapers if there's a baby in the house. Janitors' sinks are used to fill buckets and rinse mops, so they will have bacteria from whatever people tracked into the building on their shoes, often including animal (dog, bird, etc.) droppings.
All this may make it sound as if sinks are dangerous to have around, but they really work very hard to make our lives safe. Ask some of the people whose water supply got "drowned" in the recent floods in North Carolina, or some of the people in parts of the world where water supplies are always questionable, either as to quantity or quality. Life is very different when you don't have safe water, faucets and sinks. Faucets and sinks always have bacteria on them, but this is because water doesn't have to be sterile to be safe, and because we use our water to take harmful bacteria (and other things) off of our food, our bodies, and our clothing. If we just keep the sink reasonably clean and let the surfaces dry as much as possible (and keep the water supply and drain lines working properly), we will live at a standard of hygiene that much of world's population envies.
If you want to grow bacteria from your faucet or sink to study them, you can search on "grow bacteria" at this site for methods. Remember, though, that if bacteria like Salmonella may be in your sink, you need to handle your cultures with care — the kinds of bacteria (and other disease agents) we work with in my laboratory are very capable of "biting the hand that feeds them."
Try the links in the MadSci Library for more information on Microbiology.