There are many different kinds of microorganisms and bacteria that can get into your water supply. The three most common types of bacteria in drinking water are iron, sulfur and coliform bacteria. While iron bacteria doesn’t pose a health risk, it can cause rust-colored stains to develop in your toilet, sink and on other fixtures. It can also give your tap water a foul odor and taste. Sulfur bacteria in your water will result in a rotten egg smell, and can also produce a black sludge that can corrode pipes. Coliforms are bacteria that reside in an animal’s intestinal system, such as E. coli. The presence of coliforms in your water supply means that it has come in contact with human or animal waste which, needless to say, can be very unpleasant and can cause diarrhea, cramping and nausea.