|MadSci Network: Microbiology|
Your question relates to a very interesting microbial habitat of the human body, the oral cavity. There are many bacteria that inhabit the oral cavity. Most of them are anaerobes, bacteria which do not use oxygen to grow. Common ones are streptococci and lactobacilli. Our saliva contains some compounds which act to inhibit the growth of these organisms. However, the presence of food particles in our mouths and growth adaptations by the bacteria allow the bacteria to flourish. Because of this it is necessary for us to brush our teeth. When we brush our teeth we do not kill all of the bacteria in our mouths, we do not even come close. Instead, by brushing our teeth we control the growth of the bacteria so that they do not become so numerous that they begin to damage our teeth ( an interesting side topic you may want to pursue is to learn how these bacteria damage our teeth when they grow). By brushing our teeth we remove food particles and dental plaque from our teeth. Dental plaque is formed when bacteria use our teeth as a place to grow. So the toothbrush acts as a method to remove the dental plaque and therefore remove bacteria from your teeth. Some toothpaste brands have compounds like fluoride which act to kill bacteria. However, the toothpaste will never kill all the bacteria, therefore there will always be some survivors that will replenish the population. Ultimately we are trying to control bacterial growth by removing there source of food. For example food particles stuck in our teeth. We are also trying to remove the bacteria that have stuck to our teeth in the form of dental plaque.
Microbiology of the human mouth is very interesting and I encourage you to learn more about this fascinating topic. A good start would be any introductory microbiology textbook such Biology of Microorganisms published by Prentice Hall.
I hope this helps.
Try the links in the MadSci Library for more information on Microbiology.