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. By brushing our teeth we remove food particles and dental plaque from our teeth.