from Michael F. Cole, Katherine A. Wirth and George H. Bowden writing in Oral Microbial Ecology: Current Research and New Perspectives:
In this review we consider the biology of the viridans streptococci in the human oropharynx with a particular focus on the pioneer bacterium Streptococcus mitis. We show that, although this species is a constant component of the human oral cavity, each person harbors a unique and diverse population of stains that appear not to be shared within a family and, apparently, are rarely transmitted from mother to neonate. The population of stains of S. mitis within the mouth of each individual exhibits turnover perhaps in response to pressure exerted by the mucosal immune system since it has been shown that some secretory immunoglobulin A (SIgA) antibodies are clone-specific. We assert that the strains that are successful in establishing in the mouth are physiologically adapted to occupy their niche within their habitat. While it is clear that in vitro experiments and animal models have provided useful information, they are no substitute for studying commensal oral bacteria in their environment, the human oral cavity.
Further reading: Oral Microbial Ecology: Current Research and New Perspectives