The human oral cavity is the highly selective environment, which can be colonized by certain microbes. Viridans group streptococci are the most common bacterial isolates along with many other aerobic and anaerobic bacteria. S. salivarius is the most common viridians group streptococcus followed by S. mitis, S. sanguis, S. milleri, S. gordonii and S. mutans. The diverse habitat unique to the oral cavity together with a variety of nutrients is supportive to mixed population. These members of the resident oral flora, including viridians group streptococci, are responsible for colonization resistance, i.e., they prevent colonization / invasion by more pathogenic exogenous bacterial pathogens. Disruption of the oral ecosystem may lead to impairment of colonization resistance leading to higher rates of colonization by more pathogenic exogenous bacterial pathogens. Thus, viridians group streptococci normally present in the oral cavity play a significant role in oral health.