Viridans streptococci are a heterogeneous group of microorganisms that produce (partial) α-hemolysis or no hemolysis when grown on sheep blood agar. Various species can be distinguished according to their physiologic and biochemical characteristics, more than 20 of which have been associated with human infections. Typically, however, clinical laboratories speciate isolates only if they are recovered from blood or other usually sterile sites. Isolates recovered in mixed culture or from mucosal surfaces are normally reported only as nonhemolytic or α-hemolytic streptococci. Table 151.1 lists the most common species of viridans streptococci isolated from blood cultures.

Viridans streptococci are an important part of the normal microbial flora in humans. They are indigenous to the oral cavity, the upper respiratory tract, the female genital tract, and the gastrointestinal tract. Because these microorganisms lack traditional virulence factors, such as endo- and exotoxins, viridans streptococci are often considered to be of low virulence. However, a propensity to adhere to endovascular tissues accounts for their ability to produce endocarditis; extracellular dextran plays an important role in adherence and propagation of viridans streptococci on cardiac valves. Furthermore, some viridans streptococci are known to cause abscess formation, particularly the Streptococcus milleri group, composed of Streptococcus intermedius, Streptococcus constellatus, and Streptococcus anginosus.