infectious diseases
Streptococcal Infections
Classification of Streptococci
Three different types of streptococci are initially differentiated by their appearance when they are grown in culture on sheep blood agar: Beta-hemolytic streptococci produce zones of clear hemolysis around each colony. Alpha-hemolytic streptococci (commonly called viridans streptococci) are surrounded by green discoloration resulting from incomplete hemolysis. Gamma-hemolytic streptococci are nonhemolytic. *). In the Lancefield classification, Enterococcal Infections were initially included among the group D streptococci but are now classified as a separate genus even though they do express Lancefield group D antigens. Lancefield groups K through V are streptococcal species of limited virulence that can cause infections in people who are immunocompromised. Viridans streptococci form a separate group that is difficult to classify. Some streptococci such as Pneumococcal Infections are alpha-hemolytic, ie, they are a type of viridans streptococci and do not express Lancefield antigens. 