The designations 'group C Streptococcus' and 'group G Streptococcus' are used by clinical microbiology laboratories to denote clinical isolates of streptococci that react with Lancefield group C or G typing serum and, like S. pyogenes (group A Streptococcus), form large colonies on sheep blood agar, typically surrounded by a zone of beta-hemolysis.
Group C streptococci (GCS) and group G streptococci (GGS) are normal commensal flora of the human upper airway and frequently are asymptomatic colonizers of the skin, gastrointestinal tract and female genital tract. They are also implicated in mono- and polymicrobial infections of the skin and soft tissues, pharyngitis, bacteremia and endocarditis, septic arthritis and osteomyelitis, puerperal infections, and meningitis.
GCS and GGS of human origin are now considered to constitute a single subspecies, Streptococcus dysgalactiae subsp. equisimilis. A comparison of the complete genome sequence of a clinical isolate of GGS S. dysgalactiae subsp. equisimilis with that of other streptococcal species demonstrated it is most closely related to S. pyogenes, with 72 percent sequence similarity . S. dysgalactiae subsp. equisimilis shares many virulence determinants with S. pyogenes, including the antiphagocytic M protein, streptolysin O, streptolysin S, streptokinase, and one or more pyrogenic exotoxins similar to those implicated in streptococcal toxic shock [1,2].
GCS and GGS of animal origin are occasionally associated with human infection; these bacteria belong to distinct species. (See 'Microbiology' below.)
Streptococci in the S. anginosus or S. milleri group may also react with C or G typing sera, but these organisms are distinguished by the fact that they form small (<0.5 mm) colonies on sheep blood agar. They are normally identified by diagnostic laboratories as belonging to the S. anginosus or S. milleri group, not as GCS or GGS. (See "Infections due to the Streptococcus anginosus (Streptococcus milleri) group".)