Streptococcal infections are extremely common despite the overall susceptibility of these organisms to antibiotics, especially penicillin. While infections caused by the Lancefield group A streptococcus (GAS or Streptococcus pyogenes) have dominated the streptococcal medical literature, Lancefield groups C and G share many microbiologic and clinical characteristics with GAS . (See related topic reviews on GAS).
Work in the early 20th century described S. pyogenes as an exclusively human pathogen and detailed the frequency of carriers and the most characteristic infections . The pioneering work of Rebecca Lancefield led to the identification of a number of groups of streptococci including A, C and G, producing hemolysis on sheep blood agar, exhibiting different biochemical properties and isolated from a variety of animal species . Classification methods based upon the Lancefield methodology were established soon thereafter .
Among this diverse group, the group C and group G streptococci have assumed more important clinical roles. For a number of reasons, they can be considered together, separate from other members of the genus Streptococcus. Although less extensively studied, groups C and G streptococci are now appreciated to produce infections quite similar to GAS although they more commonly cause opportunistic and nosocomial infections than GAS.
- They are widely distributed in the animal kingdom and are also part of the normal human flora.
- The microbiology is similar to GAS; they are primarily large colony, beta-hemolytic, and produce pyogenic and inflammatory reactions. On plates, colonies often resemble S. pyogenes, especially when susceptible to bacitracin.
- They cause isolated exudative or common source epidemic pharyngitis and cellulitis indistinguishable clinically from GAS disease.
- Some strains contain fibrinolysins and streptolysins and infections can stimulate antistreptolysin O titers (ASO), similar to S. pyogenes.
Groups C and G streptococci are typical chaining cocci that are Gram positive, facultative anaerobes which produce small or large colonies on sheep blood agar. They are usually, but not exclusively, beta hemolytic. Unlike GAS, which are inhibited by 0.04 units/mL of bacitracin, the group C and G isolates are extremely variable in their bacitracin sensitivity, ranging from as few as 6 to 8 percent  to as many as 30 to 67 percent . This variability should discourage the use of bacitracin as a screening step.