Clinical Pathology: Microbiology

• Beta-hemolytic streptococci have been classified as group A (Streptococcus pyogenes), group B (S. agalactiae), and groups C and G. Recently the large colony variants (>0.5 mm) of Streptococcus groups C and G were reclassified as S. dysgalactiae subsp. equisimilis (SDSE). These strains are pyogenic and often cause pharyngitis in adolescents. The small colony variants (≤0.5 mm) of groups C and G have been placed in the S. anginosus group, which are often harmless commensals that can occasionally cause abscesses.

• Group A Streptococcus (GAS) remains the primary cause of bacterial pharyngitis, particularly in children. There are a multitude of rapid GAS antigen detection tests and they vary greatly in their performance characteristics. False-negative reactions may be related to poor sensitivity, inadequate specimen collection, or a low bacterial load at the site of infection. Therefore, it is recommended that negative rapid tests be backed up with culture. Culture also offers the advantage of detecting non-GAS causes of pharyngitis.

• SDSE is a common cause of upper respiratory tract infections and also infects skin and soft tissues, causing cellulitis, abscesses, and necrotizing fasciitis. SDSE shares many of the same virulence factors as GAS, including adhesions, pyogenic exotoxins, the M protein, and streptokinases.

• Other than colony size, SDSE strains can be differentiated from members of the S. anginosus group by a negative Voges-Proskauer reaction.

• SDSE isolates are uniformly susceptible to penicillin, which is the drug of choice. Severe infections may require the addition of an aminoglycoside; clindamycin is typically added for cases of toxic shock syndrome.