Streptococci

Streptococci are spherical, gram-positive bacteria that divide along a single axis and therefore appear in chains or pairs on Gram stain. The defining laboratory feature separating them from staphylococci, which cluster in grape-like groups, is that streptococci are catalase-negative while staphylococci are catalase-positive.

Classification proceeds by hemolysis pattern on blood agar. Alpha-hemolytic organisms partially lyse red cells, producing a green discoloration; this group includes Streptococcus pneumoniae (optochin-sensitive, bile-soluble) and the viridans streptococci (optochin-resistant). Beta-hemolytic organisms lyse red cells completely, producing a clear zone; this group is further split by Lancefield group antigen into group A Streptococcus pyogenes (bacitracin-sensitive) and group B Streptococcus agalactiae (bacitracin-resistant, CAMP test positive). Gamma-hemolytic organisms cause no hemolysis and include the non-enterococcal group D streptococci.

Clinically, the species differ sharply. S. pyogenes causes pharyngitis, impetigo, erysipelas, cellulitis, scarlet fever, and necrotizing fasciitis, and its infections can be followed by the immune-mediated sequelae of rheumatic fever and post-streptococcal glomerulonephritis. S. agalactiae is a leading cause of neonatal sepsis and meningitis, which is why pregnant patients are screened for colonization. S. pneumoniae is a leading cause of community-acquired pneumonia, otitis media, sinusitis, and bacterial meningitis. Viridans streptococci contribute to dental caries and subacute bacterial endocarditis.

Streptococci are heavily tested on USMLE Step 1 microbiology, where questions typically supply a Gram stain description plus a hemolysis or sensitivity result and expect species identification. They also appear on the CCMA exam in the context of skin lesions and infectious diseases of the integumentary system, such as impetigo and cellulitis.