MICROBIOLOGY

Streptococci and enterococci are gram-positive cocci that form chains when grown in liquid media.

• Culture on blood agar reveals three hemolytic patterns:
  • α-Hemolysis results in partial hemolysis that imparts a greenish appearance to agar. This pattern is seen with S. pneumoniae and viridans streptococci.
  • β-Hemolysis results in complete hemolysis around a colony. This pattern is seen with streptococci of Lancefield groups A, B, C, and G, which all form large (≥0.5-mm) colonies. Lancefield grouping is based on cell-wall carbohydrate antigens.
  • γ-Hemolysis describes the absence of hemolytic ability. This pattern is typical of enterococci, nonenterococcal group D streptococci, and anaerobic streptococci.
• Streptococci and enterococci colonize the respiratory, GI, and genitourinary tracts as part of the normal flora. Several of these species are also important causes of human diseases.