The genus Streptococcus consists of gram-positive, "chain" forming, spherical bacterial cells. Cocci are each approximately one micron in diameter. The genomes of streptococci are typically AT-rich. Complex media are required for their in vitro cultivation. Streptococci are typically facultative anaerobes.
Based on their activity on the surface of blood agar, Streptococci are differentiated as alpha, beta, and gamma types. alpha-hemolytic streptococcal colonies are surrounded by a small zone of hemolysis that shows green discoloration based on the hemolysin's action on the hemoglobin; beta-hemolyticstreptococci show a larger well defined clear zone of hemolysis around the colony; gamma-hemolytjc streptococci have no effect on the red blood cells. Streptococcus pyogenes are nearly always beta-hemolytic whereas closely related Groups B and C streptococci usually appear as beta hemolytic colonies, but different strains can vary in their hemolytic activity. S. pneumoniae are nearly always alpha-hemolytic but have been shown to exhibit ß-hemolysis during anaerobic incubation. In contrast, most oral streptococci and enterococci are non hemolytic, and thus considered gamma types. The property of hemolysis is used in rapid screens for identification of S. pyogenes and S. pneumoniae, but may be unreliable for general differentiation of other streptococci.