ABSTRACT
Streptococci will appear under the microscope as round bacteria arranged in pairs or in chains. By the Gram-staining technique, they will be Gram-positive. The chaining characteristic is best observed when organisms are grown in liquid media or isolated from infected body fluids such as blood. Streptococci are differentiated as alpha, beta, and gamma types based on their activity on the surface of blood agar, which could differ somewhat based on the species and age of the red blood cells. Alpha-hemolytic streptococcal colonies are surrounded by a narrow zone of hemolysis that shows green discoloration based on the hemolysin's action on the hemoglobin; beta-hemolytic streptococci show a well-defined clear zone of hemolysis around the colony; gamma-hemolytic 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. Nearly all strains of Streptococcus pneumoniae are alpha-hemolytic but have been shown to exhibit beta-hemolysis during anaerobic incubation. Most oral streptococci and enterococci are nonhemolytic, 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. In general, strains that produce a hemolysin are professional pathogens, responsible for direct human and animal diseases, whereas nonhemolytic streptococci are mostly opportunistic pathogens.
