Authored by: Vincent A. Fischetti , Patricia Ryan

Practical Handbook of Microbiology

Print publication date:  June  2015
Online publication date:  June  2015

Print ISBN: 9781466587397
eBook ISBN: 9781466587403
Adobe ISBN:


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Streptococci will appear under the microscope as nonmotile 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 blood cells visualized 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; and gamma-hemolytic streptococci have no effect on the red blood cells. Streptococcus pyo-genes (or group A streptococci) 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 α-hemolytic but have been shown to exhibit β-hemolysis during anaerobic incubation. Most oral streptococci and enterococci are nonhemolytic and thus considered γ-types. The property of hemolysis is used in rapid screens for the identification of S. pyogenes and S. pneumoniae, but may be unreliable for general differentiation of other streptococci.

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