Hemolysis is the breakdown of red blood cells (RBC). A substance that causes hemolysis is a hemolysin. Brown (1919) introduced three terms alpha, beta and gamma to indicate three types of streptococci based on haemolytic reactions observed on blood agar plates.
Beta-hemolysis (β-hemolysis) is associated with complete lysis of red cells surrounding the colony. Beta hemolysis is caused by two hemolysins O and S; the former is inactive in the presence of oxygen. Thus, stabbing of the plate increases the intensity of the hemolysis reaction. S is an oxygen-stable cytotoxin.
It exhibit a wide zone (2-4 mm wide). Beta hemolysis is more marked when the plate has been incubated anaerobically. They are generally commensals of throat and causes opportunistic infections.
Examples: Streptococcus pyogenes, or Group A beta-hemolytic Strep (GAS).
Weakly beta-hemolytic species: Streptococcus agalactiae, Clostridium perfringens, Listeria monocytogenes
Alpha-hemolysis (α-hemolysis) is a partial or “green” hemolysis associated with reduction of red cell hemoglobin. Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green methemoglobin.
It exhibit incomplete haemolysis with 1-2 mm wide. Persistence of some unhaemolysed RBC’s can be seen microscopically.
Examples: Streptococcus pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci)
Gamma-hemolytic (Non-haemolytic) Streptococci
Colonies show neither typical alpha nor beta haemolysis. There may be, however, slight discoloration in the medium. The streptococci included in this group are usually not pathogenic.
Examples: Enterococcus faecalis (formerly called “Group D Strep”)