Alpha-hemolysis When
alpha-hemolysis (α-hemolysis) is present, the agar under the colony is light and greenish.
Streptococcus pneumoniae and a group of oral streptococci (
Streptococcus viridans or viridans streptococci) display alpha-hemolysis. This is sometimes called
green hemolysis because of the color change in the agar. Other synonymous terms are
incomplete hemolysis and
partial hemolysis. Alpha-hemolysis is caused by the bacteria's production of
hydrogen peroxide, which oxidizes
hemoglobin and produces the green oxidized derivative
methemoglobin.
Beta-hemolysis Beta-hemolysis (β-hemolysis), sometimes called
complete hemolysis, is a complete
lysis of red cells in the media around and under the colonies: the area appears lightened (yellow) and transparent.
Streptolysin, an
exotoxin, is the enzyme produced by the bacteria which causes the complete lysis of red blood cells. There are two types of streptolysin: streptolysin O (SLO) and streptolysin S (SLS). Streptolysin O is an oxygen-sensitive cytotoxin secreted by most Group A streptococcus (GAS) and
Streptococcus dysgalactiae; it interacts with cholesterol in the membrane of eukaryotic cells (mainly red and white blood cells, macrophages, and platelets), usually resulting in β-hemolysis under the surface of blood agar. Streptolysin S is an oxygen-stable cytotoxin also produced by most GAS strains which results in clearing on the surface of blood agar. SLS affects immune cells, including
polymorphonuclear leukocytes and lymphocytes, and is thought to prevent the host immune system from clearing infection.
Streptococcus pyogenes, or Group A beta-hemolytic Strep (GAS), as well as
Streptococcus agalactiae display beta-hemolysis. The hemolysis of some weakly beta-hemolytic organisms is enhanced when streaked close to certain beta hemolytic strains of
Staphylococcus aureus. This phenomenon is the mechanism behind the
CAMP test, a test that was historically used for the identification of
Streptococcus agalactiae and
Listeria monocytogenes. A modified version of this test called the reverse CAMP test, utilizing
S. agalactiae instead of
S. aureus, can also be used to identify
Clostridium perfringens.
Gamma-hemolysis If an organism does not induce hemolysis, the agar under and around the colony is unchanged and the organism is called
non-hemolytic or said to display
gamma-hemolysis (γ-hemolysis).
Enterococcus faecalis (formerly called "Group D Strep"),
Staphylococcus saprophyticus, and
Staphylococcus epidermidis display gamma-hemolysis. ==Hemodigestion==