Some bacteria are capable of producing substances (Hemolysin) that breakdown Red blood cells and this ability is useful in classifying these bacteria(Hemolysis test) particular streptococcal species which is more demonstrated in growth media like blood agar.Therefore in order to manifest hemolysis,blood or red blood cells must be involved
The action of hemolysins is to damage the host cytoplasmic membrane, causing cell lysis and death. Hemolysin activity is most easily observed with assays involving the lysis of red blood cells (erythrocytes). Some hemolysins attack the phospholipid of the host cytoplasmic membrane. Because the phospholipid lecithin (phosphatidylcholine) is often used as a substrate, these enzymes are called lecithinase or phospholipases. Some hemolysins also affects the sterols of the host cytoplasmic membrane.
In bacteriology,there are three types of hemolysis observed in some bacteria as discuss below:
The presence of alpha hemolysis (α-hemolysis) is indicated when the agar under the suspected colony is dark and greenish. Alpha hemolysis is a sort of partial or incomplete hemolysis and is sometimes refers to as green hemolysis because of the color change in the agar.Alpha hemolysis is caused by hydrogen peroxide produced by some bacteria which oxidizes hemoglobin producing the green oxidized derivative know as methemoglobin.
Beta hemolysis is a complete lysis of blood product specifically red cells in the media around and under the colonies.It is also known as complete hemolysis and the area with beta hemolysis appears lightened (yellow) and transparent.
This is commonly caused by an exotoxin enzyme called streptolysin produced by beta hemolytic bacteria which causes the complete lysis of red blood cells.
The’re two types of these streptolysin enzymes which are
- Streptolysin O (SLO)
- Streptolysin S (SLS).
The enzyme Streptolysin O is an oxygen-sensitive cytotoxin, It is by most Group A streptococcus and Streptococcus dysgalactiae which interacts with cholesterol in the membrane of eukaryotic cells (mainly red and white blood cells, macrophages, and platelets), and usually results in β-hemolysis which is observed under the surface of blood agar.
Meanwhile Streptolysin S is an oxygen-stable cytotoxin also which is also produced by most Group A streptococcus strains which results in clearing on the surface of blood agar. streptolysin S affects immune cells, including polymorphonuclear leukocytes and lymphocytes, and is thought to prevent the host immune system from clearing infection.Bacteria species like Streptococcus pyogenes, or Group A beta-hemolytic Streptococci displays beta hemolysis.
There are also some weakly beta-hemolytic species that causes intense beta hemolysis when grown together with a strain of Staphylococcus in a CAMP test.
Also Streptococcus agalactiae displays this property. Clostridium perfringens can be identified presumptively with this test. Listeria monocytogenes is also positive beta hemolytic on sheep’s blood agar.
Those organism unable to display Alpha or Beta hemolysis during
hemolysis test are term Gamma hemolytic organism. the agar under and around the colony remains unchanged, and the organism is called non-hemolytic or said to display gamma hemolysis because they don’t possesses hemolytic enzymes.Examples of such bacteria are Enterococcus faecalis, Staphylococcus saprophyticus, and Staphylococcus epidermidis
Hemodigestion is a nonspecific killing of blood cells by the action of metabolic by-products of bacteria. It can be observed on a blood agar plate, when the blood surrounding the confluent part of the streak turns green, but there is no visible change around single colonies. Hemodigestion is mostly seen with Vibrio cholerae.
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