Hemolysin

Hemolysins are exotoxin protein produced by bacteria which causes lysis of red blood cells in vitro. Visualization of hemolysis of red blood cells in agar plates facilitates the categorization of some pathogenic bacteria such as Streptococcus. Although the lytic activity of some hemolysins on red blood cells may be important for nutrient acquisition or for causing certain conditions such as anemia, many hemolysin-producing pathogens do not cause significant lysis of red blood cells during infection. Although hemolysins are able to lyse red blood cells in vitro, the ability of hemolysins to target other cells, including white blood cells, often accounts for the effects of hemolysins in the host.

Purpose of hemolysin
Bacteria may use hemolysins as a way to obtain nutrients from host cells. For example, iron may be a limiting factor in the growth of various pathogenic bacteria. Since free iron may generate damaging free radicals, free iron is typically maintained at low concentrations within the body. Red blood cells are rich in iron-containing heme. Lysis of these cells releases heme into the surroundings, allowing the bacteria to take up the free iron.

The regulation of gene expression of hemolysins (such as streptolysin S) is a system repressed in the presence of iron. This ensures that hemolysin is produced only when needed.

Some hemolysins, such as listeriolysin O, allow bacteria to evade the immune system by escaping from phagosomes. Hemolysins may also mediate bacterial escape from host cells.

3D structure of hemolysin
Hemolysin forms a heptameric beta-barrel in biological membranes.