Hemolysin
Encyclopedia
Hemolysins are exotoxins produced by bacteria
that cause 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
and Staphylococcus
. 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 cell
s, often accounts for the effects of hemolysins in the host.
Most hemolysins are proteins, but others such as rhamnolipid
s are lipid biosurfactants.
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 phagosome
s. Hemolysins may also mediate bacterial escape from host cells.
It is now believed that many clinically relevant fungi also produce hemolysins.
alpha-Hemolysin forms a homo-heptameric beta-barrel in biological membranes (Protein Data Bank
:7AHL).
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that cause lysis
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
of red blood cells in vitro. Visualization of hemolysis of red blood cells in agar plates facilitates the categorization of some pathogenic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
such as Streptococcus
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...
and Staphylococcus
Staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....
. 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 cell
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
s, often accounts for the effects of hemolysins in the host.
Most hemolysins are proteins, but others such as rhamnolipid
Rhamnolipid
Rhamnolipids are a class of glycolipid produced by Pseudomonas aeruginosa, frequently cited as the best characterised of the bacterial surfactants They have a glycosyl head group, in this case a rhamnose moiety, and a 3-alkanoic acid fatty acid tail.Specifically there are two main classes of...
s are lipid biosurfactants.
Function
Bacteria may use hemolysins as a way to obtain nutrients from host cells. For example, iron may be a limiting factorLimiting factor
A limiting factor or limiting resource is a factor that controls a process, such as organism growth or species population, size, or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism...
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
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
. Lysis of these cells releases heme into the surroundings, allowing the bacteria to take up the free iron.
The regulation of gene expression
Regulation of gene expression
Gene modulation redirects here. For information on therapeutic regulation of gene expression, see therapeutic gene modulation.Regulation of gene expression includes the processes that cells and viruses use to regulate the way that the information in genes is turned into gene products...
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 phagosome
Phagosome
In cell biology, a phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested...
s. Hemolysins may also mediate bacterial escape from host cells.
It is now believed that many clinically relevant fungi also produce hemolysins.
3D structure of hemolysin
Staphylococcus aureusStaphylococcus aureus
Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...
alpha-Hemolysin forms a homo-heptameric beta-barrel in biological membranes (Protein Data Bank
Protein Data Bank
The Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids....
:7AHL).