Virulence factors (preferably known as pathogenicity factors or effectors in plant science) are cellular structures, molecules and regulatory systems that enable microbial pathogens (

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...

es,

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, and

protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

) to achieve the following: * colonization of a niche in the host (this includes movement towards and attachment to host cells) * immunoevasion, evasion of the host's immune response *

immunosuppression Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...

, inhibition of the host's immune response (this includes

leukocidin A leukocidin is a type of cytotoxin created by some types of bacteria (''Staphylococcus''). It is a type of pore-forming toxin. The model for pore formation is step-wise. First, the cytotoxin’s “S” subunit recognizes specific protein-containi ...

-mediated cell death) * entry into and exit out of cells (if the pathogen is an intracellular one) * obtain nutrition from the host Specific pathogens possess a wide array of virulence factors. Some are chromosomally encoded and intrinsic to the bacteria (e.g. capsules and

endotoxin Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer ...

), whereas others are obtained from

mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In h ...

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...

s and

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...

s (e.g. some exotoxins). Virulence factors encoded on mobile genetic elements spread through

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

, and can convert harmless bacteria into dangerous pathogens. Bacteria like '' Escherichia coli O157:H7'' gain the majority of their

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

from mobile genetic elements.

Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

secrete a variety of virulence factors at host-pathogen interface, via

membrane vesicle trafficking Membrane vesicle trafficking in eukaryotic animal cells involves movement of biochemical signal molecules from synthesis-and-packaging locations in the Golgi body to specific release locations on the inside of the plasma membrane of the secretory ...

bacterial outer membrane vesicles Bacterial outer membrane vesicles (OMVs) are vesicles of lipids released from the outer membranes of Gram-negative bacteria. These vesicles were the first bacterial membrane vesicles (MVs) to be discovered, while Gram-positive bacteria release ...

for invasion, nutrition and other cell-cell communications. It has been found that many pathogens have converged on similar virulence factors to battle against

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

host defenses. These obtained bacterial virulence factors have two different routes used to help them survive and grow: * The factors are used to assist and promote colonization of the host. These factors include adhesins,

invasin Invasins are a class of proteins associated with the penetration of pathogens into host cells. Invasins play a role in promoting entry during the initial stage of infection. In 2007, Als3 was identified as a fungal invasion allowing ''Candida alb ...

s, and antiphagocytic factors. * The factors, including

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

hemolysin Hemolysins or haemolysins are lipids and proteins that cause lysis of red blood cells by disrupting the cell membrane. Although the lytic activity of some microbe-derived hemolysins on red blood cells may be of great importance for nutrient acqu ...

s and

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

s, bring damage to the host.

Attachment, immunoevasion, and immunosuppression

Bacteria produce various

adhesins Adhesins are cell-surface components or appendages of bacteria that facilitate adhesion or adherence to other cells or to surfaces, usually in the host they are infecting or living in. Adhesins are a type of virulence factor. Adherence is an essent ...

including

lipoteichoic acid Lipoteichoic acid (LTA) is a major constituent of the cell wall of gram-positive bacteria. These organisms have an inner (or cytoplasmic) membrane and, external to it, a thick (up to 80 nanometer) peptidoglycan layer. The structure of LTA varies be ...

, trimeric autotransporter adhesins and a wide variety of other surface proteins to attach to host tissue. Capsules, made of carbohydrate, form part of the outer structure of many bacterial cells including ''

Neisseria meningitidis ''Neisseria meningitidis'', often referred to as meningococcus, is a Gram-negative bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life-threatening sepsis. The bacterium is referred to as a ...

''. Capsules play important roles in immune evasion, as they inhibit

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

, as well as protecting the bacteria while outside the host. Another group of virulence factors possessed by bacteria are

immunoglobulin An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

(Ig)

s. Immunoglobulins are antibodies expressed and secreted by hosts in response to an infection. These immunoglobulins play a major role in destruction of the pathogen through mechanisms such as

opsonization Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. ...

. Some bacteria, such as ''

Streptococcus pyogenes ''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus ''Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They are ...

'', are able to break down the host's immunoglobulins using proteases. Viruses also have notable virulence factors. Experimental research, for example, often focuses on creating environments that isolate and identify the role of "

niche Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development *Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ecology, the ...

-specific virulence genes". These are genes that perform specific tasks within specific tissues/places at specific times; the sum total of niche-specific genes is the virus'

. Genes characteristic of this concept are those that control latency in some viruses like herpes.

Murine The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families excep ...

gamma herpesvirus 68 (γHV68) and human herpesviruses depend on a subset of genes that allow them to maintain a chronic infection by reactivating when specific environmental conditions are met. Even though they are not essential for

lytic The lytic cycle ( ) is one of the two cycles of viral reproduction (referring to bacterial viruses or bacteriophages), the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane. Bacteri ...

phases of the virus, these latency genes are important for promoting chronic infection and continued replication within infected individuals.

Destructive enzymes

Some bacteria, such as ''

'', ''

Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...

'' and ''

Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerugi ...

'', produce a variety of enzymes which cause damage to host tissues. Enzymes include

hyaluronidase Hyaluronidases are a family of enzymes that catalyse the degradation of hyaluronic acid (HA). Karl Meyer classified these enzymes in 1971, into three distinct groups, a scheme based on the enzyme reaction products. The three main types of hyal ...

, which breaks down the connective tissue component

hyaluronic acid Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...

; a range of proteases and

lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...

s; DNases, which break down DNA, and hemolysins which break down a variety of host cells, including red blood cells.

GTPases

A major group of virulence factors are proteins that can control the activation levels of

GTPase GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a pro ...

s. There are two ways in which they act. One is by acting as a GEF or GAP, and proceeding to look like a normally eukaryotic cellular protein. The other is covalently modifying the GTPase itself. The first way is reversible; many bacteria like Salmonella have two proteins to turn the GTPases on and off. The other process is irreversible, using toxins to completely change the target GTPase and shut down or override gene expression. One example of a bacterial virulence factor acting like a eukaryotic protein is Salmonella protein SopE it acts as a GEF, turning the GTPase on to create more GTP. It does not modify anything, but overdrives normal cellular internalization process, making it easier for the Bacteria to be colonized within a host cell. YopT (''Yersinia'' outer protein T) from

Yersinia ''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of ''Ye ...

is an example of modification of the host. It modifies the proteolytic cleavage of carboxyl terminus of RhoA, releasing RhoA from the membrane. The mislocalization of RhoA causes downstream effectors to not work.

Toxins

A major category of virulence factors are bacterial toxins. These are divided into two groups:

endotoxins Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer me ...

and

exotoxins An exotoxin is a toxin secreted by bacteria. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host. Exotoxins may be secreted, or, simi ...

Endotoxins

Endotoxin is a component ( lipopolysaccharide (LPS)) of the cell wall of gram-negative bacteria. It is the

lipid A Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also called endotoxin molecule, and its hydrophobic nature allows it ...

part of this LPS which is toxic. Lipid A is an endotoxin. Endotoxins trigger intense inflammation. They bind to receptors on

monocytes Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also infl ...

causing the release of inflammatory mediators which induce

degranulation Degranulation is a cellular process that releases antimicrobial cytotoxic or other molecules from secretory vesicles called granules found inside some cells. It is used by several different cells involved in the immune system, including granulo ...

. As part of this immune response cytokines are released; these can cause the fever and other symptoms seen during disease. If a high amount of LPS is present then septic shock (or endotoxic shock) may result which, in severe cases, can lead to death. As glycolipids (as opposed to peptides), endotoxins are not bound by B or T-cell receptors and do not elicit an adaptive immune response.

Exotoxins

Exotoxins are actively secreted by some bacteria and have a wide range of effects including inhibition of certain biochemical pathways in the host. The two most potent known exotoxins are the tetanus toxin (

tetanospasmin Tetanus toxin (TeNT) is an extremely potent neurotoxin produced by the vegetative cell of ''Clostridium tetani'' in anaerobic conditions, causing tetanus. It has no known function for clostridia in the soil environment where they are normally ...

) secreted by ''

Clostridium tetani ''Clostridium tetani'' is a common soil bacterium and the causative agent of tetanus. Vegetative cells of ''Clostridium tetani'' are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped when ...

'' and the

botulinum toxin Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromusc ...

secreted by ''

Clostridium botulinum ''Clostridium botulinum'' is a Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce the neurotoxin botulinum. The botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans an ...

''. Exotoxins are also produced by a range of other bacteria including ''

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

''; ''

Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...

'' (causative agent of

cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...

); ''

Clostridium perfringens ''Clostridium perfringens'' (formerly known as ''C. welchii'', or ''Bacillus welchii'') is a Gram-positive, rod-shaped, anaerobic, spore-forming pathogenic bacterium of the genus '' Clostridium''. ''C. perfringens'' is ever-present in nature an ...

'' (common causative agent of

food poisoning Foodborne illness (also foodborne disease and food poisoning) is any illness resulting from the spoilage of contaminated food by pathogenic bacteria, viruses, or parasites that contaminate food, as well as prions (the agents of mad cow disease) ...

as well as

gas gangrene Gas gangrene (also known as clostridial myonecrosis and myonecrosis) is a bacterial infection that produces tissue gas in gangrene. This deadly form of gangrene usually is caused by ''Clostridium perfringens'' bacteria. About 1,000 cases of gas g ...

) and '' Clostridium difficile'' (causative agent of

pseudomembranous colitis Colitis is swelling or inflammation of the large intestine ( colon). Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases. In a medical context, the label ''colitis'' (without qualification) ...

). A potent three-protein virulence factor produced by ''

Bacillus anthracis ''Bacillus anthracis'' is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent ( obligate) pathogen within the genus ''Bacillus''. Its infection is a ...

'', called

anthrax toxin Anthrax toxin is a three-protein exotoxin secreted by virulent strains of the bacterium, ''Bacillus anthracis''—the causative agent of anthrax. The toxin was first discovered by Harry Smith in 1954. Anthrax toxin is composed of a cell-binding ...

, plays a key role in

anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day and more than two months after the infection is contracted. The sk ...

pathogenesis. Exotoxins are extremely immunogenic meaning that they trigger the humoral response (antibodies target the toxin). Exotoxins are also produced by some

as a competitive resource. The toxins, named

mycotoxin A mycotoxin (from the Greek μύκης , "fungus" and τοξίνη , "toxin") is a toxic secondary metabolite produced by organisms of kingdom Fungi and is capable of causing disease and death in both humans and other animals. The term 'mycotoxin' ...

s, deter other organisms from consuming the food colonised by the fungi. As with bacterial toxins, there is a wide array of fungal toxins. Arguably one of the more dangerous mycotoxins is

aflatoxin Aflatoxins are various poisonous carcinogens and mutagens that are produced by certain molds, particularly ''Aspergillus'' species. The fungi grow in soil, decaying vegetation and various staple foodstuffs and commodities such as hay, sweetcorn ...

produced by certain species of the genus ''

Aspergillus ' () is a genus consisting of several hundred mold species found in various climates worldwide. ''Aspergillus'' was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Miche ...

'' (notably '' A. flavus''). If ingested repeatedly, this toxin can cause serious liver damage.

Examples

Examples of virulence factors for ''

'' are

coagulase Coagulase is a protein enzyme produced by several microorganisms that enables the conversion of fibrinogen to fibrin. In the laboratory, it is used to distinguish between different types of '' Staphylococcus'' isolates. Importantly, '' S. aureus' ...

deoxyribonuclease Deoxyribonuclease (DNase, for short) refers to a group of glycoprotein endonucleases which are enzymes that catalyze the hydrolytic cleavage of phosphodiester linkages in the DNA backbone, thus degrading DNA. The role of the DNase enzyme in cells ...

s and enterotoxins. Examples for ''

'' are M protein,

capsule, destructive enzymes (including

streptokinase Streptokinase (SK) is a thrombolytic medication activating plasminogen by nonenzymatic mechanism. As a medication it is used to break down clots in some cases of myocardial infarction (heart attack), pulmonary embolism, and arterial thromboemboli ...

, streptodornase, and

), and

(including

streptolysin Streptolysins are two hemolytic exotoxins from ''Streptococcus''. Types include streptolysin O (SLO; ''slo''), which is oxygen-labile, and streptolysin S (SLS; ''sagA''), which is oxygen-stable. SLO is part of the thiol-activated cytolysin fami ...

). Examples for ''

Listeria monocytogenes ''Listeria monocytogenes'' is the species of pathogenic bacteria that causes the infection listeriosis. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. It can grow and reproduce inside the host' ...

'' include internalin A, internalin B,

listeriolysin O Listeriolysin O (LLO) is a hemolysin produced by the bacterium ''Listeria monocytogenes'', the pathogen responsible for causing listeriosis. The toxin may be considered a virulence factor, since it is crucial for the virulence of ''L. monocytogenes ...

, and actA, all of which are used to help colonize the host. Examples for ''

Yersinia pestis ''Yersinia pestis'' (''Y. pestis''; formerly '' Pasteurella pestis'') is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both ''Yersinia pseudotuberculosis'' and ''Yersinia enterocolitica''. It is a facult ...

'' are an altered form of lipopolysaccharide, type three secretion system, and YopE and YopJ pathogenicity. The cytolytic peptide

Candidalysin Candidalysin is a cytolytic 31-amino acid α-helical amphipathic peptide toxin secreted by the opportunistic pathogen ''Candida albicans''. This toxin is a fungal example of a classical virulence factor. Hyphal morphogenesis in ''C. albicans'' is ...

is produced during

hyphal A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

formation by ''

Candida albicans ''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is us ...

''; it is an example of a virulence factor from a fungus. Other virulence factors include factors required for

biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...

formation (e.g.

sortase Sortase refers to a group of prokaryotic enzymes that modify surface proteins by recognizing and cleaving a carboxyl-terminal sorting signal. For most substrates of sortase enzymes, the recognition signal consists of the motif LPXTG (Leu-Pro-any ...

s) and

integrin Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, ...

s (e.g. beta-1 and 3).

Inhibition and control

Strategies to target virulence factors and the genes encoding them have been proposed.

Small molecule Within the fields of molecular biology and pharmacology, a small molecule or micromolecule is a low molecular weight (≤ 1000 daltons) organic compound that may regulate a biological process, with a size on the order of 1 nm. Many drugs ar ...

s being investigated for their ability to inhibit virulence factors and virulence factor

expression Expression may refer to: Linguistics * Expression (linguistics), a word, phrase, or sentence * Fixed expression, a form of words with a specific meaning * Idiom, a type of fixed expression * Metaphorical expression, a particular word, phrase, o ...

include

alkaloid Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

s, and

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...

s. Experimental studies are done to characterize specific bacterial pathogens and to identify their specific virulence factors. Scientists are trying to better understand these virulence factors through identification and analysis to better understand the infectious process in hopes that new diagnostic techniques, specific antimicrobial compounds, and effective vaccines or toxoids may be eventually produced to treat and prevent infection. There are three general experimental ways for the virulence factors to be identified: biochemically, immunologically, and genetically. For the most part, the genetic approach is the most extensive way in identifying the bacterial virulence factors. Bacterial DNA can be altered from pathogenic to non-pathogenic, random mutations may be introduced to their genome, specific genes encoding for membrane or secretory products may be identified and mutated, and genes that regulate virulence genes maybe identified. Experiments involving ''

Yersinia pseudotuberculosis ''Yersinia pseudotuberculosis'' is a Gram-negative bacterium that causes Far East scarlet-like fever in humans, who occasionally get infected zoonotically, most often through the food-borne route. Animals are also infected by ''Y. pseudotuber ...

'' have been used to change the virulence phenotype of non-pathogenic bacteria to pathogenic. Because of horizontal gene transfer, it is possible to transfer the a clone of the DNA from ''Yersinia'' to a non-pathogenic ''E. coli'' and have them express the pathogenic virulence factor.

Transposon A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transpo ...

, a DNA element inserted at random, mutagenesis of bacteria DNA is also a highly used experimental technique done by scientists. These transposons carry a marker that can be identified within the DNA. When placed at random, the transposon may be placed next to a virulence factor or placed in the middle of a virulence factor gene, which stops the expression of the virulence factor. By doing so, scientists can make a library of the genes using these markers and easily find the genes that cause the virulence factor.

References

{{DEFAULTSORT:Virulence Factor