Virulence is a

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...

's or

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...

'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 cause

disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

—is determined by its virulence factors. In the specific context of gene for gene systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host. The

noun A noun () is a word that generally functions as the name of a specific object or set of objects, such as living creatures, places, actions, qualities, states of existence, or ideas.Example nouns for: * Living creatures (including people, alive, ...

''virulence'' derives from the

adjective In linguistics, an adjective ( abbreviated ) is a word that generally modifies a noun or noun phrase or describes its referent. Its semantic role is to change information given by the noun. Traditionally, adjectives were considered one of the ...

''virulent'', meaning disease severity. The word ''virulent'' derives from the Latin word ''virulentus'', meaning "a poisoned wound" or "full of poison." From an ecological standpoint, virulence is the loss of fitness induced by a parasite upon its host. Virulence can be understood in terms of proximate causes—those specific traits of the pathogen that help make the host ill—and ultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.

Virulent bacteria

The ability of

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 am ...

to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Many virulence factors are so-called effector proteins that are injected into the host cells by specialized secretion apparati, such as the

type three secretion system Type three secretion system (often written Type III secretion system and abbreviated TTSS or T3SS, also called Injectisome) is a protein appendage found in several Gram-negative bacteria. In pathogenic bacteria, the needle-like structure is u ...

. Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered (e.g.,

cytokine storm A cytokine storm, also called hypercytokinemia, is a physiological reaction in humans and other animals in which the innate immune system causes an uncontrolled and excessive release of pro-inflammatory signaling molecules called cytokines. Norma ...

). The virulence factors of bacteria are typically proteins or other molecules that are synthesized by

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

s. These proteins are coded for by genes in

chromosomal A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

DNA,

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". Bac ...

DNA or plasmids. Certain bacteria employ mobile genetic elements and horizontal gene transfer. Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed. Bacteria use quorum sensing to synchronise release of the molecules. These are all proximate causes of morbidity in the host.

Methods by which bacteria cause disease

; Adhesion: Many bacteria must first bind to host cell surfaces. Many bacterial and host molecules that are involved in the adhesion of bacteria to host cells have been identified. Often, the host

cell surface receptors Cell surface receptors (membrane receptors, transmembrane receptors) are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving (binding to) extracellular molecules. They are specialized integral m ...

for bacteria are essential proteins for other functions. Due to the presence of

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

lining and of anti-microbial substances around some host cells, it is difficult for certain pathogens to establish direct contact-adhesion. ; Colonization: Some virulent bacteria produce special proteins that allow them to colonize parts of the host body. '' Helicobacter pylori'' is able to survive in the acidic environment of the human stomach by producing the enzyme urease. Colonization of the stomach lining by this bacterium can lead to

gastric ulcers Peptic ulcer disease (PUD) is a break in the inner lining of the stomach, the first part of the small intestine, or sometimes the lower esophagus. An ulcer in the stomach is called a gastric ulcer, while one in the first part of the intestines i ...

and

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

. The virulence of various strains of ''Helicobacter pylori'' tends to correlate with the level of production of urease. ; Invasion: Some virulent bacteria produce proteins that either disrupt host cell membranes or stimulate their own endocytosis or

macropinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...

into host cells. These virulence factors allow the bacteria to enter host cells and facilitate entry into the body across epithelial tissue layers at the body surface. ; Immune response inhibitors: Many bacteria produce virulence factors that inhibit the host's immune system defenses. For example, a common bacterial strategy is to produce proteins that bind host antibodies. The polysaccharide capsule of '' Streptococcus pneumoniae'' inhibits phagocytosis of the bacterium by host immune cells. ; Toxins: Many virulence factors are proteins made by bacteria that poison host cells and cause tissue damage. For example, there are many food poisoning

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 ...

s produced by bacteria that can contaminate human foods. Some of these can remain in "spoiled" food even after cooking and cause illness when the contaminated food is consumed. Other bacterial toxins are chemically altered and inactivated by the heat of cooking.

Virulent viruses

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 Ivanovsk ...

virulence factors allow it to replicate, modify host defenses, and spread within the host, and they are toxic to the host. They determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are endocytosed and the bound virus then enters the host cell. Virulent viruses such as HIV, which causes AIDS, have mechanisms for evading host defenses. HIV infects

T-helper cells The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are consider ...

, which leads to a reduction of the adaptive immune response of the host and eventually leads to an immunocompromised state. Death results from opportunistic infections secondary to disruption of the immune system caused by AIDS. Some viral virulence factors confer ability to replicate during the defensive inflammation responses of the host such as during virus-induced

fever Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using val ...

. Many viruses can exist inside a host for long periods during which little damage is done. Extremely virulent strains can eventually evolve by mutation and

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...

within the virus population inside a host. The term " neurovirulent" is used for viruses such as

rabies Rabies is a viral disease that causes encephalitis in humans and other mammals. Early symptoms can include fever and tingling at the site of exposure. These symptoms are followed by one or more of the following symptoms: nausea, vomiting, ...

and herpes simplex which can invade the

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

and cause disease there. Extensively studied model organisms of virulent viruses include virus T4 and other

T-even bacteriophages Escherichia virus T4 is a species of bacteriophages that infect ''Escherichia coli'' bacteria. It is a double-stranded DNA virus in the subfamily '' Tevenvirinae'' from the family Myoviridae. T4 is capable of undergoing only a lytic lifecycle ...

which infect

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 '' Esc ...

and a number of related

. The lytic life cycle of virulent bacteriophages is contrasted by the temperate lifecycle of temperate bacteriophages.Encyclopædia Britannica Online, 2009
"lytic phage"
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Evolution

According to

evolutionary medicine Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. Modern biomedical research and practice have focused on the molecular and physiological mechanisms underlying he ...

optimal virulence Optimal virulence is a concept relating to the ecology of hosts and parasites. One definition of virulence is the host's parasite-induced loss of fitness. The parasite's fitness is determined by its success in transmitting offspring to other hos ...

increases with horizontal transmission (between non-relatives) and decreases with vertical transmission (from parent to child). This is because the fitness of the host is bound to the fitness in vertical transmission but is not so bound in horizontal transmission.

References

{{Medical research studies Virology

Virulent bacteria

Methods by which bacteria cause disease

Virulent viruses

Evolution

See also

References