Virulence

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Virulence is a pathogen's or microbe's ability to infect or damage a host.

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In the context of gene for gene systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host. [1] In most other contexts, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. [2] The pathogenicity of an organism - its ability to cause disease - is determined by its virulence factors. [3] [4] The noun virulence derives from the adjective virulent. Virulent can describe either disease severity or a pathogen's infectivity. [5] The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison." [5] [6]

In ecology, virulence is the host's parasite-induced loss of fitness. 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. [7]

Virulent bacteria

The ability of bacteria 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 special secretion machines such as the type 3 secretion system. 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.

The virulence factors of bacteria are typically proteins or other molecules that are synthesized by enzymes. These proteins are coded for by genes in chromosomal DNA, bacteriophage 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. [8] 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

Virulent viruses

Virus virulence factors allow it to replicate, modify host defenses, and spread within the host, and they are toxic to the host. [9]

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, 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. 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 within the virus population inside a host. The term "neurovirulent" is used for viruses such as rabies and herpes simplex which can invade the nervous system and cause disease there.

Extensively studied model organisms of virulent viruses include virus T4 and other T-even bacteriophages which infect Escherichia coli and a number of related bacteria.

The lytic life cycle of virulent bacteriophages is contrasted by the temperate lifecycle of Temperate bacteriophages. [10] [11]

Evolution

According to evolutionary medicine, optimal virulence 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.

See also

Related Research Articles

Bacteriophage virus that infects and replicates within bacteria

A bacteriophage, also known informally as a phage, is a virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν, meaning "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have structures that are either simple or elaborate. Their genomes may encode as few as four genes and as many as hundreds of genes. Phages replicate within the bacterium following the injection of their genome into its cytoplasm.

Virology study of viruses

Virology is the study of viral – submicroscopic, parasitic particles of genetic material contained in a protein coat – and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine.

<i>Yersinia pestis</i> Species of bacteria, cause of plague

Yersinia pestis is a gram-negative, non-motile, rod-shaped, coccobacillus bacterium, with no spores. It is a facultative anaerobic organism that can infect humans via the Oriental rat flea. It causes the disease plague, which takes three main forms: pneumonic, septicemic, and bubonic.

Infection Invasion of an organisms body tissues by disease-causing agents

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Plant pathology Scientific study of plant diseases

Plant pathology is the scientific study of diseases in plants caused by pathogens and environmental conditions. Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrate, or other pests that affect plant health by eating of plant tissues. Plant pathology also involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, and management of plant diseases.

Prophage

A prophage is a bacteriophage genome inserted and integrated into the circular bacterial DNA chromosome or exists as an extrachromosomal plasmid. This is a latent form of a phage, in which the viral genes are present in the bacterium without causing disruption of the bacterial cell. Pro means ''before'', so, prophage means the stage of a virus in the form of genome inserted into host DNA before being activated inside the host.

Viral pathogenesis is the study of how biological viruses cause diseases in their target hosts, usually carried out at the cellular or molecular level. It is a specialized field of study in virology.

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.

Lysogenic cycle

Lysogeny, or the lysogenic cycle, is one of two cycles of viral reproduction. Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome or formation of a circular replicon in the bacterial cytoplasm. In this condition the bacterium continues to live and reproduce normally. The genetic material of the bacteriophage, called a prophage, can be transmitted to daughter cells at each subsequent cell division, and at later events can release it, causing proliferation of new phages via the lytic cycle. Lysogenic cycles can also occur in eukaryotes, although the method of DNA incorporation is not fully understood.

Innate immune system defense responses mediated by germline encoded components that directly recognize components of potential pathogens.

The innate immune system is one of the two main immunity strategies found in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms.

Virulence factors are molecules produced by bacteria, viruses, fungi, and protozoa that add to their effectiveness and enable them to achieve the following:

<i>Aeromonas hydrophila</i> species of bacterium

Aeromonas hydrophila is a heterotrophic, Gram-negative, rod-shaped bacterium mainly found in areas with a warm climate. This bacterium can be found in fresh or brackish water. It can survive in aerobic and anaerobic environments, and can digest materials such as gelatin and hemoglobin. A. hydrophila was isolated from humans and animals in the 1950s. It is the most well known of the species of Aeromonas. It is resistant to most common antibiotics and cold temperatures and is oxidase- and indole-positive.

Host tropism is the infection specificity of certain pathogens to particular hosts and host tissues. This type of tropism explains why most pathogens are only capable of infecting a limited range of host organisms.

<i>Pasteurella multocida</i> species of bacterium

Pasteurella multocida is a Gram-negative, nonmotile, penicillin-sensitive coccobacillus belonging to the Pasteurellaceae family. Strains belonging to the species are currently classified into five serogroups based on capsular composition and 16 somatic serovars (1–16). P. multocida is the cause of a range of diseases in mammals and birds, including fowl cholera in poultry, atrophic rhinitis in pigs, and bovine hemorrhagic septicemia in cattle and buffalo. It can also cause a zoonotic infection in humans, which typically is a result of bites or scratches from domestic pets. Many mammals and birds harbor it as part of their normal respiratory microbiota.

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.

Microbial toxins are toxins produced by micro-organisms, including bacteria and fungi. Microbial toxins promote infection and disease by directly damaging host tissues and by disabling the immune system. Some bacterial toxins, such as Botulinum neurotoxins, are the most potent natural toxins known. However, microbial toxins also have important uses in medical science and research. Potential applications of toxin research include combating microbial virulence, the development of novel anticancer drugs and other medicines, and the use of toxins as tools in neurobiology and cellular biology.

In biology, a pathogen in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.

The host-pathogen interaction is defined as how microbes or viruses sustain themselves within host organisms on a molecular, cellular, organismal or population level. This term is most commonly used to refer to disease-causing microorganisms although they may not cause illness in all hosts. Because of this, the definition has been expanded to how known pathogens survive within their host, whether they cause disease or not.

Actinobacillus pleuropneumoniae, is a Gram-negative, facultative anaerobic, respiratory pathogen found in pigs. It was first reported in 1957, and was formally declared to be the causative agent of porcine pleuropneumonia in 1964. It was reclassified in 1983 after DNA studies showed it was more closely related to A. lignieresii.

Antivirulence is the concept of blocking virulence factors. In regards to bacteria, the idea is to design agents that block virulence rather than kill bacteria en masse, as the current regime results in much more selective pressure.

References

  1. Thrall, Peter H.; Burdon, Jeremy J. (2003-03-14). "Evolution of Virulence in a Plant Host-Pathogen Metapopulation". Science. 299 (5613): 1735–1737. doi:10.1126/science.1080070. ISSN   0036-8075. PMID   12637745.
  2. Pirofski LA, Casadevall A (2012). "Q and A: What is a pathogen? A question that begs the point". BMC Biology. 10: 6. doi:10.1186/1741-7007-10-6. PMC   3269390 . PMID   22293325.
  3. MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010
  4. Biology Online. "Virulence". Biology Online.
  5. 1 2 Compact Oxford English Dictionary virulent
  6. A Latin Dictionary virulentus
  7. Encyclopædia Britannica Online, 25 May 2009. "plant disease development"
  8. Keen, E. C. (December 2012). "Paradigms of pathogenesis: Targeting the mobile genetic elements of disease". Frontiers in Cellular and Infection Microbiology. 2: 161. doi:10.3389/fcimb.2012.00161. PMC   3522046 . PMID   23248780.
  9. Flint, S.Jane; Enquist, Lynn W.; Racaniello, Vincent R.; Skalka, Anna Marie (2009). Principles of Virology. Vol. II Pathogenesis and Control (3rd ed.). Washington, D.C.: ASM. pp. 42–47. ISBN   978-1-55581-480-9.
  10. Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN   978-0-13-144329-7.CS1 maint: extra text: authors list (link)
  11. Encyclopædia Britannica Online, 2009. "lytic phage"