Virulence is, by disease - is determined by its virulence factors. The noun virulence derives from the adjective virulent. Virulent can describe either disease severity or a pathogen's infectivity. The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison."
In an ecological context, virulence can be defined as 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.
Virulent bacteria 1
- Methods by which bacteria cause disease 1.1
- Virulent viruses 2
- Evolution 3
- See also 4
- References 5
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. 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 receptors for bacteria are essential proteins for other functions. Due to presence of mucous 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 ulcer and cancer. 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 macro-pinocytosis 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.
Virus virulence factors 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.
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.
- Host-pathogen interface
- Membrane vesicle trafficking
- Bacterial effector protein
- Infectious disease
- Optimal virulence
- Verotoxin-producing Escherichia coli
- Virulence factor
- MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010
- Compact Oxford English Dictionary virulent
- A Latin Dictionary virulentus
- Encyclopædia Britannica Online, 25 May 2009. "plant disease development"
- Keen, E. C. (December 2012). "Paradigms of pathogenesis: Targeting the mobile genetic elements of disease". Frontiers in Cellular and Infection Microbiology 2: 161.
- YashRoy R C (2007) Mechanism of infection of human isolated Salmonella (3,10,r:-) in chicken ileum: Ultrastructural study. Indian Journal of Medical Research, vol. 126, pp. 558-566.http://icmr.nic.in/ijmr/2007/december/1211.pdf
- YashRoy R C (2003) Eucaryotic cell intoxication by Gram-negative pathogens: A novel bacterial outermembrane-bound vesicular exocytosis model for Type III secretion system. Toxicology International, vol. 10(1), pp. 1-9.http://www.researchgate.net/publication/230793514_Eukaryotic_cell_intoxication_by_Gram-negative_pathogens_A_novel_bacterial_outermembrane-bound_nanovesicular_exocytosis_model_for_Type-III_secretion_system._Toxicology_International_vol._10_No._1_pages_1-9_year_2003?ev=prf_pub
- Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms (11th ed.). Prentice Hall.
- Encyclopædia Britannica Online, 2009. "lytic phage"