VIRULENCE is a pathogen or microbe's ability to infect or damage a
In the context of gene for gene systems, often in plants, virulence
refers to a pathogen's ability to infect a resistant host. In most
other contexts, 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 - 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.
* 1 Virulent bacteria
* 1.1 Methods by which bacteria cause disease
* 2 Virulent viruses
* 4 See also
* 5 References
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
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 Colonization 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
* 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.
* 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 toxins 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. Some
bacterial toxins are chemically altered and inactivated by the heat of
Virus virulence factors allow it replicate, modify host defenses,
allow it to spread within the host, and 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,
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.
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.
Membrane vesicle trafficking
Bacterial effector protein
Wikimedia Commons has media related to VIRULENCE .
* ^ Thrall, Peter H.; Burdon, Jeremy J. (2003-03-14). "
Virulence in a Plant Host-Pathogen Metapopulation". Science. 299
(5613): 1735–1737. ISSN 0036-8075 . PMID 12637745 . doi
* ^ Pirofski LA, Casadevall A (2012). "Q and A: What is a pathogen?
A question that begs the point" . BMC Biology. 10: 6. PMC 3269390 .
PMID 22293325 . doi :10.1186/1741-7007-10-6 .
* ^ MeSH - Medical Subject Headings, Karolinska Institute, 13 April
* ^ A B Compact Oxford English Dictionary virulent
* ^ A Latin Dictionary virulentus
* ^ Encyclopædia Britannica Online, 25 May 2009. "plant disease
* ^ Keen, E. C. (December 2012). "Paradigms of pathogenesis:
Targeting the mobile genetic elements of disease" . Frontiers in
Cellular and Infection Microbiology. 2: 161. PMC 3522046 . PMID
23248780 . doi :10.3389/fcimb.2012.00161 .
* ^ 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 .
* ^ Madigan M, Martinko J (editors) (2006). Brock Biology of
Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1 . CS1
maint: Extra text: authors list (link )
* ^ Encyclopædia Britannica Online, 2009. "lytic phage"
Clinical research and experimental design
* Trial protocols
Adaptive clinical trial
Academic clinical trials
Clinical study design
(EBM I to II-1; A to B )
Randomized controlled trial
* Scientific experiment
(EBM II-2 to II-3; B to C )
Cross-sectional study vs.
Longitudinal study ,
Case-control study (
Nested case-control study )
* occurrence : Incidence (
Cumulative incidence )
* association : absolute (
Absolute risk reduction , Attributable
Attributable risk percent )
* relative (
Relative risk ,
Odds ratio ,
Hazard ratio )
* other :
Case fatality rate
Specificity and sensitivity
* Pre/post-test probability
Animal testing on non-human primates
ANALYSIS OF CLINICAL TRIALS
INTERPRETATION OF RESULTS
Correlation does not imply causation
* LIST OF TOPICS