are molecules produced by bacteria
, and protozoa
that add to their effectiveness and enable them to achieve the following:
- colonization of a niche in the host (this includes attachment to cells)
- immunoevasion, evasion of the host's immune response
- immunosuppression, inhibition of the host's immune response
- 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
), whereas others are obtained from mobile genetic elements
s and bacteriophage
s (e.g. some exotoxins). Virulence factors encoded on mobile genetic elements spread through horizontal gene transfer
, and can convert harmless bacteria into dangerous pathogens. Bacteria like
gain the majority of their virulence
from mobile genetic elements. Gram-negative bacteria
secrete a variety of virulence factors at host-pathogen interface
, via membrane vesicle trafficking
as bacterial outer membrane vesicles
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 host defenses. These obtained bacterial virulence factors have two different routes used to help them survive and grow:
Attachment, immunoevasion, and immunosuppression
Bacteria produce various adhesins
including lipoteichoic acid
, 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
. Capsules play important roles in immune evasion, as they inhibit phagocytosis
, as well as protecting the bacteria while outside the host.
Another group of virulence factors possessed by bacteria are immunoglobulin
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
. Some bacteria, such as Streptococcus pyogenes
, 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
-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' virulence
. Genes characteristic of this concept are those that control latency
in some viruses like herpes. Murine
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
phases of the virus, these latency genes are important for promoting chronic infection and continued replication within infected individuals.
Some bacteria, such as Streptococcus pyogenes
, Staphylococcus aureus
and Pseudomonas aeruginosa
, produce a variety of enzymes which cause damage to host tissues. Enzymes include hyaluronidase
, which breaks down the connective tissue component hyaluronic acid
; a range of proteases and lipase
, which break down DNA, and hemolysins
which break down a variety of host cells, including red blood cells. Virulence Factors basically Include the Antigenic Structure and The Toxins produced by the organisms.
Virulence factors dealing in the role of GTPases
A major group of virulence factors are proteins that can control the activation levels of GTPase
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.
outer protein T) from Yersinia
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.
A major group of virulence factors are bacterial toxins. These are divided into two groups: endotoxins
Endotoxin is a component ( lipopolysaccharide (LPS)
) of the cell wall of gram-negative bacteria. It is the lipid A
part of this LPS which is toxic. Lipid A is an endotoxin. Endotoxins trigger intense inflammation. They bind to receptors on monocytes causing the release of inflammatory mediators which induce degranulation
. 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. Endotoxins are non-immunogenic, and therefore do not have an acquired immune response.
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
) secreted by Clostridium tetani
and the botulinum toxin
secreted by Clostridium botulinum
. Exotoxins are also produced by a range of other bacteria including Escherichia coli
; Vibrio cholerae
(causative agent of cholera
); Clostridium perfringens
(common causative agent of food poisoning
as well as gas gangrene
) and Clostridium difficile
(causative agent of pseudomembranous colitis
). A potent three-protein virulence factor produced by Bacillus anthracis
, called anthrax toxin
, plays a key role in anthrax
pathogenesis. Exotoxins are extremely immunogenic meaning that they trigger the humoral response (antibodies target the toxin).
Exotoxins are also produced by some fungi
as a competitive resource. The toxins, named 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
produced by certain species of the genus Aspergillus
(notably A. flavus
). If ingested repeatedly, this toxin can cause serious liver damage.
Examples of virulence factors for Staphylococcus aureus
s and enterotoxins
. Examples for Streptococcus pyogenes
are M protein
, lipoteichoic acid
, hyaluronic acid
capsule, destructive enzymes (including streptokinase
, and hyaluronidase
), and exotoxins
). Examples for Listeria monocytogenes
include internalin A, internalin B, lysteriolysin O
, and actA, all of which are used to help colonize the host. Examples for Yersinia pestis
are an altered form of lipopolysaccharide, type three secretion system, and YopE and YopJ pathogenicity. The cytolytic peptide Candidalysin
is produced during hyphal
formation by Candida albicans
; it is an example of a virulence factor from a fungus. Other virulence factors include factors required for biofilm
formation (e.g. sortase
s) and integrin
s (e.g. beta-1 and 3). https://www.hindawi.com/journals/jpath/2011/601905/
Targeting virulence factors as a means of infection control
Strategies to target virulence factors and the genes encoding them have been proposed. Small molecule
s being investigated for their ability to inhibit virulence factors and virulence factor expression
s, and peptide
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 alter from pathogenic to non-pathogenic, random mutations may be introduce 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
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.
, 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.