Pathogenicity islands (PAIs), as termed in 1990, are a distinct class of
genomic island
A genomic island (GI) is part of a genome that has evidence of horizontal origins. The term is usually used in microbiology, especially with regard to bacteria. A GI can code for many functions, can be involved in symbiosis or pathogenesis, an ...
s acquired by
microorganisms through
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
.
Pathogenicity islands are found in both animal and plant pathogens.
Additionally, PAIs are found in both
gram-positive
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
Gram-positive bact ...
and
gram-negative bacteria
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
.
They are transferred through horizontal gene transfer events such as transfer by a
plasmid,
phage
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". Bacter ...
, or conjugative
transposon
A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Tra ...
.
Therefore, PAIs contribute to microorganisms' ability to evolve.
One species of bacteria may have more than one PAI. For example, ''Salmonella'' has at least five.
An analogous genomic structure in
rhizobia is termed a ''
symbiosis island''.
Properties
Pathogenicity islands (PAIs) are gene clusters incorporated in the
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
, chromosomally or extrachromosomally, of pathogenic organisms, but are usually absent from those
nonpathogenic organisms
Nonpathogenic organisms are those that do not cause disease, harm or death to another organism. The term is usually used to describe bacteria. It describes a property of a bacterium – its inability to cause disease. Most bacteria are nonpathoge ...
of the same or closely related species.
They may be located on a bacterial chromosome or may be transferred within a
plasmid or can be found in bacteriophage genomes.
The
GC-content and codon usage of pathogenicity islands often differs from that of the rest of the genome, potentially aiding in their detection within a given DNA sequence, unless the donor and recipient of the PAI have similar GC-content.
PAIs are discrete genetic units flanked by direct repeats, insertion sequences or tRNA genes, which act as sites for recombination into the DNA. Cryptic mobility genes may also be present, indicating the provenance as transduction.
PAIs are flanked by direct repeats; the sequence of bases at two ends of the inserted sequence are the same. They carry functional genes, such as
integrases,
transposase
A transposase is any of a class of enzymes capable of binding to the end of a transposon and catalysing its movement to another part of a genome, typically by a cut-and-paste mechanism or a replicative mechanism, in a process known as transpositio ...
s, phagocytosis, or part of
insertion sequence Insertion element (also known as an IS, an insertion sequence element, or an IS element) is a short DNA sequence that acts as a simple transposable element. Insertion sequences have two major characteristics: they are small relative to other transp ...
s, to enable insertion into host DNA.
PAIs are often associated with
tRNA
Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ...
genes, which target sites for this integration event.
They can be transferred as a single unit to new bacterial cells, thus conferring virulence to formerly benign strains.
PAIs, a type of
mobile genetic element
Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In h ...
, may range from 10-200
kb and encode
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s which contribute to the
virulence
Virulence is a pathogen's or microorganism'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 ...
of the respective pathogen.
Pathogenicity islands carry genes encoding one or more virulence factors, including, but not limited to,
adhesins,
secretion systems (like
type III secretion system
The type III secretion system (T3SS or TTSS), also called the injectisome, is one of the bacterial secretion systems used by bacteria to secrete their effector proteins into the host's cells to promote virulence and colonisation. The T3SS is a ...
),
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,
invasin
Invasins are a class of proteins associated with the penetration of pathogens into host cells. Invasins play a role in promoting entry during the initial stage of infection.
In 2007, Als3 was identified as a fungal invasion allowing ''Candida alb ...
s,
modulins,
effector
Effector may refer to:
*Effector (biology), a molecule that binds to a protein and thereby alters the activity of that protein
* ''Effector'' (album), a music album by the Experimental Techno group Download
* ''EFFector'', a publication of the El ...
s,
superantigens
Superantigens (SAgs) are a class of antigens that result in excessive activation of the immune system. Specifically it causes non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. SAgs are ...
, iron uptake systems, o-antigen synthesis, serum resistance,
immunoglobulin A
Immunoglobulin A (Ig A, also referred to as sIgA in its secretory form) is an antibody that plays a role in the immune function of mucous membranes. The amount of IgA produced in association with mucosal membranes is greater than all other typ ...
proteases,
apoptosis,
capsule synthesis, and plant tumorigenesis via ''
Agrobacterium tumefaciens''.
There are various combinations of regulation involving pathogenicity islands. The first combination is that the pathogenicity island contains the genes to regulate the virulence genes encoded on the PAI.
The second combination is that the pathogenicity island contains the genes to regulate genes located outside of the pathogenecity island.
Additionally, regulatory genes outside of the PAI may regulate virulence genes in the pathogenicity island.
Regulation genes typically encoded on PAIs include AraC-like proteins and two-component response regulators.
PAIs can be considered unstable DNA regions as they are susceptible to deletions or mobilization.
This may be due to the structure of PAIs, with direct repeats, insertion sequences and association with tRNA that enables deletion and mobilization at higher frequencies.
Additionally, deletions of pathogenicity islands inserted in the genome can result in disrupting tRNA and subsequently affect the metabolism of the cell.
Examples
*The
P fimbriae P fimbriae (also known as pyelonephritis-associated pili) or P pili or Pap are chaperon-usher type (specifically of the π family) fimbrial appendages found on the surface of many ''Escherichia coli'' bacteria. The P fimbriae is considered to be o ...
island contains virulence factors such as haemolysin, pili, cytotoxic necrosing factor, and uropathogenic specific protein (USP).
*''
Yersinia pestis
''Yersinia pestis'' (''Y. pestis''; formerly '' Pasteurella pestis'') is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both ''Yersinia pseudotuberculosis'' and ''Yersinia enterocolitica''. It is a facult ...
'' high pathogenicity island I has genes regulating iron uptake and storage.
*''
Salmonella'' SP1 and SP2 sites.
*''
Rhodococcus equi
''Rhodococcus equi'' is a Gram-positive coccobacillus bacterium. The organism is commonly found in dry and dusty soil and can be important for diseases of domesticated animals (horses and goats). The frequency of infection can reach near 60%. '' ...
'' virulence plasmid pathogenicity island encodes virulence factors for proliferation in macrophages.
*The
SaPI family of ''Staphylococcus aureus'' pathogenicity islands, mobile genetic elements, encode
superantigens
Superantigens (SAgs) are a class of antigens that result in excessive activation of the immune system. Specifically it causes non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. SAgs are ...
, including the gene for
toxic shock syndrome
Toxic shock syndrome (TSS) is a condition caused by bacterial toxins. Symptoms may include fever, rash, skin peeling, and low blood pressure. There may also be symptoms related to the specific underlying infection such as mastitis, osteomyeli ...
toxin, and are mobilized at high frequencies by specific
bacteriophages.
*Phage encoded Cholera toxin of ''
Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...
'', Diphtheria toxin of ''
Corynebacterium diphtheriae'', Neurotoxins of ''
Clostridium botulinum
''Clostridium botulinum'' is a Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce the neurotoxin botulinum.
The botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans ...
'' and Cytotoxin of ''
Pseudomonas aeruginosa
''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerug ...
''.
*''
Helicobacter pylori
''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophilic, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, helicobacter, derives) is thoug ...
'' has two strains, one being more virulent than the other due to the presence of the ''Cag'' pathogenicity island.
References
External links
BAC definition of pathogenicity{{Repeated sequence
Genetics