Pathogenicity island
Encyclopedia
Pathogenicity islands are a distinct class of genomic island
s acquired by microorganisms through horizontal gene transfer
. They are incorporated in the genome
of pathogenic organisms but are usually absent from those non-pathogenic organisms of the same or closely related species. These mobile genetic elements
may range from 10-200 kb and encode gene
s which contribute to the virulence
of the respective pathogen. Typical examples are adherence factors, toxin
s, iron uptake systems, invasion factors and secretion systems. Pathogenicity islands 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.
One species of bacteria may have more than one PAI (i.e. Salmonella has at least 5). They are transferred through horizontal gene transfer
events such as transfer by plasmid
, phage
, or a conjugative transposon
.
An analogous genomic structure in rhizobia
is termed a symbiosis island.
s, toxins, or invasins. They may be located on a bacterial chromosome or may be transferred within a plasmid
. The GC-content
of pathogenicity islands often differs from that of the rest of the genome
, potentially aiding in their detection within a given DNA sequence.
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 integrase
s, transposase
s, or part of insertion sequence
s to enable insertion into host DNA. PAIs are often associated with tRNA 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.
Genomic island
A Genomic island 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, and may help an organism's adaptation. Many sub-classes of...
s acquired by microorganisms through horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
. They are incorporated in the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
of pathogenic organisms but are usually absent from those non-pathogenic organisms of the same or closely related species. These mobile genetic elements
Mobile genetic elements
Mobile genetic elements are a type of DNA that can move around within the genome. They include:*Transposons **Retrotransposons**DNA transposons**Insertion sequences*Plasmids...
may range from 10-200 kb and encode gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s which contribute to the virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
of the respective pathogen. Typical examples are adherence factors, toxin
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...
s, iron uptake systems, invasion factors and secretion systems. Pathogenicity islands 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.
One species of bacteria may have more than one PAI (i.e. Salmonella has at least 5). They are transferred through horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
events such as transfer by plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
, phage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
, or a conjugative transposon
Transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...
.
An analogous genomic structure in rhizobia
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
is termed a symbiosis island.
Properties
Pathogenicity islands (or PAIs) carry genes encoding one or more virulence factors including but not limited to adhesinAdhesin
Adhesins are cell-surface components or appendages of bacteria that facilitate bacterial adhesion or adherence to other cells or to inanimate surfaces. Adhesins are a type of virulence factor....
s, toxins, or invasins. They may be located on a bacterial chromosome or may be transferred within a plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
. The GC-content
GC-content
In molecular biology and genetics, GC-content is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine . This may refer to a specific fragment of DNA or RNA, or that of the whole genome...
of pathogenicity islands often differs from that of the rest of the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
, potentially aiding in their detection within a given DNA sequence.
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 integrase
Integrase
Retroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
s, transposase
Transposase
Transposase is an enzyme that binds to the ends of a transposon and catalyzes the movement of the transposon to another part of the genome by a cut and paste mechanism or a replicative transposition mechanism....
s, or part of insertion sequence
Insertion sequence
An insertion sequence is a short DNA sequence that acts as a simple transposable element...
s to enable insertion into host DNA. PAIs are often associated with tRNA 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.
Examples
- The UPEC P fimbriae island contains virulence factors such as haemolysin, pili, cytotoxic necrosing factor, and uropathogenic specific protein (usp).
- Yersinia pestisYersinia pestisYersinia pestis is a Gram-negative rod-shaped bacterium. It is a facultative anaerobe that can infect humans and other animals....
High Pathogenicity Island I has genes regulating iron uptake and storage - SalmonellaSalmonellaSalmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...
SP1 and SP2 sites. - Rhodococcus equiRhodococcus equiRhodococcus 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 . The frequency of infection can reach near 60 percent. R. equi is an important pathogen causing pneumonia in foals. Since...
virulence plasmid pathogenicity island encoding virulence factors for proliferation in macrophages - The SaPISaPISaPIs are a family of mobile genetic elements resident in the genome of some strains of Staphylococcus aureus. Much like bacteriophages, SaPIs can be transferred to uninfected cells and integrate into the host chromosome...
family of Staphylococcus aureus pathogenicity islands. These mobile genetic elements encode superantigens including the gene for toxic shock syndromeToxic shock syndromeToxic shock syndrome is a potentially fatal illness caused by a bacterial toxin. Different bacterial toxins may cause toxic shock syndrome, depending on the situation. The causative bacteria include Staphylococcus aureus and Streptococcus pyogenes...
toxin and are mobilized at high frequencies by specific bacteriophageBacteriophageA bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
s.