Pathogenicity Islands: Properties and Types

 Pathogenicity islands (PAIs) are large groups of mobile genetic elements that are associated with pathogenicity and are located on the bacterial chromosome. These genetic elements are thought to have evolved from lysogenic bacteriophages and plasmids and are transferred by horizontal gene transfer. The concept of PAI was founded in the late 1980s by Jörg Hacker and colleagues.

PAIs are typically comprised of one or more virulence-associated genes and “mobility” genes (i.e., integrases and transposases) that mediate movements between various genetic elements (e.g., plasmids, and chromosomes) and among different bacterial strains.

The presence of pathogenicity islands (PAIs) in the genomes of bacterial pathogens is one of the main features that differentiate them from closely related nonpathogenic strains or species.

The major properties of PAIs are as follows:

1. Pathogenicity islands have one or more virulence genes
2. PAIs are large organized groups of genes, usually 100 to 200 kb in size.
3. They are found with parts of the genome associated with mobile genetic elements and cause genetic instability
4. These islands typically have different guanine plus cytosine content than the rest of the bacterial genome. 

Few examples of the very large number of pathogenicity islands of human pathogens are:

 Source: LANGE Medical Microbiology/PubMed Central