Pathogenicity Island

Pathogenicity Island

Pathogenicity islands (PAIs) are a distinct class of genomic islands acquired by microorganisms through horizontal gene transfer. They are incorporated in the genome of pathogenic organisms, but are usually absent from those nonpathogenic organisms of the same or closely related species. These mobile genetic elements may range from 10-200 kb and encode genes which contribute to the virulence of the respective pathogen. Typical examples are adherence factors, toxins, 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 a plasmid, phage, or conjugative transposon.

An analogous genomic structure in rhizobia is termed a symbiosis island.