Pathogenicity islands (PAIs), as termed in 1990, are a distinct class of genomic islands acquired by microorganisms through horizontal gene transfer.[1][2] Pathogenicity islands are found in both animal and plant pathogens.[2] Additionally, PAIs are found in both gram-positive and gram-negative bacteria.[2] They are transferred through horizontal gene transfer events such as transfer by a plasmid, phage, or conjugative transposon.[3] 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.[citation needed]
^Hacker, J; Bender, L; Ott, M; et al. (1990). "Deletions of chro- mosomal regions coding for fimbriae and hemolysins occur in vivo and in vitro in various extraintestinal Escherichia coli iso- lates". Microb. Pathog. 8 (3): 213–25. doi:10.1016/0882-4010(90)90048-U. PMID1974320.