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A pathogenicity island is defined as a region in the chromosome of bacteria responsible for pathogenicity and/or virulence in that particular organism. Pathogenicity islands (PAIs) are the regions of bacterial chromosome (usually of foreign origin) that contain clusters of genes that provoke virulence in the pathogenic microbe. They are usually acquired by pathogenic bacteria by horizontal gene transfer in the environment. It is noteworthy that only pathogenic bacteria harbour PAIs as they are normally absent in non-virulent bacteria of the same species. PAIs are cassettes or clusters of genes that encode a series of virulence factors in pathogenic bacteria; and they are mainly found in the chromosomes of bacteria especially those that are pathogenic (i.e., disease-causing) in nature. Virulence factors in these organisms are usually encoded by the genes found on the pathogenicity islands present in the genome of pathogenic microbes that have them.


PAIs usually contain gene clusters that mediate virulence in a pathogenic microbe, and they are obtained from other pathogenic bacteria usually through means of genetic transfer. These virulence gene clusters may be located in either the plasmids or chromosomes of the bacterium that has them, and they are foreign because this group of genes is somewhat different from the overall genetic make-up of the harbouring organism. Because the base sequence of these clusters of genes are different from the rest of the genome and they are only found in some strains of a certain group of bacterial species (i.e., pathogenic bacteria), it is widely believed that pathogenicity islands were acquired and are not actually innate make-up of the organisms genome. Examples of bacteria that have pathogenicity islands in their genome include enteropathogenic Escherichia coli, Salmonella typhimurium, Shigella flexneri and Yersinia species.

PAIs encode genes which contribute to the virulence of pathogenic bacteria. Though PAIs are commonly found in Gram-negative bacteria as exemplified above, they have also been reported in some Gram-positive bacteria. Bacterial cells usually have more than one pathogenicity islands, and these gene clusters are very advantageous to bacteria in that they assist in the secretion of certain types of virulent protein molecules that increases their disease mechanism processes in vivo and damage host cells. These clustered genes for virulence found in the genome of bacteria are usually acquired from other pathogenic organisms in their environment especially through genetic transfer mechanisms such as conjugation and transduction in the form of plasmids or phages. Symbiosis and biodegradation are other functions carried out by bacterial pathogenicity (chromosomal) islands. They generally increase the virulence of bacteria, and are lacking in non-pathogenic strains of organisms in the same genus or species level with those pathogenic bacteria harbouring them. 

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.


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