J. Hacker et al., PATHOGENICITY ISLANDS OF VIRULENT BACTERIA - STRUCTURE, FUNCTION AND IMPACT ON MICROBIAL EVOLUTION, Molecular microbiology, 23(6), 1997, pp. 1089-1097
Virulence genes of pathogenic bacteria, which code for toxins, adhesin
s, invasins or other virulence factors, may be located on transmissibl
e genetic elements such as transposons, plasmids or bacteriophages. In
addition, such genes may be part of particular regions on the bacteri
al chromosome, termed 'pathogenicity islands' (Pais). Pathogenicity is
lands are found in Gram-negative as well as in Gram-positive bacteria.
They are present in the genome of pathogenic strains of a given speci
es but absent or only rarely present in those of non-pathogenic varian
ts of the same or related species. They comprise large DNA regions (up
to 200 kb of DNA) and often carry more than one virulence gene, the G
+C contents of which often differ from those of the remaining bacteria
l genome. In most cases, Pais are flanked by specific DNA sequences, s
uch as direct repeats or insertion sequence (IS) elements. In addition
, Pais of certain bacteria (e.g. uropathogenic Escherichia cell, Yersi
nia spp., Helicobacter pylori) have the tendency to delete with high f
requencies or may undergo duplications and amplifications. Pais are of
ten associated with tRNA loci, which may represent target sites for th
e chromosomal integration of these elements. Bacteriophage attachment
sites and cryptic genes on Pais, which are homologous to phage integra
se genes, plasmid origins of replication or IS elements, indicate that
these particular genetic elements were previously able to spread amon
g bacterial populations by horizontal gene transfer, a process known t
o contribute to microbial evolution.