Vokes, S.A., S.A. Reeves, A.G. Torres, and S.M. Payne. 1999. The
aerobactin iron transport system genes in Shigella flexneri are present within a
pathogenicity island.
Molecular
Microbiology. 33(1):63-73
Genes encoding the synthesis and transport of aerobactin, a hydroxamate
siderophore associated with increased virulence of enteric bacteria, were mapped
within a pathogenicity island in Shigella flexneri. The island, designated SHI-2
for Shigella pathogenicity island 2, was located downstream of selC, the site of
insertion of pathogenicity islands in several other enteric pathogens. DNA
sequence analysis revealed the presence of multiple insertion sequences upstream
and downstream of the aerobactin genes and an integrase gene that was nearly
identical to an int gene found in Escherichia coli O157:H7. SHI-2 sequences
adjacent to selC were similar to sequences at the junction between selC and
pathogenicity islands found in E. coli O157:H7 and in enteropathogenic E. coli,
but the junctions between the island and downstream yic genes were variable.
SHI-2 also encoded immunity to the normally plasmid-encoded colicins I and V,
suggesting a common origin for the aerobactin genes in both S. flexneri and E.
coli pColV. Polymerase chain reaction and Southern hybridization data indicate
that SHI-2 is present in the same location in Shigella sonnei, but the
aerobactin genes are not located within SHI-2 in Shigella boydii or
enteroinvasive E. coli. Shigella dysenteriae type 1 strains do not produce
aerobactin but do contain sequences downstream of selC that are homologous to
SHI-2. The presence of the aerobactin genes on plasmids in E. coli pColV and
Salmonella, on a pathogenicity island in S. flexneri and S. sonnei and in a
different chromosomal location in S. boydii and some E. coli suggests that these
virulence-enhancing genes are mobile, and they may constitute an island within
an island in S. flexneri.