Shigella and Shigellosis (page 3)
This chapter has 4 pages
© Kenneth Todar, PhD
The Large Virulence Plasmid of Shigella
All virulent strains of Shigella flexneri possess a large
plasmid that mediates its virulence properties. This so-called the invasion
plasmid has been shown to encode the genes for several aspects of Shigella
- Adhesins that are involved in the adherence of bacteria onto the
of target epithelial cells
- The production of invasion plasmid antigens (Ipa) that have a
role in the Shigella invasion process
- Transport or processing functions that ensure the correct surface
expression of the Ipa proteins
- The induction of endocytic uptake of bacteria and disruption of
- The intra- and inter-cellular spreading phenotypes
- The regulation of plasmid-encoded
The presence of this plasmid was discovered in the 1980s, after
the observation that essentially the entire chromosome of S.
could be transferred to E. coli without reconstituting the
phenotype of the donor. However, the ability to invade tissue culture
was transferred to E. coli by the conjugal mobilization of this
plasmid from S. flexneri. (see below)
3. Circular map of the large virulence plasmid of Shigella.
Outer ring depicts ORFs and their
orientations, color coded according to functional category: 1.
identical or essentially identical to known virulence-associated
proteins (red); 2, homologous to known pathogenesis-associated proteins
(pink); 3. highly homologous to IS elements or transposases (blue); 4.
weakly homologous to IS elements or transposases (light blue); 5.
homologous to proteins involved in replication, plasmid maintenance, or
other DNA metabolic functions (yellow); 6. no significant similarity to
any protein or ORF in the database (brown); 7. homologous or identical
to conserved hypothetical ORFs, i.e., proteins of unknown function
(orange); and 8. Tn501
insertion-associated genes (green). The
second ring shows complete IS elements. The third ring graphs G+C
content, calculated for each ORF and plotted around the mean value for
all ORFs, with each value color coded for the corresponding ORF. Scale
is in base pairs. The figure was generated by Genescene (DNASTAR).
Venkatesan, M.M., et al. Complete DNA Sequence and Analysis of the
Large Virulence Plasmid of Shigella flexneri. Infect Immun.
2001 May; 69(5): 3271�3285.
The invasion locus on the virulence plasmid of Shigella is
pathogenicity island-like cluster that consists of 38 ORFs of the ipa-mxi-spa
operons within a stretch of 32 kb of the plasmid. Genes within this
locus are critical for Shigella
invasion of mammalian cells, although certain genes outside this region
are required for optimal invasion of tissue culture cells.
Genes and Functions Encoded by the Large Shigella Virulence
||Protein Product MW
||positive regulators of the virG and
invasion (orients ipa gene products in outer membrane
||Same as above
||Same as above
invasion: mediates endocytic uptake of
||Same as above
||Same as above
||Not necessary for
invasion (role unknown)
of the virG and ipa-mxi-spa
assembles actin tails that
propel the bacteria through the cell cytoplasm and into adjacent cells
||has 5 alleles;
IpaH7.8 facilitates the escape of Shigella
from phagocytic vacuoles
the Shigella virulence plasmid
genetic analyses suggest that shigellae do not constitute a distinct
genus as traditionally believed but rather are within the genus of E.
coli, much like the enteric pathogenic E. coli. These
analyses indicate that Shigella emerged from E. coli
seven or eight independent times during evolution, leading to three
clusters of Shigella,
each of which contains serotypes from multiple traditional species, and
four or five additional forms, each of which contains one traditional
serotype. The three main Shigella
clusters are estimated to have evolved 35,000 to 270,000 years ago,
which predates the development of agriculture and makes shigellosis one
of the early infectious diseases of humans.
The defining event each time Shigella
arose was almost certainly the acquisition of an historical precursor
of the current-day virulence plasmid. The data also suggest that the
loss of specific catabolic pathways (inability to utilize lactose and
mucate and to decarboxylate lysine), loss of motility, and expansion of
O-antigen diversity that are characteristic of Shigella
strains occurred more recently than the acquisition of the plasmid.
Since the plasmid was acquired at distinct times, one would predict
that differences reflecting the evolution of the plasmid could be
obtained by genetic comparison of virulence plasmids of the seven
different Shigella evolutionary groups. Subsequent to the
acquisition of the virulence plasmid, divergence of Shigella
clones from E. coli
would involve clonal divergence (accumulation of mutations by base
substitution), horizontal transfer of genetic material from other
species, and loss of gene sequences that interfere with pathogenicity.
Certain horizontal gene transfer events have been key to the
evolution of Shigella. A quintessential feature of Shigella
is its ability to invade mammalian cells and access the cell cytoplasm,
defining a niche unique among enteric Gram-negative bacteria, with the
exception of enteroinvasive E. coli. Thus, the acquisition
and evolution of the ipa-mxi-spa
pathogenicity island, which encodes all of the genes required for cell
invasion and phagolysosomal lysis, permitted a major alteration in
pathogenesis. Likewise, the acquisition of virG (icsA),
which mediates actin assembly on Shigella, and virF
and virB, the regulators of the virG and ipa-mxi-spa
loci, were key to the emergence of Shigella. Since all Shigella
serotypes contain these loci, they were probably all present on the
prototypic virulence plasmid.