Biology Reference
In-Depth Information
normal flora
E. coli
, EIEC are non-motile and 70% of isolates are unable to
ferment lactose (
Silva et al., 1980
;
van den Beld and Reubsaet, 2012
). These
are also features of
Shigella
. More striking is the observation that strains of
EIEC are almost universally negative for lysine decarboxylase (LDC) activity
whereas almost 90% of normal flora
E. coli
are positive. In this respect, EIEC
also resemble
Shigella,
which uniformly lack LDC activity. Some serotypes of
EIEC even share identical O-antigens with
Shigella
(
Sansonetti et al., 1985
). By
contrast, EIEC resemble
E. coli
in their ability to ferment xylose and to produce
gas from glucose, both traits for which
Shigella
are negative (
Silva et al., 1980
).
It is now generally accepted that the present day strains of
Shigella
arose
multiple times from as many as seven independent ancestral strains of
E. coli
(
Pupo et al., 2000
;
Yang et al., 2005
). EIEC probably evolved later than
Shigella
and from different
E. coli
ancestors (
Lan et al., 2004
). However, the common
seminal event in the evolution of both groups of pathogens was the acquisition
of a large plasmid that encodes the genes necessary for invasion of mammalian
cells (see below) (
Lan et al., 2001
).
Evolution of
Shigella
species and EIEC
The EIEC were recognized as a heterogeneous group of pathogens that resem-
bled
Shigella
in their pathogenic potential (and in certain metabolic traits) but
were more closely related to
E. coli
. Early genetic studies demonstrated that
essentially all the virulence factors required for expression of the invasive phe-
notype are encoded on a large plasmid present in all
Shigella
and EIEC iso-
lates examined (
Sansonetti et al., 1981, 1982b
;
Harris et al., 1982
). Therefore,
from the pathogenesis perspective, EIEC are closely related to
Shigella
. These
observations led many investigators to propose, incorrectly, that the EIEC rep-
resented a missing link in the evolution from
E. coli
to
Shigella
.
Several studies have established that, like the EIEC, the four species of
Shigella
are so closely related to
E. coli
that they should all be included in a
single species. The chromosomes of these organisms are largely co-linear and
are more than 90% homologous (
Brenner et al., 1969
). Therefore,
Shigella
, like
EIEC, are a group of pathogenic
E. coli.
In fact, several investigators using
different approaches have established that the four species of
Shigella
evolved
from separate
E. coli
strains (
Pupo et al., 1997, 2000
;
Rolland et al., 1998
).
Seven different
Shigella
lineages have been identified through sequence analy-
sis of multiple chromosomal loci. Bacteria expressing the
Shigella
phenotype
were generated through horizontal transfer of the virulence plasmid from an
unknown donor bacterium to commensal
E. coli
(
Pupo et al., 2000
). Thus,
horizontal transfer of the
Shigella
virulence plasmid to commensal
E. coli
has occurred multiple times, each time giving rise to new
Shigella
clones.
These findings suggest that traits unique to and shared by
Shigella
species and
EIEC are the result of convergent evolution either through gain-of-function
mutations (e.g. horizontal transfer of the virulence plasmid) or loss-of-function
Search WWH ::
Custom Search