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as well as in underdeveloped countries. The global burden of shigellosis has
been estimated at over 163 million cases per year with nearly 1 million deaths,
the majority occurring in children less than 5 years of age (
Kotloff et al., 1999
).
The appropriateness of a chapter on
Shigella
in a topic devoted to pathogenic
E. coli
derives from the fact that EIEC cause dysentery that is clinically indis-
tinguishable from that caused by members of the
Shigella
genus. In addition,
an abundance of studies starting with the DNA-DNA hybridization work of
Brenner et al. (1969)
to multilocus enzyme electrophoresis (
Pupo et al., 1997
)
and whole genome sequencing (
Touchon et al., 2009
;
Sims and Kim, 2011
)
unequivocally establish that
Shigella
spp. are clones of
E. coli
. In this chap-
ter, we will consider these pathogens together. Since the vast majority of the
research on the pathogenesis of bacillary dysentery has been done on
Shigella
,
we will focus on these organisms. No single review can be completely compre-
hensive. Therefore, the reader is encouraged to refer to several excellent recent
reviews for additional information (
Parsot, 2005
;
Schroeder and Hilbi, 2008
).
Classification and biochemical characteristics
The Japanese microbiologist Shiga isolated an organism (
Shigella dysente-
riae
) from the dysenteric stool of a stricken individual in 1898. Three more
Shigella
species (
S. boydii, S. flexneri
, and
S. sonnei
) were subsequently identi-
fied and grouped by serotype and metabolic activities. The four species of the
genus
Shigella
are grouped serologically (41 serotypes) based on their somatic
O-antigens:
S. dysenteriae
(group A),
S. flexneri
(group B),
S. boydii
(group C)
,
and
S. sonnei
(group D). As members of the family
Enterobacteriaceae
, they
are closely related to the salmonellae (
Ochman et al., 1983
).
Shigella
are non-
motile, Gram-negative rods. Some important biochemical characteristics that
distinguish these bacteria from other enterics are their inability to utilize citric
acid as a sole carbon source and their inability to ferment lactose, although some
strains of
S. sonnei
may ferment lactose slowly. They are oxidase-negative, do
not produce H
2
S (except for
S. flexneri
6 and
S. boydii
serotypes 13 and 14),
and do not produce gas from glucose.
Shigella
spp. are inhibited by potassium
cyanide and do not synthesize lysine decarboxylase (
Ewing, 1986
).
Based on the cumulative sequence evidence from 16S rRNA genes, multiple
housekeeping genes, and whole genome comparisons, the pathogenic
Shigella
can be considered a subset of non-pathogenic
E. coli
(
Cilia et al., 1996
;
Pupo
et al., 1997
;
Shu et al., 2000
;
Jin et al., 2002
). Although phylogenetically it would
be more accurate to treat
Shigella
as pathotypes of
E. coli
, the members of the
genus
Shigella
continue to be divided for historical and medical purposes into
the four species or subgroups described above (
Strockbine and Maurelli, 2005
).
Enteroinvasive
E. coli
(EIEC) isolates that cause a diarrheal illness identical
to
Shigella
dysentery were identified in the 1970s (
DuPont et al., 1971
;
Tulloch
et al., 1973
). The pathogenic and biochemical properties that EIEC share with
Shigella
pose a problem for distinguishing these pathogens. For example, unlike
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