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Cumulative evidence by several groups confirmed that the two outbreaks in
Germany and France were caused by a STEC O104:H4, with virulence features
common to the EAEC pathogroup (
Bielaszewska et al., 2011
;
Scheutz et al.,
2011
). This combination is very rare and was previously described in strains of
serotype O111:H2 involved in a small outbreak of HUS in children in France
(
Morabito et al., 1998
). The German outbreak strain, like the 55989 EAEC
strain, was found to possess several virulence factors found in extraintestinal
pathogenic
E. coli
as well as to have acquired resistance to numerous antibiot-
ics, including third-generation cephalosporins, due to the presence of plasmid-
encoded multi-antibiotic resistance cassettes (
Denamur, 2011
). The general
consensus indicates that the
E. coli
O104:H4 outbreak strain is a recombinant
of two pathogenic
E. coli
types, STEC and EAEC, and as a result, two mod-
els for
E. coli
O104:H4 evolution have been proposed. The first one indicates
that
E. coli
O104:H4 and the African EAEC isolate 55989 (see below) evolved
from a common Shiga toxin-producing
E. coli
O104:H4 progenitor strain by
stepwise gain and loss of chromosomal and plasmid-encoded virulence factors
(
Mellmann et al., 2011
). This process generated a highly pathogenic new hybrid
strain, that has been proposed to be renamed 'Entero-Aggregative-Hemorrhagic
E. coli
(EAHEC)' (
Brzuszkiewicz et al., 2011
), but is also known as Shiga-toxin-
producing
E. coli
serotype O104:H4. The second model is derived from com-
parative genomic analysis among the sequence of the German outbreak strain
and the sequences of other O104:H4 EAEC from Africa and reference EAEC
strains from serotypes different than O104:H4 (
Rasko et al., 2011
). This analy-
sis indicated that the German
E. coli
O104:H4 strain belongs to the enteroaggre-
gative clade, but could be distinguished from other
E. coli
O104:H4 strains due
to the fact that it contains a recently acquired prophage encoding Shiga toxin
2 genes and a set of additional virulence and antibiotic-resistance factors. The
second model supports the idea of horizontal genetic exchange, which allowed
the emergence of the highly virulent Shiga-toxin-producing enteroaggregative
E. coli
O104:H4 strain (
Rasko et al., 2011
).
Evolution
Rapid identification of
E. coli
O104:H4 as the causative agent of the outbreak,
followed by the application of high-throughput sequencing technologies,
allowed draft genome sequences of several isolates (
Brzuszkiewicz et al., 2011
;
Mellmann et al., 2011
;
Rasko et al., 2011
;
Rohde et al., 2011
). A worldwide
effort led to rapid phylogenetic analyses which indicated that a close relation-
ship existed between the outbreak of
E. coli
O104:H4 isolates and EAEC 55989
(
Rohde et al., 2011
), a strain originally isolated in the Central African Republic
from an HIV-infected individual who experienced persistent diarrhea (
Bernier
et al., 2002
). Interestingly, Shiga-toxin-producing EAEC strains had been iden-
tified in the same population from which strain 55989 was isolated (
Mossoro
et al., 2002
). Further comparisons of the outbreak isolate genome sequence with
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