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that promote secretion or transport of its virulence factors need to be studied.
Finally, appropriate infection models to study virulence and fitness under differ-
ent conditions should be established. However, some clues regarding virulence
can be deduced by the analysis of the genome sequence of E. coli O104:H4 and
extrapolation of the role of similar virulence factors in other pathogenic E. coli .
Shiga toxin
Characterization of the epidemic strain showed that it carries the stx2 genes
but, in contrast to typical EHEC, it lacks the LEE pathogenicity island (genetic
marker for Shiga-toxin-producing E. coli and other attaching and effacing
E. coli strains; for further details, see Chapters 4 and 5) ( Bielaszewska et al.,
2011 ; Scheutz et al., 2011 ). E. coli O104:H4 acquired the bacteriophage encod-
ing Stx and further subtyping showed that the gene corresponds to the stx2a
variant ( Scheutz et al., 2011 ). Sequence analysis of the E. coli O104:H4 stx2a
gene showed 100% amino acid identity of the Stx2a protein found in the proto-
type STEC O157:H7 isolate EDL933, but differed by one nucleotide, making
the nucleotide sequence identical to that found in sorbitol-fermenting STEC
O157 strains from Germany ( Scheutz et al., 2011 ). Therefore, the unusually
high level of HUS development associated with the E. coli O104:H4 strain is
not related to differences in toxin activity, but rather might be explained by the
enteroaggregative adherence phenotype displayed by this strain, allowing the
bacteria to efficiently colonize the intestinal mucosa, increasing the exposure of
the tissue in the affected patient to the toxin.
Adherence
The E. coli O104:H4 strains isolated from these outbreaks were PCR-positive
for different combinations of the aggR, aatA, aaiC, aap, astA, sepA, pic, sigA,
aggA genes, commonly found in EAEC strains (for further details see Chapter 8)
( Scheutz et al., 2011 ). The aggA gene encodes the major component of the
aggregative adherence fimbriae (AAF/I), which is an adhesin associated with
production of biofilms and hemagglutination of human erythrocytes ( Harrington
et al., 2006 ). In addition to the AAF/I fimbrial operon, E. coli O104:H4 also
possesses the master virulence regulator gene aggR , typical of EAEC strains
( Harrington et al., 2006 ). In general, limited evidence about the acquisition of
AAF/I fimbrial operon by this serotype is available, because the closely related
EAEC 55989 strain possesses instead the aggregative adherence fimbriae type
III (AAF/III, encoded by the agg3A gene). It has been hypothesized that AAF-
mediated enteroaggregative adherence allows E. coli O104:H4 colonization of
the intestinal mucosa of infected patients ( Scheutz et al., 2011 ). However, puta-
tively inferred differences in the mechanism of adhesion in this isolate might
explain why this strain is more likely to cause severe disease in adults rather
than in children (i.e. they might differ in their susceptibility to the adherence
and/or colonization properties of E. coli O104:H4).
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