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polysialic acid expression is significantly reduced, and by the time they reach
the meninges it is absent entirely (
Zelmer et al., 2008
). This is not due to
a selection for capsule-deficient strains, since isolating and culturing these
bacteria leads to restoration of the encapsulated phenotype (
Zelmer et al.,
2008
). While many regulatory mechanisms for the inducement of capsule
synthesis upon switching from 20°C to 37°C have been described, it is not
known if any of these mechanisms are responsible for the loss of capsule
when the bacteria colonize the brain. The endo-neuraminidase is an enzyme
that hydrolyzes polysialic acid. Transcriptome analysis of
E. coli
K1 shows
that the gene encoding the endo-neuraminidase (found near the polysialic
acid O-acetyltransferase gene,
neuO
, on the CUS-3 bacteriophage insertion)
is significantly up-regulated upon binding to human microvascular endothe-
lial cells (
Xie et al., 2008
). However, more research is needed to determine
whether this protein is responsible for the observed loss of capsule upon
reaching the cerebrospinal fluid.
CONCLUSIONS
In summary, LPS and CPS molecules fulfill diverse roles in the biology and
pathogenesis of
E. coli.
In some cases a function may be related to a precise
structural motif or feature, whereas in other cases the presence of a hydrated
layer provides protection regardless of the exact glycan structure. While signifi-
cant progress has been made in understanding the structures and biosynthesis of
LPS and CPS, there are still significant gaps in the details of their interactions
with elements of the innate and acquired immune system.
REFERENCES
activity. Carbohydr. Res. 185, 211-223
.
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