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Transient Darwinian selection in Salmonella enterica serovar Paratyphi A during 450 years of global spread of enteric fever.
[typhoid]
Multiple
epidemic
diseases
have
been
designated
as
emerging
or
reemerging
because
the
numbers
of
clinical
cases
have
increased
.
Emerging
diseases
are
often
suspected
to
be
driven
by
increased
virulence
or
fitness
,
possibly
associated
with
the
gain
of
novel
genes
or
mutations
.
However
,
the
time
period
over
which
humans
have
been
afflicted
by
such
diseases
is
only
known
for
very
few
bacterial
pathogens
,
and
the
evidence
for
recently
increased
virulence
or
fitness
is
scanty
.
Has
Darwinian
(
diversifying
)
selection
at
the
genomic
level
recently
driven
microevolution
within
bacterial
pathogens
of
humans
?
Salmonella
enterica
serovar
Paratyphi
A
is
a
major
cause
of
enteric
fever
,
with
a
microbiological
history
dating
to
1898
.
We
identified
seven
modern
lineages
among
149
genomes
on
the
basis
of
4
,
584
SNPs
in
the
core
genome
and
estimated
that
Paratyphi
A
originated
450
y
ago
.
During
that
time
period
,
the
effective
population
size
has
undergone
expansion
,
reduction
,
and
recent
expansion
.
Mutations
,
some
of
which
inactivate
genes
,
have
occurred
continuously
over
the
history
of
Paratyphi
A
,
as
has
the
gain
or
loss
of
accessory
genes
.
We
also
identified
273
mutations
that
were
under
Darwinian
selection
.
However
,
most
genetic
changes
are
transient
,
continuously
being
removed
by
purifying
selection
,
and
the
genome
of
Paratyphi
A
has
not
changed
dramatically
over
centuries
.
We
conclude
that
Darwinian
selection
is
not
responsible
for
increased
frequency
of
enteric
fever
and
suggest
that
environmental
changes
may
be
more
important
for
the
frequency
of
disease
.
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"and the evidence for recently increased virulence or fitness is scanty"
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