Combined topical use of silver sulfadiazine and antibiotics as a possible solution to bacterial resistance in burn wounds


Modak, S.M.; Sampath, L.; Fox, C.L.

Journal of Burn Care and Rehabilitation 9(4): 359-363

1988


The superior efficacy of quinolones (norfloxacin, pefloxacin, and enoxacin) in controlling burn wound infections signals the discovery of new topical agents. However, there are a few reports on the emergence of resistant mutants to quinolones. Since attempts to develop AgSD resistant strains in vitro were unsuccessful and the emergence of AgSD resistance in vivo is a rare occurrence, we decided to investigate if the combined use of AgSD with other effective antibiotics, especially quinolones, would minimize the development of resistant bacteria. Our in vitro results indicate that when Ps. aeruginosa cultures were serially transferred 10 times through subinhibitory concentrations of norfloxacin, pefloxacin, etc., the MIC increased 40 times while when the cultures were passed through a combination of AgSD and these quinolones, the MIC of quinolones increased only tenfold. In vivo, when burned mice infected with either AgSD sensitive or resistant Ps. aeruginosa strains were treated with a topical cream containing 10mM silver sulfadiazine and 5mM norfloxacin or 5mM pefloxacin, the mortality was much lower than that of 10mM silver sulfadiazine alone or 5mM quinolones alone. Thus, combined use of silver sulfadiazine and quinolones appears to diminish the ability of Ps. aeruginosa strains to form resistant mutants. Furthermore, when the combination is used as a topical agent in burn wounds, lesser amounts of the individual drug are needed to control infection thereby reducing the toxic effects, if any, associated with these drugs. This combination does not in any way interfere with the antifungal or antibacterial properties of these individual drugs.

Combined.
Topical
Use
of
Silver
Sulfadiazine
and
Antibiotics
as
a
Possible
Solution
to
Bacterial
Resistance
in
Burn
Wounds
Shanta
M.
Modak,
PhD;
Lester
Sampath,
BA;
C.
L.
Fox,
Jr.,
MD
Columbia
University
College
of
Physicians
&
Surgeons
The
superior
efficacy
of
quinolones
(norfloxacin,
pefloxacin,
and
enoxacin)
in
con-
trolling
burn
wound
infections
signals
the
discovery
of
new
topical
agents.
How-
ever,
there
are
a
few
reports
on
the
emergence
of
resistant
mutants
to
quinolones.
Since
attempts
to
develop
AgSD
resistant
strains
in
vitro
were
unsuccessful
and
the
emergence
of
AgSD
resistance
in
vivo
is
a
rare
occurrence,
we
decided
to
investi-
gate
if
the
combined
use
of
AgSD
with
other
effective
antibiotics,
especially
quino-
lones,
would
minimize
the
development
of
resistant
bacteria.
Our
in
vitro
results
indicate
that
when
Ps.
aeruginosa
cultures
were
serially
transferred
10
times
through
subinhibitory
concentrations
of
norfloxacin,
pefloxacin,
etc.,
the
MIC
increased
40
times
while
when
the
cultures
were
passed
through
a
combination
of
AgSD
and
these
quinolones,
the
MIC
of
quinolones
increased
only
tenfold.
In
vivo,
when
burned
mice
infected
with
either
AgSD
sensitive
or
resistant
Ps.
aeruginosa
strains
were
treated
with
a
topical
cream
containing
10mM
silver
sulfadiazine
and
5mM
norfloxacin
or
5mM
pefloxacin,
the
mortality
was
much
lower
than
that
of
10mM
silver
sulfadiazine
alone
or
5mM
quinolones
alone.
Thus,
combined
use
of
silver
sulfadiazine
and
quinolones
appears
to
diminish
the
ability
of
Ps.
aeruginosa
strains
to
form
resistant
mutants.
Furthermore,
when
the
combination
is
used
as
a
topical
agent
in
burn
wounds,
lesser
amounts
of
the
individual
drug
are
needed
to
control
infection
thereby
reducing
the
toxic
effects,
if
any,
associated
with
these
drugs.
This
combination
does
not
in
any
way
interfere
with
the
antifungal
or
anti-
bacterial
properties
of
these
individual
drugs.
Numerous
systemic
antimicrobials
have
been
developed
and
used
effectively
whereas
topical
antimicrobials
are
few
in
number.
Since
resistant
mutants
frequently
emerge
after
topical
application,
use
of
highly
effective,
broad
spectrum
antibiotics
has
been
limited
exclusively
to
systemic
use.
Silver
sulfadiazine
(AgSD)'
is
one
of
the
most
successful
developments
in
the
long
history
of
topi-
cal
burn
care.
AgSD
differs
physically
from
other
silver
compounds
in
that
it
remains
stable
in
powder
form
and
in
oint-
ments.
In
contrast
with
silver
nitrate,
AgSD
does
not
react
rapidly
with
chloride,
sulf
hydryl
groups,
or
protein.
Thus
its
antibacterial
action
is
not
diminished
in
the
wound.
Furthermore,
unlike
sulfonamides,
the
action
of
AgSD
is
not
inhibited
competitively
by
PABA,
a
compo-
nent
of
folic
acid.'
-
'
Additional
in
vitro
studies
showed
that
AgSD
reacted
rapidly
with
DNA.
Furthermore,
bacteria
incubated
with
radioactive
AgSD
acquired
radioactive
silver
in
propor-
tion
to
their
DNA
content.'
Subsequently,
when
Pseu-
Reprint
requests
should
be
sent
to:
Shanta
M.
Modak,
PhD,
Columbia
University,
College
of
Physicians
and
Surgeons,
Black
Building
Room
1734,630
West
168
Street,
New
York,
NY
10032,
(212)
305-4060.
domonas
aeruginosa
was
grown
in
media
containing
silver
sulfadiazine
and
then
separated
into
protein,
RNA,
and
DNA
fractions,
maximum
uptake
of
silver
occurred
in
the
DNA
fraction.'
In
contrast,
when
sulfur-labeled
AgSD
(AgSD)
was
used
in
the
growth
media,
no
labeled
sulfur
was
found
in
the
DNA
fraction
or
in
the
whole
bacteria.
6
These
results
indicate
that
the
action
of
silver
sulfadiazine
is
mainly
at
the
DNA
level.
An
often
overlooked
feature
of
this
silver
binding
is
the
ability
of
silver
atoms
to
avoid
numerous
phosphate
groups
in
DNA.
The
atoms
obtain
their
perch
in
place
of
some
of
the
hydrogen
bonds
connecting
the
two
strands
of
the
double
helix'
This
unexpected
reaction
appears
to
be
facilitated
by
the
polymeric
structure
of
AgSD.
X-ray
crystallographic
analyses
by
Baenziger
and
Struss"'
disclosed
a
polymeric
structure
composed
of
six
silver
atoms
bonded
to
six
sulfadiazine
molecules
by
link-
age
of
the
silver
atoms
to
the
nitrogens
of
the
pyrimidine
ring.
This
unique
polymeric
structure
may
contribute
to
the
stability
and
special
reactivity
of
AgSD.
(Incidentally,
this
structure
is
restricted
to
unsubstituted
pyrimidine
nitrogens.)
This
suggested
mode
of
action
also
may
account
for
the
conspicuous
epithelial
regeneration
that
occurs
in
the
presence
of
AgSD.
There
is
approximately
100
times
July/August
1988
359
more
DNA
in
mammalian
cells
than
in
bacterial
cells.
9
Thus,
the
ratio
of
inhibitory
concentrations
of
AgSD
to
bacterial
DNA
is
high
enough
to
prevent
bacterial
divi-
sion.
However,
the
resulting
ratio
of
AgSD
to
epithelial
DNA
is
too
low
to
block
epithelial
cell
regeneration
(healing),
which
also
is
encouraged
by
suppression
of
competing
bacterial
growth.
Obviously
there
are
other
factors
that
also
participate
in
epithelial
cell
regeneration.
Low
solubility
and
large
molecular
size
of
AgSD
facili-
tate
maintaining
a
relatively
high
mass
but
low
molar
concentration
in
wound
exudates.
In
experimental
burns,
practically
all
of
the
silver
remains
in
the
eschar.'°
In
wound
exudates
in
patients,
measurement
of
sulfadiazine
24
hours
after
topical
treatment
showed
levels
of
90
to
100
mg
percent
(over
1,000
times
the
minimal
inhibitory
concentration).
At
the
same
time,
blood
levels
were
less
than
0.2
to
1.0
mg
percent
and
urine
levels
40
to
60
mg
per
100
mL.'
These
low
concentrations
in
blood
and
urine
indicate
limited
but
definite
tissue
penetration
with
transfer
of
some
sulfadiazine
from
wound
to
blood.
No
detectable
silver
levels
were
found
in
organs
or
blood
in
the
burned
animal.
Thus
AgSD
has
proven
itself
to
be
a
safe
and
effective
topical
agent
in
burn
wound
management.
Recently
a
few
reports
on
the
in
vivo
emergence
of
bacteria
resistant
to
AgSD
have
appeared."'
Our
search
continues
for
new
drugs
that
are
effective
against
these
AgSD-resistant
mutants.
We
found
that
certain
new
quinolone
com-
pounds
such
as
Norfloxacin
(NF),"
Pefloxacin
(PF),'
'
and
Enoxacin
(Enox)"
and
their
silver
salts
were
highly
effec-
tive
in
vivo
against
experimental
burn
wound
infections
caused
by
these
resistant
Ps.
aeruginosa
strains.
However,
bacteria
resistant
to
these
quinolones
can
be
developed
in
vitro
by
repeated
transfer
of
cultures
through
sub-
inhibitory
concentrations
of
these
drugs.'
6
17
Since
attempts
to
develop
AgSD-resistant
strains
in
vitro
were
unsuccessful
and
emergence
of
AgSD
resist-
ance
in
vivo
is
a
relatively
rare
occurrence,
we
investigated
the
effect
of
AgSD
on
development
of
resistant
bacteria
when
used
along
with
other
effective
antibiotics,
espe-
cially
quinolones.
We
have
already
reported
the
synergis-
tic
effect
of
piperacillin
and
AgSD
8
and
silver
Norflox-
acin
(AgNF)
and
AgSD'
9
when
used
topically
in
burned
and
infected
mice.
In
the
latter
report,
use
of
Norfloxacin
in
combination
with
silver
sulfadiazine
also
has
been
proposed.
This
report
demonstrates
(1)
reduction
in
the
emergence
of
resistant
bacteria
in
vitro
by
the
use
of
AgSD
along
with
antibiotics;
and
(2)
the
efficacy
of
some
antibiotic-AgSD
combinations
in
controlling
burn
wound
infection
caused
by
resistant
Ps.
aeruginosa.
Materials
and
Methods
Compounds
were
supplied
by:
Norfloxacin
Merck,
Sharp
and
Dohme,
Rahway,
NJ:
Pefloxacin
Norwich-Eaton
Pharmaceuticals,
Norwich,
NY;
Enox-
acin
Warner-Lambert
Company,
Morris
Plains,
NJ;
Ofloxacin
Ortho
Pharmaceuticals
Corp.,
Raritan,
NJ;
DJ6783
(Keto
Quinolone)
Daiichi
Seiyako
Co.
Ltd.,
Tokyo,
Japan;
Aztreonam
(SQ26,776)
Squibb
Institute
for
Medical
Research,
Princeton,
NJ;
Fortaz
Glaxo,
Inc.,
Research
Triangle.
Park,
NC.
Bacterial
strains.
Ps.
aeruginosa
(Boston)
was
the
strain
used
in
our
previous
investigations.
Ps.
aeruginosa
(181)
was
a
resistant
clinical
isolate
from
the
Hospital
de
los
Nirios
in
Lima,
Peru.
Patients
infected
with
this
strain
did
not
respond
to
silver
sulfadiazine
therapy.
In
vitro
assay
of
microbial
inhibitions.
Inhibition
indices
were
obtained
by
tube
dilution
tests
using
nutrient
broth.
Growth
in
the
presence
and
absence
of
drugs
was
observed
by
turbidity
measurements
and
colony
counts
after
incubation
at
37°C
for
24
to
48
hours.
In
vitro
development
of
drug
resistance.
Attempts
were
made
to
produce
Ps.
aeruginosa
mutants
resistant
to
quinolones
(Norfloxacin,
Pefloxacin,
Ofloxacin,
DJ6783),
beta
lactam
antibiotics
(Fortaz)
and
AgSD.
This
was
carried
out
by
serially
transferring
cultures
through
sub-
inhibitory
concentrations
of
AgSD
alone,
antibiotics
alone,
or
a
combination
of
AgSD
and
antibiotic
(2:1
molar
ratio
of
AgSD:antibiotic).
After
10
transfers,
these
cultures
were
subcultured
on
a
blood
agar
plate
and
then
onto
nutrient
agar
slants
and
stored.
Susceptibility
of
these
cultures
to
the
corresponding
drugs
to
which
they
were
exposed
was
then
tested.
Animal
experiments—
scald
burn.
Mice
(female)
Swiss,
18
to
22
gm)
were
subjected
to
standardized
scald
using
methods
reported
previously.'
One
hour
postburn,
wounds
were
contaminated
with
freshly
prepared
18-
to
20-hour
broth
culture
of
Pseudomonas
diluted
to
optical
density
0.30.
Mice
were
infected
by
immersing
the
tail
in
the
culture.
Treatment
was
begun
four
hours
or
24
hours
after
infection
by
thoroughly
rubbing
a
medicated
cream
over
the
burned
surfaces.
All
drugs
were
mixed
into
the
placebo
base
of
Silvadene.
Thereafter,
animals
were
observed
and
treated
once
daily;
the
primary
criterion
was
survival.
Animals
that
died
were
autopsied
and
cardiac
blood
was
cultured
to
verify
the
presence
of
Pseudomonas.
In
conducting
the
research
described
in
this
report,
investigators
followed
precisely
the
1987
Guide
of
Labo-
ratory
Animal
Facilities
and
Care,
as
promulgated
by
the
Committee
on
the
Guide
for
Laboratory
Animal
Re-
sources,
National
Academy
of
Sciences,
National
Research
Council.
Results
In
vitro
development
of
drug
resistance.
The
main
objective
was
to
determine
(a)
the
pattern
of
resistance
formation
by
Ps.
aeruginosa
to
various
quinolones
and
360
JBCR,
Vol.
9,
No.
4
other
classes
of
antibiotics
such
as
beta
lactarns
and
cephalosporins;
and
(b)
effect
of
the
addition
of
AgSD
to
the
antibiotics
on
the
formation
of
resistance.
Ps.
aeruginosa
cultures
were
serially
transferred
10
times
through
sub-inhibitory
concentrations
of
Norflox-
acin,
Pefloxacin,
Ofloxacin
(OF),
DJ6783,
Fortaz,
and
AgSD.
Simultaneously,
cultures
also
were
passed
through
sub-inhibitory
concentrations
of
a
combination
of
AgSD
and
the
above
antibiotics.
Minimal
inhibitory
concentra-
tion
(MIC)
of
the
individual
antibiotic
and
the
AgSD-
antibiotic
combination
was
determined
prior
to
and
after
10
passages.
Results
presented
in
Table
1
show
that
the
MIC
of
all
the
antibiotics
except
AgSD
increased
40
to
130
fold
while
the
increase
for
AgSD
was
minimal
(1.6
fold).
The
addition
of
AgSD
to
the
antibiotics,
however,
had
a
signif-
icant
effect
on
the
MIC
as
judged
by
the
smaller
(6.6
to
16
fold)
increase
in
resistance
to
the
antibiotics
as
well
as
antibiotic-AgSD
combination.
Table
1.
Altered
Susceptibility
of
Ps.
aeruginosa
to
Drugs
after
10
Passages
through
Sub-Inhibitory
Concentrations
MIC
(nmol/mL)
Before
After
10
Decrease
in
Drug
Passage
Passages
Susceptibility
AgSD
50.0
80
1.6
Fold
NF
5.0
200 40.0
Fold
NF
AgSD
7.5
75
10.0
Fold
PF
5.0
200 40.0
Fold
PF
AgSD
7.5
75
10.0
Fold
OF
5.0
500
100.0
Fold
OF
AgSD
3.0
50
16.7
Fold
DJ6783
6.0
250
41.7
Fold
DJ6783
AgSD
15.0
100
6.6
Fold
Fortaz
1.5
200
133.0
Fold
Fortaz
AgSD
12.0
100
8.3
Fold
Ratio
of
antibiotic
to
AgSD
is
1:2.
AgSD
=
silver
sulfadiazine
NF
=
Norfloxacin
OF
=
Ofloxacin
PF
=Pefloxacin
The
most
important
observation
from
these
studies
was
that
formation
of
resistance
by
Ps.
aeruginosa
seems
to
level
off
in
10
passages
through
the
AgSD-drug
com-
bination,
whereas
the
MIC
appears
to
increase
steadily
in
the
presence
of
the
antibiotic
alone.
Furthermore,
when
the
cultures
made
resistant
by
passage
10
times
in
the
presence
of
the
combination
were
tested
against
the
individual
antibiotics,
the
MIC
was
found
to
be
lower
than
that
obtained
against
cultures
made
resistant
to
antibio-
tics
alone.
Efficacy
of
AgSD-antibiotic
combination
in
burned
mice.
AgSD,
when
used
in
combination
with
antibio-
tics,
appears
to
slow
down
the
formation
of
resistant
bacteria
in
vitro.
In
order
to
determine
that
mixing
AgSD
with
these
highly
effective
antibiotics
does
not
in
any
way
diminish
their
activity
in
vivo
especially
against
AgSD-
resistant
strains,
these
drugs
were
used
individually
and
in
combination
with
AgSD
in
a
topical
ointment
and
the
efficacy
was
investigated
in
burned
mice
infected
with
sensitive
and
resistant
Ps.
aeruginosa
strains.
Efficacy
of
combination
topical
therapy.
In
mice
infected
with
AgSD-sensitive
Pseudomonas
strain,
the
percentage
mortality
was
lower
in
groups
receiving
a
combination
of
smaller
concentrations
of
AgSD
and
quinolones
or
AgSD
and
Aztreonam
(ATN)
than
either
of
these
drugs
alone
(Table
2A).
For
example,
the
groups
treated
with
10
mm
AgSD
and
5
mm
NF
or
PF
had
a
mortality
of
21%.
When
10
mm
AgSD
alone
or
5
mm
NF
or
PF
alone
were
used
for
treatment,
the
mortality
was
40%,
47
%,
and
52%,
respectively.
Similar
results
were
also
obtained
in
the
case
of
ATN-
AgSD
combination.
The
group
receiving
treatment
with
15
mm
ATN
had
a
mortality
of
40%
while
10
mm
AgSD
+
15
mm
ATN
group
had
only
10%
mortality.
Table
2B
shows
the
mortality
for
different
treatment
groups
in
an
Table
2.
Topical
Therapy
of
Burned
Mice
Infected
with
Ps.
aeruginosa
(Sensitive
Strain)
A
First
Treatment
Four
Hours
Post-Infection
Group
No.
of
Mice
%
Mortality
(Days
Postburn)
2
4
6
8
Control
20
50
95
100
100
10
mm
AgSD
30
3
17
37
40
30
mm
AgSD
20
0 0
0
0
5
mm
NF
30
3
10
37
47
5
mm
NF
10
mm
AgSD
33
3
9
12
21
5
mm
PF
29
3
28
48
52
5
mm
PF
10
mm
AgSD
34
0
9
18
21
15
mm
ATN
25
20
30
40
15
mm
ATN
10
mm
AgSD
25
0
10
10
B
First
Treatment
24
Hours
Post-Infection
Group
No.
of
Mice
%
Mortality
(Days
Postburn)
2
4
6
8
Control
20
40
60
100
100
30
mm
AgSD
20
20
40
60
60
10
mm
NF
10
mm
NF
30
mm
AgSD
20
20
20
5
40
10
40
10
45
10
30
mm
NF
20
0
15
20
25
10
mm
PF
10
mm
PF
30
mm
AgSD
20
20
25
10
50
25
60
25
65
25
30
mm
PF
AgSD
=
silver
sulfadiazine
NF
=
Norfloxacin
PF
=Pefloxacin
ATN
=
Aztreonam
20
10
10
10
20
July/August
1988
361
experiment_
where
the
first
treatment
was
started
24
hours
post-infection.
The
group
of
mice
receiving
30
mm
AgSD
+
10
mm
NF
had
only
10%
mortality
while
the
group
treated
with
AgSD
alone
or
NF
alone
had
a
mortal-
ity
of
60%
and
45%,
respectively.
In
the
AgSD
PF
treated
group,
the
mortality
was
25%,
while
the
group
treated
with
PF
alone
had
mortality
of
65%.
Efficacy
of
combination
therapy
in
burned
mice
infected
with
Ps.
aeruginosa
strain
known
to
be
resistant
to
AgSD
in
patients
is
shown
in
Table
3.
Even
with
the
resistant
strain,
groups
treated
with
15
mm
AgSD
+
10
mm
quinolones
(NF,
PF,
or
Enox)
showed
lower
mortal-
ity
(7
to
25%)
as
compared
to
the
quinolones
alone
(23
to
53%).
Table
3.
Topical
Therapy
of
Burned
Mice
Infected
with
Ps.
aeruginosa
Using
Strain
Known
to
Be
Resistant
in
Vivo
Group
No.
of
Mice
%
Mortality
(Days
Postburn)
2
4
6
8
Control
50
20
60
100
100
15-30
mm
AgSD
50
40
80
100
100
10
mm
NF
10
mm
NF
15
mm
AgSD
30
30
7
0
21
3
23
7
23
7
30
mm
NF
30
0
0
0
0
10
mm
PF
10mm
PF
15
mm
AgSD
35
35
9
0
11
9
29
11
37
11
30
mm
PF
35
0
0 0
0
5-10
mm
Enox
5-10
mm
Enox
15
mm
AgSD
20
20
0
5
5
10
50
20
50
25
30
mm
Enox
20
0
0
0
0
30
mm
ATN
30
mm
ATN
15
mm
AgSD
AgSD
=
silver
sulfadiazine
NP
=
Norfloxacin
Enox
=
Enoxacin
PF
=Pefloxacin
ATN
=
Aztreonam
30
30
7
0
14
7
23
13
23
17
Effect
of
AgSD-antibiotic
combination
on
Candida
albicans.
AgSD
is
unique
in
that
it
exhibits
both
anti-
bacterial
and
antifungal
activity.
In
order
to
investigate
whether
AgSD
would
retain
its
antifungal
activity
when
used
in
combination
with
antibiotics
that
do
not
show
any
antifungal
activity,
the
MIC
of
these
combinations
against
Candida
albicans
were
determined.
The
MIC
of
AgSD-NF
or
AgSD-PF
combination
(2:1
molar
ratio)
was
150
nmol/mL
(100
nmol
AgSD).
AgSD
alone
also
had
similar
MIC
(100
nmol/mL)
(Table
4).
NF
or
PF,
even
at
a
concentration
of
500
nmol/mL
was
not
active
against
C.
albicans.
The
(MBC)
of
silver
sulfadia-
zine
when
used
in
combination
was
also
the
same
as
that
of
AgSD
alone
(200
nmol/mL)
(Table
4).
All
other
antibiotics
tested
in
combination
with
AgSD
showed
similar
results.
Discussion
The
superior
efficacy
of
quinolones
and
other
antibacte-
rial
agents
such
as
Aztreonam
and
Fortaz
signals
the
discovery
of
new
topical
agents.
Development
of
antibio-
tic
resistance
is
a
continuous
problem
in
the
effective
control
of
bacterial
infections.
We
have
initiated
some
studies
to
investigate
whether
the
development
of
resist-
ance
to
antibiotics
can
be
reduced
or
eliminated
by
using
them
along
with
silver
sulfadiazine.
Incidence
of
AgSD-
resistant
mutants
is
low
in
spite
of
its
extensive
use
for
over
a
decade.
Because
the
silver
radical
of
this
compound
plays
an
important
role
in
its
antibacterial
activity
by
directly
binding
to
DNA,
this
drug
was
chosen
for
the
combined
use
with
antibiotics.
Our
studies
reported
here
confirmed
the
hypothesis
that
when
antibiotics
such
as
the
quinolones,
beta
lactams,
and
cephalosporins
were
used
in
combination
with
AgSD,
development
of
resistance
by
Ps.
aeruginosa
was
not
only
reduced
but
leveled
off
at
much
lower
concentra-
tion
of
antibiotics
(Table
1).
Silver
appears
to
play
an
important
role
in
halting
the
resistance
formation.
When
sulfadiazine
was
used
alone
or
along
with
the
antibiotic,
no
influence
was
observed
on
the
occurrence
of
resistance.
This
may
be
the
result
of
the
difference
in
the
mode
of
action
of
silver
and
sulfadiazine.
Sulfadiazine
is
bacterio-
static
and
when
complexed
with
silver
acts
as
a
stable
reservoir
of
silver
ions.
Silver
is
bacteriocidal
even
at
very
low
concentrations.
Thus,
bacteria
exposed
to
AgSD
and
antibiotics
do
not
survive
at
or
above
the
MBC
of
silver,
which
is
.05
to
0.1
pmole/mL.
This
prevents
the
prolifer-
ation
of
any
bacteria
that
could
develop
resistance
to
antibiotics.
Another
important
observation
made
from
these
studies
is
that
resistant
strains
(developed
by
10
repeated
exposures
to
sub-inhibitory
concentrations)
still
exhibit
identical
sensitivity
to
AgSD
as
their
parent
strains.
This
suggests
that
there
is
no
cross-resistance
between
these
antibiotics
and
AgSD.
Table
4.
Effect
of
Silver
Sulfadiazine-Norfloxacin
Combination
on
Candida
albicans
Drug
MIC
(nmol/mL)
MBC
(nmol/mL)
AgSD
100
200
NF
AgSD
150
300
NF
>500
NF
AgSD
150
300
PF
>500
DJ6783
AgSD
150
300
DJ6783
>500
Fortaz
AgSD
150
300
Fortaz
>500
Ratio
of
antibiotic
to
AgSD
is
1:2.
AgSD
=
silver
sulfadiazine
NF
=
Norfloxacin
PP
=Pefloxacin
This
also
was
confirmed
by
carrying
out
in
vivo
exper-
iments
with
NF-resistant
strains.
In
vivo,
burned
mice
were
infected
with
Norfloxacin-resistant
Ps.
aeruginosa
362
JBCR,
Vol.
9,
No.
4
strain
(made
resistant
by
repeated
exposure
to
NF)
and
treated
with
different
topical
agents.
Mortality
in
the
group
treated
with
silver
sulfadiazine
was
10%
while
the
NF-treated
group
had
50%
mortality
(Table
5).
We
also
have
demonstrated
that
NF
is
effective
against
AgSD
resistant
Ps.
aeruginosa
in
experimental
burn
wound
infection.
Table
5.
Topical
Therapy
of
Burned
Mice
Infected
with
Norfloxacin-Resistant
Ps.
aeruginosa
No.
of
Group
Mice
%
Mortality
(Days
Postburn)
2
4
6
8
Control
10
60
100
100
100
30
mm
AgSD
10
0
0
10
10
30
mm
NF
10
0
20
50
50
30
mm
NF
30
mm
AgSD
10
0 0
0
10
AgSD
=
silver
sulfadiazine
NF
=
Norfloxacin
These
in
vitro
and
in
vivo
studies
indicate
that
use
of
a
topical
preparation
containing
silver
sulfadiazine
and
quinolones
or
other
effective
antibiotics
may
delay
or
even
eliminate
the
emergence
of
resistant
bacteria.
Furthermore,
when
AgSD
and
any
one
of
the
above
drugs
are
used
in
combination,
lesser
amounts
of
the
individual
drug
are
needed
to
control
infection,
thereby
reducing
the
toxic
effects
associated
with
larger
doses.
Ointments
containing
silver
sulfadiazine-antibiotic
combination
also
exhibit
prolonged
stability.
When
an
aqueous
suspension
of
15
mm
AgSD
was
mixed
with
10
mm
quinolones
and
incubated
at
37°C
for
one
month,
no
reaction
between
these
two
compounds
(as
determined
by
the
sulfadiazine
released
in
the
supernatant)
was
noted.
The
non-reactivity
of
these
two
compounds
in
the
ointment
may
contribute
to
the
stability
of
the
AgSD
quinolone
preparation.
Systemic
infection
with
fungi,
especially
with
C.
albi-
cans,
has
been
reported
from
several
burn
centers
as
an
important
and
apparently
increasing
hazard
in
the
man-
agement
of
severely
burned
patients.
2
"
.2
'
The
combination
of
AgSD
and
antibiotic
may
have
a
broader
spectrum
due
to
the
antifungal
activity
of
AgSD.
None
of
the
quino-
lones
or
other
antibiotics
are
effective
against
fungi.
In
conclusion,
AgSD
along
with
the
highly
effective
new
antibacterial
agents
such
as
the
quinolones,
Fortaz,
Aztreonam,
etc.,
may
prevent
or
decrease
the
emergence
of
resistant
organisms.
This
appears
to
be
the
result
of
the
bacteriocidal
activity
of
silver
ions
released
from
AgSD,
which
binds
to
DNA
thus
interfering
in
the
bacterial
proliferation.
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July/August
1988
363