J. Biomedical Science and Engineering, 2011, 4, 535-542
doi:10.4236/jbise.2011.48069 Published Online August 2011 (http://www.SciRP.org/journal/jbise/
Published Online August 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Prevention of Staphylococcus aureus biofilm formation on me-
tallic surgical implants via controlled release of gentamicin
David J. McMillan1#, Cameron Lutton2#, Natalie Rosenzweig1,2, Kadaba S. Sriprakash1, Ben Goss2,
Michaela Stemberger2, Michael A. Schuetz2,3, Roland Steck 2
1Bacterial Pathogenesis Laboratory, The Queensland Institute of Medical Research, Brisbane, Australia;
2Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Australia;
3Trauma Services, Dept of Orthopaedics, Princess Alexandra Hospital, Woolloongabba, Australia.
Email: r.steck@qut.edu.au
Received 24 May 2011; revised 18 June 2011; accepted 4 July 2011.
Implant associated infections are a critical health con-
cern following orthopaedic surgery. Sustained local
delivery of antibiotics has been suggested as a means
of preventing these infections. Poly(D,L-lactide)
(PDLLA) is a biodegradable polymer that has been
used to coat implants for the delivery of antibiotics
and other bioactive molecules. While effective, these
studies show that antibiotics are released in a burst
profile. Here we evaluated a method for controlled
release of gentamicin from implant surfaces using the
palmitate alkyl salt to decrease its solubility in aque-
ous solution. Steel Kirschner wires (K-wires) were
coated with Gentamicin-palmitate (GP)-PDLLA,
gentamicin sulphate (GS)-PDLLA or vancomycin
sulphate (VS)-PDLLA, and elution of antibiotics
from coated K-wires investigated using HPLC/MS/
MS. In contrast to burst antibiotic release from the
GS-PDLLA and VS-PDLLA groups, GP was released
in a slower sustained manner. Colonisation and ini-
tial attachment of Staphylococcus aureus Xen29 to
gentamicin-coated K-wires was reduced by 90%
when compared to the non-coated control group.
However there was no statistical difference in recov-
ery of bacteria from GS or GP groups. Bacteria re-
covered from VS-PDLLA coated K-wires decreased
by 36%. Bioluminescence emitted by S. aureus Xen29
was also reduced over seven days in the antibiotic
control groups, demonstrating that growth and
biofilm development over the longer term was im-
paired by antibiotic-PDLLA coating. These results
indicate that using alkyl salts of antibiotics may be an
effective strategy for contro lling the release of antibi-
otics fr om implants .
Keywords: Implant Infections; Poly(D,L-lactide); Biofilm;
Gentamicin; Palmitate
Medical implants and devices are a vital part of modern
medical practice, including orthopaedics and trauma care,
with an estimated 600,000 joint prostheses and two mil-
lion fracture fixation devices used each year in the
United States alone [1]. While improving health out-
comes for patients, implant associated infections have
become a leading cause of nosocomial infection. Be-
tween 2% of elective orthopaedic surgeries [2], and up to
44% of individuals with open fractures develop post-
operative infections [3]. Treatment of these infections
includes long term antibiotic therapy, possible removal
of the device and/or debridement of surrounding tissue
[4,5]. In the most severe cases, amputation may be re-
quired. In addition to the increased health burden, treat-
ment of implant associated infections poses a significant
economic cost to the health sector [6].
Treatment of open fracture associated infections has
added complications. The initial trauma exposes nor-
mally sterile body sites to the external environment and
increases the chance of infection. The presence of ne-
crotic anaerobic tissue also promotes infection, and
damage to vasculature around the wound site can pre-
vent systemically administered antibiotics from reaching
the site of infection [7]. Osteomyelitis is another signifi-
cant complication associated with open fractures, and
delivery of antibiotics at concentrations required to kill
bacteria in the intramedullary cavity is difficult to estab-
lish and maintain. Staphylococcus aureus and coagulase
negative staphylococci are the major bacterial pathogens
associated with osteomyelitis [8-11].
Current approaches towards reduction of implant re-
lated infection include inhibition of initial bacterial ad-
#These two authors contributed equally to this work.
D. J. McMillan et al. / J. Biomedical Science and Engineering 4 (2011) 535-542
hesion, coating implants with polymers which have in-
herent antibacterial activity or by loading of polymers
with antimicrobial agents [12-16]. Polymer-antibiotic
coatings are a particularly attractive option for treatment
of open fractures as these coatings will prevent initial
bacterial attachment and colonisation of the implant, and
release of antibiotics from the implant surface may also
eradicate bacteria from the surrounding tissues. Poly-D,
L-lactide (PDLLA) is a biodegradable non-toxic poly-
mer that has been used to coat implants with antibiotics
and other bioactive molecules [17,18]. PDLLA-genta-
micin sulphate combinations have previously been
shown to reduce implant colonisation by S. epidermidis
in in vitro and in vivo models [15]. However, immisci-
bility between the antibiotic salt and the polymer may
lead to phase separation during coating, with sustained
release profiles being difficult to achieve. Previous stud-
ies have reported a rapid burst release of gentamicin
sulphate from the surface of implants in the first hour
after immersion in PBS [15]. Using poorly water soluble
salts of antibiotics has been suggested as a method to
improve miscibility during coating and their temporal
prophylactic efficiency [19]. In the present study we
compared the ability of a PDLLA/gentamicin palmitate
coating to provide sustained release and inhibit S. aureus
colonisation and biofilm formation on the surface of
steel Kirschner wires, and compared its performance
with PDLLA coatings containing gentamicin sulphate
and vancomycin sulphate.
2.1. Bacterial Strains and Media
Staphylococcus aureus Xen29 is a derivative of the
biofilm forming S. aureus 12600 that has been recombi-
nantly altered by the introduction of a lu x operon de-
rived from Photorhabdus luminescens [20], resulting in
expression of luciferase. Unless otherwise indicated S.
aureus Xen29 was grown in Tryptic Soy Broth (TSB),
Tryptic Soy Agar (TSA) or Columbia agar at 37˚C. The
minimum inhibitory concentrations (MIC) of gentamicin
and vancomycin for S. aureus Xen 29 are reported as
12.5 mg·L–1 and 0.78 mg·L–1 respectively [21]. The MIC
for gentamicin palmitate was determined to be 25
mg· L –1.
2.2. PDLLA-Antibiotic Coating of Kirschner
Three antibiotic-PDLLA combinations, gentamicin sul-
phate (GS), gentamicin palmitate (GP) and vancomycin
sulphate (VS) were used in this study. The PDLLA used
(Resomer R203) was a racemic mixture of the D- and
L-enantiomers of lactic acid with a molecular weight of
29 kDa. Polymer-antibiotic solutions were prepared with
the aim of maximising the antibiotic concentration
whilst simultaneously maintaining solubility of all
components. Gentamicin sulphate (Sigma-Aldrich AG
Pty. Ltd., Castle Hill, Australia) was first dissolved in a
50:50 mixture of dH2O and PEG 1000 (Sigma-Aldrich)
to improve miscibility with the polymer at a concentra-
tion of 22 mg·ml–1. PDLLA was dissolved in DMSO to
a final concentration of 62.5 mg·ml–1. The gentamicin
sulphate solution and PDLLA solutions were mixed in
a 1:10 ratio with stirring at 50˚C to produce a clear
solution. The PDDLA-gentamicin palmitate (pentakis
alkyl salt) solution was prepared in a similar fashion.
However the GP was first dissolved in chloroform,
rather than dH2O at a concentration of 29 mg·ml–1. The
PDDLA-vancomycin solution was prepared by dissolv-
ing vancomycin (22 mg·ml–1) and PDLLA (133.3
mg· m l–1) in DMSO. Steel Kirschner wires (K-wires, 1.8
mm diameter) were manufactured by Synthes (Switzer-
land). The K-wires were cut to a length of approxi-
mately 30 mm and dipcoated in 5 ml of coating solution.
The wire was removed and dried in a vacuum oven for
24 hr at 50˚C, and then the coating process repeated to
increase antibiotic content on each K-wire. Each wire
was inspected visually after coating for completeness of
the coating. Coated wires were promptly used for all
2.3. Abrasion Test
K-wires were cut to 30 mm length and weighed prior to,
and after coating. The K-wires were then inserted and
passed through 1.8 mm diameter drill holes in 25 mm
thick bovine cortical bone specimens and reweighed.
After gently washing in methanol to remove dislodged
polymer and bone, the samples were weighed a fourth
time. The total and percentage loss of coating mass for
each K-wire was then calculated.
2.4. Elution Assays
To determine the rate of release of antibiotics from im-
plant surface, coated K-wires, cut into three 50 mm long
segments, were placed in 10 ml tubes containing 10 ml
of PBS (three segments per tube). The samples were
incubated at 37˚C and 500 µl aliquots collected at 0 h,
10 min, 1 h, 2 h, 3 h, 4 h, 8 h, 24 h, and 48 h and 7 days.
Antibiotic concentrations in the samples were deter-
mined by HPLC/MS/MS using an AB/Sciex API4000Q
(AB/Sciex Concord, Ontario, Canada) mass spectrome-
ter equipped with an electrospray (TurboV) interface
coupled to a Shimadzu Prominence HPLC system (Shi-
madzu Corp., Kyoto, Japan), resulting in a detection
limit of 0.5 mg·L–1.
2.5. Bacterial Attachment Assays
To measure attachment to K-wires, S. aureus Xen29 was
opyright © 2011 SciRes. JBiSE
D. J. McMillan et al. / J. Biomedical Science and Engineering 4 (2011) 535-542 537
grown overnight in TSB. After washing, the bacteria
were diluted in PBS to OD595 of 0.1. This suspension
was further diluted 1:10 in PBS. A 1.3 ml aliquot of this
suspension, containing approximately 106 CFU of S.
aureus was then added to 1.5 ml microfuge tubes, and
individual K-wires added to each tube. After incubation
at 37˚C for two hours, the K-wires were recovered,
washed three times in PBS to remove residual antibiotic
and transiently attached bacteria, and placed in 1.3 ml of
a 0.5% trypsin/PBS solution at room temperature. Bac-
teria were subsequently dislodged from the surface by
sonication at 47 KHz for 15 min at room temperature.
The suspension was then centrifuged to concentrated
bacteria, and resuspended in PBS. After serial dilution of
the resuspended samples, aliquots were plated onto TSB.
The percentage of CFU recovered from PDLLA-antibi-
otic groups when compared to the average CFU recov-
ered from the uncoated and PDLLA control groups was
determined for each set of experiments.
2.6. Inhibition of Biofilm Growth
Growth of S. aureus Xen29 biofilms on the surface of
K-wires was monitored for 7 days using an IVIS® 100
Series (Caliper Life Sciences, Hopkinton, USA) camera
[20]. Coated and uncoated K-wires were placed in
6-well tissue culture plate. Five millilitres of TSB con-
taining approximately 105 CFU of S. aureus Xen29 was
then added to each well, and the plate incubated over-
night without shaking. Next day, the K-wires were re-
covered, gently rinsed, and transferred to a new 6-well
plate containing fresh TSA. Bioluminescence was cap-
tured using the IVIS camera approximately 20 min after
transfer to the new 6-well plate. The bioluminescence in
a defined region surrounding each K-wire was then
quantified using Living image software. After visualisa-
tion, the K-wires were placed in a new 6-well plate, and
fresh media (without bacteria) added, and incubation
2.7. Statistical Analysis
T-tests were used to assess the statistical significance be-
tween mean values of experimental and control groups in
the study. Differences were considered significant at p
3.1. Loading and Mechanical Strength
The average mass of the GS, GP and VS coating on the
K-wires was 5.1 mg, 3.2 mg and 3.0 mg respectively. To
determine the mechanical robustness of the coatings in
the face of abrasive forces similar to that expected in
orthopaedic surgery, the K-wires were passed through
1.8 mm drill holes in bovine cortical bone, and re-
weighed (Figure 1).
The GS group lost an average 1.9 mg (37% of coating
mass) per K-wire, whereas the GP group lost an average
0.5 mg (16%). The difference in total loss and percent-
age loss for the two groups was not statistically signifi-
cant. Very little loss of mass (0.06 mg, 1.5%) was ob-
served in the VS group.
3.2. Antibiotic Release
Antibiotics coated to the surface of implants have two
functions. Firstly, antibiotic on the implant should pre-
vent colonisation by viable bacteria, and subsequent
biofilm development. Secondly, released antibiotic should
kill bacteria in the immediate vicinity of the implant.
The release of antibiotic from the three coating groups
was investigated by immersing coated K-wires in PBS,
and measuring concentrations of eluates over time (Fig-
ure 2). For the GS coated K-wires, the majority (71%)
of the gentamicin was rapidly released in the first 10 min.
after immersion and reached a maximum concentration
at 2 hours. The release of vancomycin was even more
rapid with it reaching 91% of its maximum solution
concentration within the first 10 min. after immersion.
Like the GS group, the maximum concentration of van-
comycin in the solution was reached after two hours. In
contrast to the burst release observed in these two groups,
the GP group had a much slower rate of release. At 10
min., the mean concentration of gentamicin in the eluate
of this group was 6.5 mg·L–1 ± 2.3 mg·L–1, seven fold
lower than measured in the GS group and 8 fold lower
than measured in VS group at the same time point. The
GP group continued to provide a sustained release for up
to 48hrs and peaked at a concentration of 24 ± 5 mg·L–1,
2.5 times less than either the GS or VS groups.
3.3. Inhibition of Colonisation of K-Wires
To test the ability of antibiotic-PDLLA to inhibit S.
aureus colonisation, K-wires were incubated in static S.
aureus suspension, and then washed to remove residual
antibiotic. Adhered bacteria were then dislodged by
Figure 1. Loss of coating mass after abrasion testing. Coated
K-wires were weighed prior to, and after being passed through
a 1.8 mm bore hole in bovine cortical bone. Data points repre-
senting individual K-wires prior to and after passing through
the bone are connected by the lines.
opyright © 2011 SciRes. JBiSE
D. J. McMillan et al. / J. Biomedical Science and Engineering 4 (2011) 535-542
sonication, serially diluted and plated onto Columbia
agar. The total and percentage CFU in the experimental
group, relative to the CFU recovered from control
groups (uncoated and PDLLA groups) was then deter-
mined for each individual experiment (Figure 3). Over
antibiotic (mg·L)
Figure 2. Elution of antibiotic from gentamicin sulphate () S
(n = 8), gentamicin palmitate () (n = 5) and vancomycin ()
coated K-wires. K-wires were immersed in PBS, and 500 µl
aliquots taken over 48 hours. Data is presented as the mean
concentration of antibiotic in the gentamicin sulphate (n = 8)
and gentamicin palmitate (n = 5) and vancomycin (n = 8) solu-
tions. Error bars represent the SEM.
Figure 3. PDLLA-antibiotic coating inhibit bacterial colonisa-
tion of K-wires. K-wires were incubated with a suspension of S.
aureus Xen29 for two hours. Bacteria were dislodged from the
K-wire by incubation with 5% trypsin and sonication. The
suspensions were serially diluted and plated on agar, incubated
overnight and CFU determined. Data is presented as the mean
percentage CFU remaining on each antibiotic-PDLLA group of
K-wires when compared to the average CFU present on the
“uncoated” and “PDLLA” coated control K-wires. All experi-
ments were performed at least in triplicate, with at least three
K-wires represented in each independent experiment. Error
bars represent SEM. Differences between the mean %CFU of
control and experimental groups were found to be statistically
all, significantly lower bacterial numbers were recovered
from the antibiotic coated K-wires. The mean bacterial
CFU recovered from uncoated and PDLLA coated
K-wires was 1.2 × 105 ± 5.8 × 104 CFU and 1.0 × 105 ±
5.8 × 104 CFU respectively. The mean CFU recovery of
bacteria from the GS (1.1 × 104 ± 6.9 × 103), GP (1.0 ×
103 ± 6.3 × 102) and VS groups (8.1 × 104 ± 4.2 × 104)
was significantly lower. There was a 93% reduction in
CFU on GS and 99% reduction of CFU on GP coated
wires. The 36% reduction in colonisation observed in the
VS group was not as pronounced but still represents a
significant decrease in comparison to the control group
(p < 0.05).
3.4. Inhibition of Biofilm Formation
The results of the colonisation studies demonstrated that
all the antibiotic coatings inhibited initial colonisation
events. To assess whether the reduced colonisation had
any impact on biofilm formation, we next incubated K-
wires with recombinant S. aureus Xen29 over a period
of seven days, and captured bioluminescence emitted by
the bacteria using an IVIS CCD camera (Figure 4). The
advantage of the IVIS system over other methods for
bacterial monitoring lies in the ability to measure the
same sample at multiple time points. For the uncoated
group, an initial signal intensity of 3.1 × 104 photon/s
was recorded on day 1. The luminescence then increased
to a maximum on the fourth day of the incubation, and
declined slowly after this time point. In contrast to other
groups, the photonic signal observed on day 1 for the
PDLLA control group exceeded 105 photons/s. The rela-
tively higher level of colonisation, as measured by lu-
minescence, was maintained throughout the experiment.
After this time point, biofilm growth continued to in-
crease over the course of the experiment. For the antibi-
Figure 4. Biofilm formation on the surface of K-wires. K-
wires were incubated with S. aureus Xen29 overnight and bio-
luminescence emitted by the bacteria monitored daily using an
IVIS CCD camera. Figure shows mean luminescent signal
intensity of control and experimental groups measured over the
seven days of the experiment.
opyright © 2011 SciRes. JBiSE
D. J. McMillan et al. / J. Biomedical Science and Engineering 4 (2011) 535-542 539
otic-PDLLA groups, the signal intensity observed on the
first day was similar to the uncoated sample.
For the GS group, signal intensity rose to a maximum
of 6.5 × 104 photon/s on day 6. However this was not
significantly greater than the intensity on day 1. Simi-
larly, no statistical increase in the intensity of signal in
the vancomycin group was observed. The GP group also
showed no increase in intensity over the first four days.
While a small increase was observed in the last two days
of the experiment, the signal intensity was still ten times
lower than the PDDLA group on comparable days. Us-
ing the IVIS camera, we were also able to detect bacteria
in the culture supernatant for each sample for each day
of the experiment (data not shown). Taken together these
results indicated that the three antibiotic groups were
effective at reducing biofilm formation on the K-wires.
Infections are still among the leading causes for compli-
cations in orthopaedic surgery, ranging in frequency
from 1% - 5% in elective, joint replacement surgery, to
higher rates in osteosynthesis of open fractures [9]. They
are painful, difficult to treat and can lead to implant-
related osteomyelitis. Furthermore, these complications
cause prolonged hospitalisation and are associated with
significantly increased costs. Therefore, antibiotic pro-
phylaxis, usually via intravenous administration, is a
routine procedure in these surgeries [22]. Although this
systemic prophylaxis is successful in most cases, deliv-
ery to wounds can be problematic due to the presence of
ischemic tissue. The difficulty in delivering antibiotics to
these tissues also increases the risk of the growth of im-
plant associated biofilms, which show high resistance to
antibiotics and the immune system and often require
implant removal [23,24].
The development of biodegradable implant coating
techniques that provide both local delivery of antibiotics
and prevent biofilm formation is an attractive approach
for the prevention of these infections. Local delivery
ensures that high concentrations of antibiotic are deliv-
ered at the wound site where infections are most likely to
occur, whilst antibiotics on the surface of the implant
may prevent bacterial colonisation. Antibiotic release
from these coatings occurs in a burst release fashion,
which is desirable in some clinical situations (e.g. open
fractures with contamination), but which on its own may
not be ideal to prevent infections caused by delayed or
repeated exposure to bacteria. To that end we investi-
gated the use of an alkyl salt of gentamicin, gentamicin
palmitate, to limit burst release and extend delivery time
from PDLLA coated implants.
Antibiotic miscibility with the coating solution had a
large effect on the film toughness. Increasing the misci-
bility of the antibiotic with PDLLA resulted in tougher
films with the gentamicin palmitate/PDLLA film as evi-
dent by its losing less mass when compared to the gen-
tamicin sulphate/PDLLA film (although the difference
was not statistically significant). Vancomycin, which
was miscible in all proportions with PDLLA, lost very
little mass under our test conditions. We attribute the
difference in mass loss to the disruption that immiscible
particles, such as gentamicin sulphate, cause to the film
structure upon drying. It is well known that the interface
between immiscible particles and polymer films has a
great influence on the toughness of the films [25,26].
Particles with no adhesive interaction with the polymer
essentially act as porogens, creating defects in the film.
Conversely, the miscibility of vancomycin provided a
more even distribution and no reduction in film tough-
ness. Whilst not specifically reported here, it was possi-
ble to include too much vancomycin in the films with
concentrations over 50 mg·ml–1 completely disrupting
stable film formation. These coatings were shed from the
implant surface (data not shown).
The release rates of the gentamicin and vancomycin in
our study are similar to those reported when no coating
was used on implants [27]. Under our time frame the
polymer did not degrade significantly or show any effect
of controlling the release as has been reported by other
authors [28]. We attribute this difference to our coating
procedure. Our procedure produced very smooth films
unlike those prepared by Aviv et al. whose coating pro-
cedure produced slightly porous films with phase sepa-
rated domains of antibiotic. In their system early pro-
longed release is most likely due to an increased path
length of antibiotic through pores in the polymer surface,
not degradation mediated release as they saw at later
time points. It is likely that our procedure of improving
the solution miscibility of the antibiotics with the poly-
mer and the smoothness of the films that were created
reduced the porosity and effectively trapped any non-
surface bound antibiotic in the polymer. Our release re-
sults, coupled with the long degradation time of PDLLA
[29,30], suggest that the release of these antibiotics over
the time frame considered is mostly due to residual anti-
biotic on the surface. We demonstrated that the use of
gentamicin palmitate does result in an extended delivery
compared to both gentamicin sulphate and vancomycin,
but in less total antibiotic release. We would expect that
the longer release to be due to a decreased solubility of
the palmitate salt in PBS. Similar to gentamicin sulphate
and vancomycin, this would be surface adsorbed gen-
tamicin palmitate that slowly dissolves in PBS. There
are two possible explanations for a lower total amount of
gentamicin palmitate released. As the salt is sparingly
soluble in PBS it may be that the solubility limit was
opyright © 2011 SciRes. JBiSE
D. J. McMillan et al. / J. Biomedical Science and Engineering 4 (2011) 535-542
reached in the volume of PBS used in the elution ex-
periment. Whilst we cannot totally rule this explanation
out, we believe that the low total amount released (24
mg· L –1) is well below the solubility limit of the antibi-
otic in PBS. The media being changed constantly in the
biofilm experiments and the increase in bacteria at four
days also suggests that the elution experiments closely
reflect the amount of antibiotic able to be released. A
more likely explanation is that our coating procedure
results in less total antibiotic adsorbed to the surface.
Care was taken to ensure that similar molar concentra-
tions of gentamicin were present in both the sulphate and
palmitate coating solutions and that the coating proce-
dures were as similar as possible. However the differing
solvents used, i.e. H20/PEG1000/DMSO combination for
the gentamicin sulphate and just chloroform for the gen-
tamicin palmitate likely resulted in differing levels of
partitioning of the antibiotic to the surface during the
coating procedure. It should be possible to increase the
amount of gentamicin palmitate on the surface simply by
increasing the coating solution concentration or increas-
ing the number of dip coatings. This would certainly
increase the total amount of antibiotic released and may
possibly extend the delivery time. The slower sustained
release of antibiotic may be particularly effective in
combating late onset osteomyelitis [31].
Despite having different delivery profiles all three an-
tibiotics reduced the numbers of attached bacteria. Both
salts of gentamicin were equally effective in reducing
bacterial attachment and were in fact better than vanco-
mycin at the two hour time point used for testing. Given
that all antibiotics were present in concentrations above
their minimum inhibitory concentrations this difference
may be due to vancomycin’s slow bactericidal activity
compared with gentamicin [32]. The longer incubation
times in the biofilm assays, and similar reduction in
bacterial load on gentamicin and vancomycin-coated
K-wires would seem to support this. Unlike the results of
Gollwitzer et al. [13] we did not see a reduction in the
attachment of viable bacteria on PDLLA-coated steel
when compared to uncoated steel. The contradicting
results may be due to the different bacterial species used,
(S. aureus vs S. epidermidis) although PDLLA coated
wires have shown no inhibitory effect on bacterial
growth in vivo [33].
Under the conditions used in the study, the presence
of antibiotics on the K-wires was able to reduce biofilm
formation for at least seven days, as determined through
measurement of bioluminescence. Although vancomycin
was less effective in preventing initial colonisation, it is
as effective as the gentamicin salts in retarding biofilm
formation compared with both the uncoated and PDLLA
coated wires. Our results also showed that an increase in
bacterial load after day 4 occurred for K-wires coated
with gentamicin palmitate, but not gentamicin sulphate.
This suggests that some viable S. aureus, below the de-
tection limits of the assay, remained on the GP coated
K-wires on days 1 - 4. The sustained elution profile of
the GP group, resulting in reduced concentrations of
gentamicin at specific time-points, may have enabled
small numbers of bacteria to survive. The subsequent
increase in bacterial numbers after four days suggests
that the majority of gentamicin had been released by this
time, allowing the growth of S. aureus. This contrasts
with the high concentrations of gentamicin in solution
associated with the burst kinetics of the gentamicin sul-
phate group, possibly killing all bacteria in the assay at
an early time-point. These observations highlight the
importance of achieving the appropriate antibiotic elu-
tion kinetics in the clinical environment. While it is de-
sirable for antibiotic to elute from implants in appropri-
ate concentrations to kill surrounding bacteria, sufficient
antibiotic must also remain attached to the implants to
help prevent later colonisation events. One of the draw-
backs of this protocol is that we did not challenge the
coated rods after the burst release was complete, as
might be expected in vivo.
Collectively these results show that the presence of
antibiotics on implants can reduce bacterial attachment
and biofilm formation and that antibiotic/polymer misci-
bility affects the coating toughness, but not the delivery
profile. We have demonstrated that gentamicin-palmitate
has a reduced rate of release from metal surgical im-
plants, but that this did not translate to a significant re-
duction in bacterial attachment and biofilm formation in
our model. Ultimately, the effect of different release
profiles on the prevention of implant-related bone infec-
tions will have to be tested and demonstrated in suitable
pre-clinical in vivo animal models. Nevertheless the sus-
tained release kinetics of gentamicin palmitate from
PDLLA, as demonstrated in our in vitro experiments, may
represent a strategy for improving the release of antibiot-
ics from polymer coatings for combating infection.
This work was supported by a grant from the Wesley Research Institute
(2007/04), Brisbane, Australia.
The authors would like to thank Prof Axel Stemberger from the Tech-
nical University, Munich (Germany) for his assistance with the concep-
tual design for this study, and Dr. Geoff Eaglesham from Queensland
Health Scientific Services for assistance with elution assays.
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