Bactericidal activity of gentamicin against S. aureus. In vitro study questions value of prolonged high concentrations

In Vitro Study of Gentamicin Elution from Tendon Grafts

Background

Though vancomycin-soaked graft reduces the infection rate after ACL reconstruction, concerns exist regarding this usage. Gentamicin has been used for graft soakage with satisfactory clinical results but gentamicin's elution characteristics are unknown.

Methods

Thirty Bovine tendon grafts were harvested from ten limbs under sterile conditions. Three tendons from each of the limbs were allotted into three groups and soaked in either saline, gentamicin or vancomycin. Pre-soakage and post-soakage swabs were cultured. Soaked grafts were immersed in a 10 ml saline solution for 5 min (initial washout) and then in another 10 ml saline solution (sustained release) for 10 min. No 1 Whatman filter paper was immersed in the solutions and placed on culture plates streaked with coagulase-negative Staphylococcus aureus (CONS) and methicillin-resistant Staphylococcus aureus (MRSA), and inhibition was noted. The difference between the two proportions was assessed by two proportion t-test for p < 0.05.

Results

No organism was cultured in the pre-soakage or post-soakage swab in any specimen. Specimens from one limb were excluded since saline-soakage showed inhibition. Elution from gentamicin-soaked graft inhibited CONS in eight out of nine samples in initial washout and all samples in sustained release solution but inhibited MRSA only in one sample in sustained release solution and the initial washout solution. Vancomycin elution inhibited both organisms in all samples.

Conclusions

Gentamicin elution from tendon graft achieves minimal inhibitory concentration against susceptible organisms. Though its clinical utility is restricted by limited antimicrobial spectrum, and it could be used where the risk of contamination by MRSA is low.

Analysis of the Ability of Different Allografts to Act as Carrier Grafts for Local Drug Delivery

Bone defects and infections pose significant challenges for treatment, requiring a comprehensive approach for prevention and treatment. Thus, this study sought to evaluate the efficacy of various bone allografts in the absorption and release of antibiotics. A specially designed high-absorbency, high-surface-area carrier graft composed of human demineralized cortical fibers and granulated cancellous bone (fibrous graft) was compared to different human bone allograft types. The groups tested here were three fibrous grafts with rehydration rates of 2.7, 4, and 8 mL/g (F(2.7), F(4), and F(8)); demineralized bone matrix (DBM); cortical granules; mineralized cancellous bone; and demineralized cancellous bone. The absorption capacity of the bone grafts was assessed after rehydration, the duration of absorption varied from 5 to 30 min, and the elution kinetics of gentamicin were determined over 21 days. Furthermore, antimicrobial activity was assessed using a zone of inhibition (ZOI) test with S. aureus. The fibrous grafts exhibited the greatest tissue matrix absorption capacity, while the mineralized cancellous bone revealed the lowest matrix-bound absorption capacity. For F(2.7) and F(4), a greater elution of gentamicin was observed from 4 h and continuously over the first 3 days when compared to the other grafts. Release kinetics were only marginally affected by the varied incubation times. The enhanced absorption capacity of the fibrous grafts resulted in a prolonged antibiotic release and activity. Therefore, fibrous grafts can serve as suitable carrier grafts, as they are able to retain fluids such as antibiotics at their intended destinations, are easy to handle, and allow for a prolonged antibiotic release. Application of these fibrous grafts can enable surgeons to provide longer courses of antibiotic administration for septic orthopedic indications, thus minimizing infections.

Controlling Antibiotic Release from Polymethylmethacrylate Bone Cement

Bone cement is used as a mortar for securing bone implants, as bone void fillers or as spacers in orthopaedic surgery. Antibiotic-loaded bone cements (ALBCs) have been used to prevent and treat prosthetic joint infections by providing a high antibiotic concentration around the implanted prosthesis. High antibiotic concentrations are, on the other hand, often associated with tissue toxicity. Controlling antibiotic release from ALBCS is key to achieving effective infection control and promoting prosthesis integration with the surrounding bone tissue. However, current ALBCs still need significant improvement in regulating antibiotic release. In this review, we first provide a brief introduction to prosthetic joint infections, and the background concepts of therapeutic efficacy and toxicity in antibiotics. We then review the current state of ALBCs and their release characteristics before focusing on the research and development in controlling the antibiotic release and osteo-conductivity/inductivity. We then conclude by a discussion on the need for better in vitro experiment designs such that the release results can be extrapolated to predict better the local antibiotic concentrations in vivo.

Risk assessment of antibiotic resistance development by antibiotic-loaded bone cements: is it a clinical concern?

Because of the risk of bacterial biofilm infections, prophylactic use of antibiotics in orthopaedic procedures involving the implantation of large prosthesis systems is considered mandatory.

A strategy based on the rationale that local antibiotics released from bone cement or other carriers establish a second antibacterial frontline in and around the prosthesis is considered complementary to the administration of systemic antibiotics.

Although less common as a consequence of the initially very high drug concentrations of local antibiotics in the tissues, a selection process of previous high resistance bacteria may occur, leading to antibiotic resistance.

The use of antibiotic combinations in bone cement is generally accepted to improve antibiotic efficacy and minimizes the treatment failure risk due to antibiotic resistance. This is important in septic revisions and/or in patients at particularly high risk of infection.

On an individual basis, the benefit of a lower infection probability with combined systemic and local antibiotic application should outweigh the risk of the selection of more resistant bacteria. Each prevented infection means that a complex and extended antibiotic therapy with risk of resistance development over time has been avoided.

On an epidemiological level there is no clinical evidence that the routine use of bone cement impregnated with appropriate bactericidal antibiotics promotes the widespread development of antibiotic resistance and thereby puts the successful treatment of a prosthetic joint infection at higher risk.

Cite this article: EFORT Open Rev 2019;4:576-584. DOI: 10.1302/2058-5241.4.180104

Increased release of gentamicin from acrylic bone cements under influence of low-frequency ultrasound

The release profile of antibiotics from antibiotic-loaded bone cement, used to prevent infections in total joint arthroplasty, is neither ideal nor complete. Ultrasound has been used to allow drugs to cross otherwise impermeable barriers. The aim of this study was to establish a possible effect of ultrasound on antibiotic release from bone cements. Samples were made of three commercially available gentamicin-loaded bone cements. Part of the samples was allowed to release gentamicin for 3 weeks before insonation. An insonation device produced an ultrasound field with a time average acoustic intensity of 167 mW/cm2 at a frequency of 46.5 kHz. The samples were exposed to the ultrasound field or not exposed to it as a control. The amount of gentamicin released was measured by fluorescence polarization immunoassay. There was a limited increase of gentamicin release with application of ultrasound in fresh samples but not in the samples that had been allowed to release gentamicin. For fresh samples, a linear regression model showed that this ultrasound effect was statistically significant. The mechanism behind these observations is not clear, but it is suggested that microstreaming or localized temperature rises may be involved.

Prophylactic use of antibiotic bone cement: an emerging standard--in opposition

Antibiotic-loaded bone cement (ABLC) is an effective delivery method of local antibiotics. ABLC for treatment should be high dose (>3.6 g per 40 g cement) for beads or spacers and 1 to 2 g antibiotic per 40 g cement for prosthesis fixation. Multiple antibiotics are required for treatment, with antibiotics being individualized according to culture susceptibilities. This approach requires hand mixing because there are no commercial high-dose products. ABLC should be low dose (< or =1 g per 40 g cement) for prophylaxis purposes. Concerns with toxicity and cement mechanical properties are inconsequential with low-dose ABLC. Available clinical evidence supports low-dose ABLC for prophylaxis in revisions and high-risk primary joints, but concerns of emerging drug-resistant organisms probably outweigh routine use of low-dose ABLC in all uncomplicated primary arthroplasties.

Staphylococcus aureus biofilm formation on different gentamicin-loaded polymethylmethacrylate bone cements

In this in vitro study, the formation of a Staphylococcus aureus biofilm on six gentamicin-loaded bone cements (CMW1, CMW3, CMW Endurance, CMW2000, Palacos, and Palamed) was determined in a modified Robbins device over a 3 days time span and related with previously (Van de Belt et al., Biomaterials 21 (2000) 1981) measured kinetics of antibiotic release by these cement brands. The influence of gentamicin release on biofilm formation was quantified by expressing the number of colony-forming units on gentamicin-loaded cement relative to the number of viable organisms on unloaded cement of the same brand. Biofilms formed on all gentamicin-loaded cements, despite the release of antibiotics, followed a consistent pattern in time with a maximum number of colony-forming units per unit cement area found between 24 and 30 h after inoculation. None of the gentamicin-loaded cements showed a reduction in biofilm formation relative to unloaded cements within 6 h after inoculation, whereas only gentamicin-loaded CMW1 and Palacos reduced biofilm formation 24 h after inoculation. Alternatively, CMW Endurance, CMW2000, and Palamed did not exhibit any initial reductions in biofilm formation, but effects started after 72, 48, and 72 h, respectively. Biofilm reduction by gentamicin-loaded CMW3 lasted the longest from 24 to 72 h. Interestingly, each cement seemed to have a different "window-of-effectiveness" with regard to reduction in biofilm formation that did not relate with the gentamicin-release kinetics. Summarising, this study demonstrates that although gentamicin loading of bone cements yields reductions in biofilm formation, S. aureus is able to grow on gentamicin-loaded bone cements.

[Tricalcium phosphate, an antibiotic carrier: a study focused on experimental osteomyelitis in rabbits]

PURPOSE OF THE STUDY

Macroporous beta tricalcium phosphate ceramic beads were elaborated to be a resorbable bone substitute and a drug delivery system carrying gentamicin or vancomycin. The aim of this study was to evaluate this implant into a rabbit experimental osteomyelitis.

MATERIAL AND METHOD

Experimentation included 24 rabbits and was performed in three stages, according to Norden's description. Induction of osteomyelitis was obtained by injection of a sclerosing agent and of Staphylococcus aureus through the lateral side of the metaphysis of the proximal tibia. Three weeks after inoculation, animals were randomly dispatched to one of the three treatment groups. After surgical debridment (to collect cinetics data), the first group received no further treatment and was considered as a control; the second group received a ceramic implant; the third group received a gentamicin-loaded ceramic implant. Euthanasia occurred between 2 days and 21 days after the debridment. Bone samples were obtained to quantify the bacterial and gentamicin bone concentrations. Gentamicin level was also measured inside the ceramic implant. Antibiotic concentration was assessed by a immunoenzymatic method.

RESULTS

Osteomyelitis was obtained in 21 of the 24 animals (87.5%). Antibiotic release was early and complete (before the third day) but gentamicin still remained in the bone for 10 days. Bacterial concentration suggested an antimicrobial activity of the implant, but not a full sterilisation of the osteomyelitis.

CONCLUSION

Norden's experimental osteomyelitis model with rabbit was proposed to assess the therapeutic activity of systemic antibiotics, but not to evaluate biomaterials. Therefore we have shifted for experimental evaluation of biomaterials to a metaphyseal osteomyelitis in the sheep, the patterns of which are close from those of the human disease.

Collagen with gentamicin for prophylaxis of postoperative infection. Staphylococcus aureus osteomyelitis studied in rabbits

In 34 rabbits, both tibiae were inoculated with Staphylococcus aureus. 14 legs received no treatment and served as controls. In 12 legs, the wound was treated with pure collagen and in 18 legs, collagen with gentamicin (Gentacoll) in a dose of 10 mg/kg body weight was applied to the wound before closure. Postoperatively 12 received 10 mg/kg body weight gentamicin intravenously and no local treatment. The animals were killed 7 days after inoculation and evaluated macroscopically and microbiologically for infection. 6 rabbits (12 legs) were used for pharmacokinetic studies only and they were killed after 2, 4, and 18 hours, respectively. 11/14 untreated legs developed a macroscopically acute osteomyelitis. No infection was found in the 18 legs treated with Gentacoll and 1/12 treated with gentamicin systemically had growth of the inoculated bacteria in tissue biopsies. The concentrations of gentamicin in the serum as well as locally reached peak values were well above the MIC value in all groups, with a maximum after 1-2 hours. No gentamicin could be detected after 18 hours, independently of the mode of administration.