The selection of appropriate bacterial strains in preclinical evaluation of infection-resistant biomaterials

Implant retention in a rabbit model of fracture-related infection

Aims

Fracture-related infection (FRI) is commonly classified based on the time of onset of symptoms. Early infections (< two weeks) are treated with debridement, antibiotics, and implant retention (DAIR). For late infections (> ten weeks), guidelines recommend implant removal due to tolerant biofilms. For delayed infections (two to ten weeks), recommendations are unclear. In this study we compared infection clearance and bone healing in early and delayed FRI treated with DAIR in a rabbit model.

Methods

Staphylococcus aureus was inoculated into a humeral osteotomy in 17 rabbits after plate osteosynthesis. Infection developed for one week (early group, n = 6) or four weeks (delayed group, n = 6) before DAIR (systemic antibiotics: two weeks, nafcillin + rifampin; four weeks, levofloxacin + rifampin). A control group (n = 5) received revision surgery after four weeks without antibiotics. Bacteriology of humerus, soft-tissue, and implants was performed seven weeks after revision surgery. Bone healing was assessed using a modified radiological union scale in tibial fractures (mRUST).

Results

Greater bacterial burden in the early group compared to the delayed and control groups at revision surgery indicates a retraction of the infection from one to four weeks. Infection was cleared in all animals in the early and delayed groups at euthanasia, but not in the control group. Osteotomies healed in the early group, but bone healing was significantly compromised in the delayed and control groups.

Conclusion

The duration of the infection from one to four weeks does not impact the success of infection clearance in this model. Bone healing, however, is impaired as the duration of the infection increases.

Development and Characterization of a Subcutaneous Implant-Related Infection Model in Mice to Test Novel Antimicrobial Treatment Strategies

Orthopedic-device-related infection is one of the most severe complications in orthopedic surgery. To reduce the associated morbidity and healthcare costs, new prevention and treatment modalities are continuously under development. Preclinical in vivo models serve as a control point prior to clinical implementation. This study presents a mouse model of subcutaneously implanted titanium discs, infected with Staphylococcus aureus, to fill a gap in the early-stage testing of antimicrobial biomaterials. Firstly, three different inocula were administered either pre-adhered to the implant or pipetted on top of it following implantation to test their ability to reliably create an infection. Secondly, the efficacy of low-dose (25 mg/kg) and high-dose (250 mg/kg) cefazolin administered systemically in infection prevention was assessed. Lastly, titanium implants were replaced by antibiotic-loaded bone cement (ALBC) discs to investigate the efficacy of local antibiotics in infection prevention. The efficacy in infection prevention of the low-dose perioperative antibiotic prophylaxis (PAP) depended on both the inoculum and inoculation method. Bacterial counts were significantly lower in animals receiving the high dose of PAP. ALBC discs with or without the additional PAP proved highly effective in infection prevention and provide a suitable positive control to test other prevention strategies.

Cocktail of isobavachalcone and curcumin enhance eradication of Staphylococcus aureus biofilm from orthopedic implants by gentamicin and alleviate inflammatory osteolysis

Orthopedic device-related infection (ODRI) caused by Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA) biofilm may lead to persist infection and severe inflammatory osteolysis. Previous studies have demonstrated that both isobavachalcone and curcumin possess antimicrobial activity, recent studies also reveal their antiosteoporosis, anti-inflammation, and immunoregulatory effect. Thus, this study aims to investigate whether the combination of isobavachalcone and curcumin can enhance the anti-S. aureus biofilm activity of gentamicin and alleviate inflammatory osteolysis in vivo. EUCAST and a standardized MBEC assay were used to verify the synergy between isobavachalcone and curcumin with gentamicin against planktonic S. aureus and its biofilm in vitro, then the antimicrobial and immunoregulatory effect of cocktail therapy was demonstrated in a femoral ODRI mouse model in vivo by μCT analysis, histopathology, quantification of bacteria in bone and myeloid-derived suppressor cell (MDSC) in bone marrow. We tested on standard MSSA ATCC25923 and MRSA USA300, 5 clinical isolated MSSA, and 2 clinical isolated MRSA strains and found that gentamicin with curcumin (62.5–250 μg/ml) and gentamicin with isobavachalcone (1.56 μg/ml) are synergistic against planktonic MSSA, while gentamicin (128 μg/ml) with curcumin (31.25–62.5, 250–500 μg/ml) and gentamicin (64–128 μg/ml) with isobavachalcone (1.56–12.5 μg/ml) exhibit synergistic effect against MSSA biofilm. Results of further study revealed that cocktail of 128 μg/ml gentamicin together with 125 μg/ml curcumin +6.25 μg/ml isobavachalcone showed promising biofilm eradication effect with synergy against USA300 biofilm in vitro. Daily intraperitoneal administration of 20 mg/kg/day isobavachalcone, 20 mg/kg/day curcumin, and 20 mg/kg/day gentamicin, can reduce inflammatory osteolysis and maintain microarchitecture of trabecular bone during orthopedic device-related MRSA infection in mice. Cocktail therapy also enhanced reduction of MDSC M1 polarization in peri-implant tissue, suppression of MDSC amplification in bone marrow, and Eradication of USA300 biofilm in vivo. Together, these results suggest that the combination of isobavachalcone and curcumin as adjuvants administrated together with gentamicin significantly enhances its antimicrobial effect against S. aureus biofilm, and can also modify topical inflammation in ODRI and protect bone microstructure in vivo, which may serve as a potential treatment strategy, especially for S. aureus induced ODRI.

3D-Printed Gentamicin-Releasing Poly-ε-Caprolactone Composite Prevents Fracture-Related Staphylococcus aureus Infection in Mice

Bacterial infections are a serious healthcare complication in orthopedic and trauma surgery worldwide. Compared to systemic, local antibiotic prophylaxis has been shown to provide a higher antibiotic dose and bioavailability at the bone site with minimum toxic effects. However, there are still not enough biomaterial and antibiotic combinations available for personalized implant sizes for patients. The aim of this study was to develop a bone fixation plate coating made of a composite of poly-ε-caprolactone, hydroxyapatite and halloysite nanotubes loaded with gentamicin sulphate and fabricated via fused filament fabrication 3D printing technology. The mechanical and thermal properties of the biomaterial were analyzed. The in vitro release kinetics of gentamicin sulphate were evaluated for 14 days showing a burst release during the first two days that was followed by a sustained release of bactericidal concentrations. The composite loaded with 2 and 5% gentamicin sulphate exhibited complete antimicrobial killing of Staphylococcus aureus in an ex vivo mouse femur fixation plate infection model. Moreover, a fixation plate of the composite loaded with 5% of gentamicin sulphate was able to prevent S. aureus infection in the bone and surrounding tissue in an in vivo mouse bone fixation plate infection model 3 days post-surgery. In conclusion, the newly developed composite material successfully prevented infection in vivo. Additionally, the ability to use fused filament fabrication 3D printing to produce patient-specific implants may provide a wider range of personalized solutions for patients.

Bioinspired drug delivery strategies for repurposing conventional antibiotics against intracellular infections

Bacteria have developed a wealth of strategies to avoid and resist the action of antibiotics, one of which involves pathogens invading and forming reservoirs within host cells. Due to the poor cell membrane permeability, stability and retention of conventional antibiotics, this renders current treatments largely ineffective, since achieving a therapeutically relevant antibiotic concentration at the site of intracellular infection is not possible. To overcome such challenges, current antibiotics are 'repurposed' via reformulation using micro- or nano-carrier systems that effectively encapsulate and deliver therapeutics across cellular membranes of infected cells. Bioinspired materials that imitate the uptake of biological particulates and release antibiotics in response to natural stimuli are recently explored to improve the targeting and specificity of this 'nanoantibiotic' approach. In this review, the mechanisms of internalization and survival of intracellular bacteria are elucidated, effectively accentuating the current treatment challenges for intracellular infections and the implications for repurposing conventional antibiotics. Key case studies of nanoantibiotics that have drawn inspiration from natural biological particles and cellular uptake pathways to effectively eradicate intracellular pathogens are detailed, clearly highlighting the rational for harnessing bioinspired drug delivery strategies.

Surface Design for Antibacterial Materials: From Fundamentals to Advanced Strategies

Healthcare-acquired infections as well as increasing antimicrobial resistance have become an urgent global challenge, thus smart alternative solutions are needed to tackle bacterial infections. Antibacterial materials in biomedical applications and hospital hygiene have attracted great interest, in particular, the emergence of surface design strategies offer an effective alternative to antibiotics, thereby preventing the possible development of bacterial resistance. In this review, recent progress on advanced surface modifications to prevent bacterial infections are addressed comprehensively, starting with the key factors against bacterial adhesion, followed by varying strategies that can inhibit biofilm formation effectively. Furthermore, "super antibacterial systems" through pre-treatment defense and targeted bactericidal system, are proposed with increasing evidence of clinical potential. Finally, the advantages and future challenges of surface strategies to resist healthcare-associated infections are discussed, with promising prospects of developing novel antimicrobial materials.

Antibiofilm efficacy of focused high-energy extracorporeal shockwaves and antibiotics in vitro

Aims

Biofilm formation is one of the primary reasons for the difficulty in treating implant-related infections (IRIs). Focused high-energy extracorporeal shockwave therapy (fhESWT), which is a treatment modality for fracture nonunions, has been shown to have a direct antibacterial effect on planktonic bacteria. The goal of the present study was to investigate the effect of fhESWT on Staphylococcus aureus biofilms in vitro in the presence and absence of antibiotic agents.

Methods

S. aureus biofilms were grown on titanium discs (13 mm × 4 mm) in a bioreactor for 48 hours. Shockwaves were applied with either 250, 500, or 1,000 impulses onto the discs surrounded by either phosphate-buffered saline or antibiotic (rifampin alone or in combination with nafcillin). The number of viable bacteria was determined by quantitative culture after sonication. Representative samples were taken for scanning electron microscopy.

Results

The application of fhESWT led to a ten-fold reduction in bacterial counts on the metal discs for all impulse numbers compared to the control (p < 0.001). Increasing the number of impulses did not further reduce bacterial counts in the absence of antibiotics (all p > 0.289). Antibiotics alone reduced the number of bacteria on the discs; however, the combined application of the fhESWT and antibiotic administration further reduced the bacterial count compared to the antibiotic treatment only (p = 0.032).

Conclusion

The use of fhESWT significantly reduced the colony-forming unit (CFU) count of a S. aureus biofilm in our model independently, and in combination with antibiotics. Therefore, the supplementary application of fhESWT could be a helpful tool in the treatment of IFIs in certain cases, including infected nonunions.

Cite this article: Bone Joint Res 2021;10(1):77–84.

Circumventing antimicrobial-resistance and preventing its development in novel, bacterial infection-control strategies

INTRODUCTION

Development of new antimicrobials with ever 'better' bacterial killing has long been considered the appropriate response to the growing threat of antimicrobial-resistant infections. However, the time-period between the introduction of a new antibiotic and the appearance of resistance amongst bacterial pathogens is getting shorter and shorter. This suggests that alternative pathways than making ever 'better' antimicrobials should be taken.

AREAS COVERED

This review aims to answer the questions (1) whether we have means to circumvent existing antibiotic-resistance mechanisms, (2) whether we can revert existing antibiotic-resistance, (3) how we can prevent the development of antimicrobial-resistance against novel infection-control strategies, including nano-antimicrobials.

EXPERT OPINION

Relying on relieving antibiotic-pressure and natural outcompeting of antimicrobial-resistant bacteria seems an uncertain way out of the antibiotic-crisis facing us. Novel, non-antibiotic, nanotechnology-based infection control-strategies are promising. At the same time, rapid development of new resistance mechanisms once novel strategies is taken into global clinical use, may not be ruled out and must be closely monitored. This suggests focusing research and development on designing suitable combinations of existing antibiotics with new nano-antimicrobials in a way that induction of new antimicrobial-resistance mechanisms is avoided. The latter suggestion, however, requires a change of focus in research and development.

Focused high-energy extracorporeal shockwaves as supplemental treatment in a rabbit model of fracture-related infection

Focused high-energy extracorporeal shockwave therapy (fhESWT) is used to improve fracture healing in cases of nonunion. In addition, it has been shown to have direct antibacterial effects. We evaluated fhESWT as an adjunct to conventional treatment in a clinically relevant rabbit model of fracture-related infection (FRI). A humeral osteotomy in 31 rabbits was fixed with a seven-hole locking compression plate. FRI was established with a clinical Staphylococcus aureus isolate. After 2 weeks, a revision surgery was performed with debridement, irrigation, and implant retention. Rabbits then received: no further treatment (controls); shockwaves (4000 impulses with 23 kV at days 2 and 6 after revision); systemic antibiotics (rifampin and nafcillin); or the combination of antibiotics and shockwaves. Treatments were applied over 1 week. Blood cultures were taken before and after shockwave sessions. After another week without treatment, rabbits were euthanized and quantitative bacteriology was performed on implants and tissues to determine infection burden. Indicator organs (brain, heart, liver, lungs, kidneys, and spleen) were cultured to assess possible bacteremia. All the rabbits were infected at revision surgery as determined by the bacteriological culture of debrided materials. fhESWT in combination with antibiotic treatment lowered the bacterial burden 100-fold compared with antibiotic treatment alone in all samples (P = .38). This effect was most prevalent for the implant sample (P = .08). No significant effect was seen for fhESWT alone compared with untreated controls. No signs of bacteremia occurred in blood cultures and organs. fhESWT appears safe and could be a helpful adjunct to conventional treatment in certain difficult-to-treat FRIs.

Local Application of a Gentamicin-Loaded Hydrogel Early After Injury Is Superior to Perioperative Systemic Prophylaxis in a Rabbit Open Fracture Model

OBJECTIVES

Open fractures are at significant risk of developing a fracture-related infection, despite the routine administration of perioperative antibiotic prophylaxis. Early application of antibiotic prophylaxis is known to reduce infection rates; however, most international guidelines focus on postoperative duration rather than prehospital administration. We compared conventional perioperative prophylaxis against early prehospital prophylaxis either as a systemic single shot of cefuroxime or a locally applied gentamicin-loaded hydrogel in a laboratory animal model.

METHODS

Thirty New Zealand white rabbits underwent a first surgical procedure to create an open wound, bone damage and contamination with Staphylococcus aureus. After a 4-hour observation period mimicking the time-to-treatment, the animals underwent a second procedure to irrigate the wound and apply a fracture fixation device. The 5 groups (n = 6 per group) received (1) no treatment; (2) conventional 24-hour cefuroxime; (3) an early single shot of cefuroxime 15 minutes after trauma; (4) a combined early and standard systemic prophylaxis; and (5) early application of a gentamicin-loaded hydrogel that was removed during irrigation.

RESULTS

Untreated animals displayed high numbers of bacteria in irrigation fluid and were all highly culture positive at euthanasia. Three of 6 animals were culture positive at euthanasia after conventional prophylaxis. Early systemic prophylaxis reduced bacterial burden in irrigation fluid by up to 100-fold, but 5/6 animals were culture positive at euthanasia. The combined prophylaxis displayed greater efficacy with only 1/6 rabbits culture positive at euthanasia. Local application of the gentamicin-loaded hydrogel reduced bacteria recovered by irrigation to just above our detection limit, and at euthanasia, all animals were culture negative at euthanasia.

CONCLUSIONS

Early systemic antibiotic administration can significantly reduce bacterial burden in the operative field and reduce culture positivity at euthanasia when continued for 24 hours after injury. The early application of a gentamicin-loaded hydrogel that was removed during irrigation displayed superior efficacy to early systemic therapy alone and postoperative conventional gold standard 24-hour systemic therapy alone. These experimental results highlight the importance of early antibiotic administration in fracture care.

Accepting higher morbidity in exchange for sacrificing fewer animals in studies developing novel infection-control strategies

Preventing bacterial infections from becoming the leading cause of death by the year 2050 requires the development of novel, infection-control strategies, building heavily on biomaterials science, including nanotechnology. Pre-clinical (animal) studies are indispensable for this development. Often, animal infection outcomes bear little relation to human clinical outcome. Here, we review conclusions from pathogen-inoculum dose-finding pilot studies for evaluation of novel infection-control strategies in murine models. Pathogen-inoculum doses are generally preferred that produce the largest differences in quantitative infection outcome parameters between a control and an experimental group, without death or termination of animals due to having reached an inhumane end-point during the study. However, animal death may represent a better end-point for evaluation than large differences in outcome parameters or number of days over which infection persists. The clinical relevance of lower pre-clinical outcomes, such as bioluminescence, colony forming units (CFUs) retrieved or more rapid clearance of infection is unknown, as most animals cure infection without intervention, depending on pathogen-species and pathogen-inoculum dose administered. In human clinical practice, patients suffering from infection present to hospital emergency wards, frequently in life-threatening conditions. Animal infection-models should therefore use prevention of death and recurrence of infection as primary efficacy targets to be addressed by novel strategies. To compensate for increased animal morbidity and mortality, animal experiments should solely be conducted for pre-clinical proof of principle and safety. With the advent of sophisticated in vitro models, we advocate limiting use of animal models when exploring pathogenesis or infection mechanisms.

Dendritic Cells Internalize Staphylococcus aureus More Efficiently than Staphylococcus epidermidis, but Do Not Differ in Induction of Antigen-Specific T Cell Proliferation

Staphylococcus aureus and Staphylococcus epidermidis are related species which can cause predominantly acute and subacute infections, respectively. Differences in human adaptive immune responses to these two species are not well understood. Dendritic cells (DCs) have an important role in the control and regulation of anti-staphylococcal T cell responses. Therefore, we aimed to compare the ability of S. aureus and S. epidermidis to influence the essential steps in human DC activation and subsequent antigen-specific CD4⁺ T cell proliferation, and to investigate the underlying mechanisms. Using multiple strains of both species, we observed that S. aureus was internalized more effectively than S. epidermidis by DCs but that both species were equally potent in activating these host cells, as evidenced by similar induction of DC maturation marker expression and antigen loading onto MHC-II molecules. The DCs stimulated by S. aureus strains not harboring superantigen (SAg) genes or by any of the S. epidermidis strains, induced low, likely physiological levels of T cell proliferation. Only DCs stimulated with S. aureus strains harboring SAg genes induced high levels of T cell proliferation. Taken together, S. aureus and S. epidermidis do not differently affect DC activation and ensuing antigen-specific T cell proliferation, unless a strain has the capacity to produce SAgs.

Intraoperative loading of calcium phosphate-coated implants with gentamicin prevents experimental Staphylococcus aureus infection in vivo

Orthopedic device-related infection (ODRI) is a potentially devastating complication arising from the colonization of the device with bacteria, such as Staphylococcus aureus. The aim of this study was to determine if intraoperative loading of a clinically approved calcium phosphate (CaP) coating with gentamicin can protect from ODRI in vivo. First, CaP-coated titanium aluminium niobium (TAN) discs were used to investigate the adsorption and release kinetics of gentamicin in vitro. Gentamicin loading and subsequent release from the coating were both rapid, with maximum loading occurring following one second of immersion, and >95% gentamicin released within 15 min in aqueous solution, respectively. Second, efficacy of the gentamicin-loaded CaP coating for preventing ODRI in vivo was investigated using a CaP-coated unicortical TAN screw implanted into the proximal tibia of skeletally mature female Wistar rats, following inoculation of the implant site with S. aureus. Gentamicin-loading prevented ODRI in 7/8 animals, whereas 9/9 of the non-gentamicin treated animals were infected after 7 days. In conclusion, gentamicin can be rapidly and simply loaded onto, and released from, CaP-based implant coatings, and this is an effective strategy for preventing peri-operative S. aureus-induced ODRI in vivo.

Biofilm Inhibition via Delivery of Novel Methylthioadenosine Nucleosidase Inhibitors from PVA-Tyramine Hydrogels while Supporting Mesenchymal Stromal Cell Viability

The rise of antibiotic resistance, coupled with increased expectations for mobility in later life, is creating a need for biofilm inhibitors and delivery systems that will reduce surgical implant infection. A limitation of some of these existing delivery approaches is toxicity exhibited toward host cells. Here, we report the application of a novel inhibitor of the enzyme, methylthioadenosine nucleosidase (MTAN), a key enzyme in bacterial metabolic pathways, which include S-adenosylmethionine catabolism and purine nucleotide recycling, in combination with a poly(vinyl alcohol)-tyramine-based (PVA-Tyr) hydrogel delivery system. We demonstrate that a lead MTAN inhibitor, selected from a screened library of 34 candidates, (2S)-2-(4-amino-5H-pyrrolo3,2-dpyrimidin-7-ylmethyl)aminoundecan-1-ol (31), showed a minimum biofilm inhibitory concentration of 2.2 ± 0.4 μM against a clinical staphylococcal species isolated from an infected implant. We observed that extracellular DNA, a key constituent of biofilms, is significantly reduced when treated with 10 μM compound 31, along with a decrease in biofilm thickness. Compound 31 was incorporated into a hydrolytically degradable photo-cross-linked PVA-Tyr hydrogel and the release profile was evaluated by HPLC studies. Compound 31 released from the PVA-hydrogel system significantly reduced biofilm formation (77.2 ± 8.4% biofilm inhibition). Finally, compound 31 released from PVA-Tyr showed no negative impact on human bone marrow stromal cell (MSC) viability, proliferation, or morphology. The results demonstrate the potential utility of MTAN inhibitors in treating infections caused by Gram-positive bacteria, and the development of a nontoxic release system that has potential for tunability for time scale of delivery.

Synergistic microbicidal effect of cationic antimicrobial peptides and teicoplanin against planktonic and biofilm-encased Staphylococcus aureus

Antibiotic resistance and biofilm formation are the main reasons for failure in treatment of bacterial infections. This study aimed to identify synergistic combinations of conventional antibiotics and novel synthetic antimicrobial and antibiofilm peptides (SAAPs) inspired by the structures of the natural human cationic peptides LL-37 and thrombocidin-1 (TC-1). The LL-37-inspired lead peptide SAAP-148 was combined with antibiotics of different classes against Staphylococcus aureus, and showed synergy with teicoplanin. Synergy with teicoplanin was also observed with LL-37, the LL-37-inspired SAAP-276 and the TC-1-inspired TC84. Interestingly, no synergy was observed against Staphylococcus epidermidis. Furthermore, teicoplanin combined with SAAP-148 or SAAP-276 showed strong interaction against S. aureus biofilms. The dltABCD operon and the mprF gene in S. aureus conferred resistance to LL-37, but SAAP-148 proved to be indifferently potent against wild-type, ΔdltA and ΔmprF S. aureus strains. When used alone, relatively high concentrations of both LL-37 and teicoplanin (30-120 µM and 4-32 mg/L, respectively) were required to kill S. aureus. Resistance to LL-37 in S. aureus was overcome by combined use of teicoplanin and LL-37. Thus, teicoplanin potentiates peptide LL-37, enhancing the efficacy of the innate defence, and combining the novel peptides with teicoplanin offers potential for enhanced efficacy of treatment of S. aureus infections, including biofilms.

Antibiotic Prophylaxis With Cefuroxime: Influence of Duration on Infection Rate With Staphylococcus aureus in a Contaminated Open Fracture Model

OBJECTIVES

The optimal duration of perioperative antibiotic prophylaxis (PAP) for open fractures remains controversial because of heterogeneous or unclear guidelines and highly variable prophylactic regimens in clinical practice. We aimed at testing different PAP durations under controlled conditions in a contaminated rabbit fracture model.

METHODS

A complete humeral osteotomy in 18 rabbits was fixed with a 7-hole locking compression plate and inoculated with Staphylococcus aureus. Cefuroxime was administered in a weight-adjusted dosage equivalent to human medicine (18.75 mg/kg). PAP was administered as a single shot only; for 24 hours; or for 72 hours in separate groups of rabbits (n = 6 per group). Infection was assessed after 2 weeks by quantitative bacteriological evaluation of the tissues and hardware.

RESULTS

Postoperative duration of PAP had a significant impact on the success of antibiotic prophylaxis in this model. Whereas the single-shot regimen completely failed to prevent infection, the 24-hour regimen showed a reduced infection rate (1 of 6 rabbits infected), but only the 72-hour course was able to prevent fracture-related infection in all animals in our model.

CONCLUSIONS

When contamination with high bacterial loads is likely (eg, in an open fracture situation), a 72-hour course of intravenous cefuroxime seems to be superior in preventing fracture-related infection in our rabbit model compared with a single-shot or 24-hour antibiotic regimen.

Influence of Material and Microtopography on the Development of Local Infection in vivo: Experimental Investigation in Rabbits

Polishing the surface of internal fracture fixation (IFF) implant materials can ease implant removal and reduce irritation to gliding tissues by reducing soft tissue adhesion and bony overgrowth. Thus, polishing the surface of these implants is expected to have significant clinical benefit in certain situations. The aim of the present study was to determine if polishing the surface of an IFF device influences susceptibility to infection.

The local infection rate associated with 4-hole 2.0 mm Synthes® locking compression plates (LCPs) composed of clinically available commercially pure titanium (cpTi) and titanium aluminium niobium (TAN) in their standard microrough form was compared with that of their test polished equivalents and also to clinically available electropolished stainless steel (EPSS). The LCPs were fixed in locking mode onto the tibia of mature, female New Zealand White rabbits and a clinical strain of Staphylococcus aureus was added to the implantation site. Twenty eight days after surgery the rabbits were euthanized and assessed for infection. The rank order based on descending ID50 was; polished TAN, standard TAN, standard cpTi, EPSS and finally polished cpTi, however, the ID50 values did not differ greatly between the groups with the same material. Using the LCP model in locking mode, polishing the surface of both cpTi and TAN was not found to influence the susceptibility to infection in our animal model.

Injectable gentamicin-loaded thermo-responsive hyaluronic acid derivative prevents infection in a rabbit model

Despite the use of systemic antibiotic prophylaxis, the surgical fixation of open fractures with osteosynthesis implants is associated with high infection rates. Antibiotic-loaded biomaterials (ALBs) are increasingly used in implant surgeries across medical specialties to deliver high concentrations of antibiotics to the surgical site and reduce the risk of implant-associated infection. ALBs which are either less or not restricted in terms of spatial distribution and which may be applied throughout complex wounds could offer improved protection against infection in open fracture care. A thermo-responsive hyaluronic acid derivative (hyaluronic acid-poly(N-isopropylacrylamide) (HApN)) was prepared by a direct amidation reaction between the tetrabutyl ammonium (TBA) salt of hyaluronic acid and amine-terminated poly(N-isopropylacrylamide) (pN). The degree of grafting, and gelation properties of this gel were characterized, and the composition was loaded with gentamicin. The rheological- and release properties of this gentamicin-loaded HApN composition were tested in vitro and its efficacy in preventing infection was tested in a rabbit model of osteosynthesis contaminated with Staphylococcus aureus. The gentamicin-loaded HApN composition was able to prevent bacterial colonization of the implant site as shown by quantitative bacteriology. This finding was supported by histopathological evaluation of the humeri samples where no bacteria were found in the stained sections. In conclusion, this gentamicin-loaded HApN hydrogel effectively prevents infection in a complex wound, simulating a contaminated fracture treated with plating osteosynthesis.

STATEMENT OF SIGNIFICANCE

Fracture fixation after trauma is associated with high infection rates. Antibiotic loaded biomaterials (ALBs) can provide high local concentrations without systemic side effects. However, the currently available ALBs have limited accessibility to contaminated tissues in open fractures because of predetermined shape. Thus, a novel thermo-responsive hyaluronan based hydrogel with control over gelation temperature is reported. The efficacy of this gentamicin loaded hyaluronan derivative is demonstrated in an in vivo fracture model in the presence of fracture fixation hardware. The bacterial burden is cleared in all of the inoculated rabbits in the presence of the ALB. Thus, the proposed injectable thermo-responsive hyaluronan presents an effective ALB for the prevention of infection.

Antibacterial Surface Design of Titanium-Based Biomaterials for Enhanced Bacteria-Killing and Cell-Assisting Functions Against Periprosthetic Joint Infection

Periprosthetic joint infection (PJI) is one of the formidable and recalcitrant complications after orthopedic surgery, and inhibiting biofilm formation on the implant surface is considered crucial to prophylaxis of PJI. However, it has recently been demonstrated that free-floating biofilm-like aggregates in the local body fluid and bacterial colonization on the implant and peri-implant tissues can coexist and are involved in the pathogenesis of PJI. An effective surface with both contact-killing and release-killing antimicrobial capabilities can potentially abate these concerns and minimize PJI caused by adherent/planktonic bacteria. Herein, Ag nanoparticles (NPs) are embedded in titania (TiO2) nanotubes by anodic oxidation and plasma immersion ion implantation (PIII) to form a contact-killing surface. Vancomycin is then incorporated into the nanotubes by vacuum extraction and lyophilization to produce the release-killing effect. A novel clinical PJI model system involving both in vitro and in vivo use of methicillin-resistant Staphylococcus aureus (MRSA) ST239 is established to systematically evaluate the antibacterial properties of the hybrid surface against planktonic and sessile bacteria. The vancomycin-loaded and Ag-implanted TiO2 nanotubular surface exhibits excellent antimicrobial and antibiofilm effects against planktonic/adherent bacteria without appreciable silver ion release. The fibroblasts/bacteria cocultures reveal that the surface can help fibroblasts to combat bacteria. We first utilize the nanoarchitecture of implant surface as a bridge between the inorganic bactericide (Ag NPs) and organic antibacterial agent (vancomycin) to achieve total victory in the battle of PJI. The combination of contact-killing and release-killing together with cell-assisting function also provides a novel and effective strategy to mitigate bacterial infection and biofilm formation on biomaterials and has large potential in orthopedic applications.

Titanium and steel fracture fixation plates with different surface topographies: Influence on infection rate in a rabbit fracture model

INTRODUCTION

Implant-related infection is a challenging complication in musculoskeletal trauma surgery. In the present study, we examined the role of implant material and surface topography as influencing factors on the development of infection in an experimental model of plating osteosynthesis in the rabbit.

METHODS

The implants included in this experimental study were composed of: standard Electropolished Stainless Steel (EPSS), standard titanium (Ti-S), roughened stainless steel (RSS) and surface polished titanium (Ti-P). Construct stability and load-to-failure of Ti-P implants was compared to that of Ti-S implants in a rabbit cadaveric model. In an in vivo study, a rabbit humeral fracture model was used. Each rabbit received one of three Staphylococcus aureus inocula, aimed at determining the infection rate at a low, medium and high dose of bacteria. Outcome measures were quantification of bacteria on the implant and in the surrounding tissues, and determination of the infectious dose 50 (ID50).

RESULTS

No significant differences were observed between Ti-S and Ti-P regarding stiffness or failure load in the cadaver study. Of the 72 rabbits eventually included in the in vivo study, 50 developed an infection. The ID50 was found to be: EPSS 3.89×10(3) colony forming units (CFU); RSS 8.23×10(3) CFU; Ti-S 5.66×10(3) CFU; Ti-P 3.41×10(3) CFU. Significantly lower bacterial counts were found on the Ti-S implants samples compared with RSS implants (p<0.001) at the high inoculum. Similarly, lower bacterial counts were found in the bone samples of animals in the Ti-S group in comparison with both RSS and EPSS groups, again at the high inoculation dose (p<0.005).

CONCLUSION

No significant differences were seen in susceptibility to infection when comparing titanium and steel implants with conventional or modified topographies. Ti-P implants, which have previously been shown in preclinical studies to reduce complications associated with tissue adherence, do not affect infection rate in this preclinical fracture model. Therefore, Ti-P implants are not expected to affect the infection rate, or influence implant stability in the clinical situation.

Monitoring immune responses in a mouse model of fracture fixation with and without Staphylococcus aureus osteomyelitis

Post-traumatic bone fractures are commonly fixed with implanted devices to restore the anatomical position of bone fragments and aid in the healing process. Bacterial infection in this situation is a challenge for clinicians due to the need for aggressive antibiotic therapy, debridement of infected tissues, and the need to maintain fracture stability. The aim of this study was to monitor immune responses that occur during healing and during Staphylococcus aureus infection, in a clinically relevant murine model of fracture fixation. Skeletally mature C57bl/6 mice received a transverse osteotomy of the femur, which was treated with commercially available titanium fracture fixation plates and screws. In the absence of infection, healing of the fracture was complete within 35days and was characterized by elevated Interleukin (IL)-4 and Interferon-gamma secretion from bone-derived cells and expression of these same genes. In contrast, mice inoculated with S. aureus could not heal the fracture within the observation period and were found to develop typical signs of implant-associated bone infection, including biofilm formation on the implant and osteolysis of surrounding bone. The immune response to infection was characterized by a TH17-led bone response, and a pro-inflammatory cytokine-led Tumor necrosis factor (TNF)-α, Interleukin (IL)-1β) soft tissue response, both of which were ineffectual in clearing implant related bone and soft tissue infections respectively. In this murine model, we characterize the kinetics of pro-inflammatory responses to infection, secondary to bone trauma and surgery. A divergent local immune polarization is evident in the infected versus non-infected animals, with the immune response ultimately unable to clear the S. aureus infection.

A doxycycline-loaded polymer-lipid encapsulation matrix coating for the prevention of implant-related osteomyelitis due to doxycycline-resistant methicillin-resistant Staphylococcus aureus

Implant-associated bone infections caused by antibiotic-resistant pathogens pose significant clinical challenges to treating physicians. Prophylactic strategies that act against resistant organisms, such as methicillin-resistant Staphylococcus aureus (MRSA), are urgently required. In the present study, we investigated the efficacy of a biodegradable Polymer-Lipid Encapsulation MatriX (PLEX) loaded with the antibiotic doxycycline as a local prophylactic strategy against implant-associated osteomyelitis. Activity was tested against both a doxycycline-susceptible (doxy(S)) methicillin-susceptible S. aureus (MSSA) as well as a doxycycline-resistant (doxy(R)) methicillin-resistant S. aureus (MRSA). In vitro elution studies revealed that 25% of the doxycycline was released from the PLEX-coated implants within the first day, followed by a 3% release per day up to day 28. The released doxycycline was highly effective against doxy(S) MSSA for at least 14days in vitro. A bolus injection of doxycycline mimicking a one day release from the PLEX-coating reduced, but did not eliminate, mouse subcutaneous implant-associated infection (doxy(S) MSSA). In a rabbit intramedullary nail-related infection model, all rabbits receiving a PLEX-doxycycline-coated nail were culture negative in the doxy(S) MSSA-group and the surrounding bone displayed a normal physiological appearance in both histological sections and radiographs. In the doxy(R) MRSA inoculated rabbits, a statistically significant reduction in the number of culture-positive samples was observed for the PLEX-doxycycline-coated group when compared to the animals that had received an uncoated nail, although the reduction in bacterial burden did not reach statistical significance. In conclusion, the PLEX-doxycycline coating on titanium alloy implants provided complete protection against implant-associated MSSA osteomyelitis, and resulted in a significant reduction in the number of culture positive samples when challenged with a doxycycline-resistant MRSA.

An overview of the methodological approach to the in vitro study of anti-infective biomaterials

Biomaterial-associated infections have an enormous impact in terms of morbidity of the patients and costs to national health systems. Perioperative antibiotics and aseptic procedures have not proved sufficient to eradicate the occurrence of this type of infections which often lead to devastating effects. Adjunctive strategies for preventing the establishment of infections are increasingly being centered on the development of new biomaterials with anti-infective properties. The creation of new anti-infective biomaterials can be obtained by alternative approaches oriented to achieve either bacteria-repellent surfaces or bioactive surfaces expressing self-sterilizing properties when not even able to treat pre-existing infections in the surrounding tissues. Here, we offer a short overview of the currently available in vitro methods that can be used to investigate and assess the performance of anti-infective biomaterials, with special emphasis on those whose mechanism of action is based on bacteria-repellent surfaces.

In vivo evaluation of the effect of intramedullary nail microtopography on the development of local infection in rabbits

BACKGROUND AND AIM

Fractures of the tibia and femoral diaphysis are commonly repaired by intramedullary (IM) nails, which are currently composed of either electropolished stainless steel (EPSS) or standard, non-polished titanium-aluminum-niobium (TAN). Once the fracture has fully healed, removal of IM nails is common, but the strong adhesion of bone to standard TAN complicates removal. Polishing the surface of TAN IM nails has been shown to reduce bony adhesion and ease implant removal without compromising fixation. Polished TAN nails are, therefore, expected to have significant clinical benefit in situations where the device is to be removed. The aim of the present study was to determine the effect of polishing TAN IM nails on susceptibility to infection in an animal model.

MATERIALS AND METHODS

Solid IM nails (Synthes, Betlach, Switzerland) composed of standard TAN were compared with polished equivalents and also to clinically available EPSS nails. The surface chemical and topographical properties of the materials were assessed by X-ray photon spectroscopy (XPS), white light profilometry, and scanning electron microscopy (SEM). An in vivo infection study was performed using a clinical isolate of Staphylococcus aureus that was characterized with respect to various virulence factors.

RESULTS

Polishing TAN IM nails caused no significant change to the chemistry of the nails, but the topography of the polished TAN nails was significantly smoother than standard TAN nails. In the infection study, the rank order based on descending infectious dose 50 (ID(50)) was: standard TAN, polished TAN, and finally EPSS. The ID(50) values did not differ greatly between any of the groups.

CONCLUSIONS

Polishing the surface TAN IM nails was not found to influence the susceptibility to infection in our animal model.

Infection in fracture fixation: can we influence infection rates through implant design?

Musculoskeletal infection is one of the most common complications associated with surgical fixation of bones fractured during trauma. These infections usually involve bacterial colonisation and biofilm formation on the fracture fixation device itself, as well as infection of the surrounding tissues. Antibiotic prophylaxis, wound debridement and postsurgical care can reduce the incidence of, but do not prevent, these infections. Much research and development has been focussed on ways to further reduce the incidence of infection and in the following short review we describe our experiences investigating the contribution of the basic design of fracture fixation devices on the susceptibility to infection. It has been shown in animal studies that device size, shape, mode of action and material and topography play an interrelated role in the susceptibility to infection. Although direct extrapolation from animal studies to the clinical setting is difficult, close consideration of the design factors that can reduce the incidence of infection in animal models is expected to help minimise the incidence of infection associated with any clinically implemented fracture fixation device.