Host tissue as a niche for biomaterial-associated infection

Interplay between environmental yielding and dynamic forcing modulates bacterial growth

Many bacterial habitats-ranging from gels and tissues in the body to cell-secreted exopolysaccharides in biofilms-are rheologically complex, undergo dynamic external forcing, and have unevenly distributed nutrients. How do these features jointly influence how the resident cells grow and proliferate? Here, we address this question by studying the growth of Escherichia coli dispersed in granular hydrogel matrices with defined and highly tunable structural and rheological properties, under different amounts of external forcing imposed by mechanical shaking, and in both aerobic and anaerobic conditions. Our experiments establish a general principle: that the balance between the yield stress of the environment that the cells inhabit, σy, and the external stress imposed on the environment, σ, modulates bacterial growth by altering transport of essential nutrients to the cells. In particular, when σy<σ, the environment is easily fluidized and mixed over large scales, providing nutrients to the cells and sustaining complete cellular growth. By contrast, when σy>σ, the elasticity of the environment suppresses large-scale fluid mixing, limiting nutrient availability and arresting cellular growth. Our work thus reveals a new mechanism, beyond effects that change cellular behavior via local forcing, by which the rheology of the environment may modulate microbial physiology in diverse natural and industrial settings.

Etiology, pathology, and host-impaired immunity in medical implant-associated infections

Host impaired immunity and pathogens adhesion factors are the key elements in analyzing medical implant-associated infections (MIAI). The infection chances are further influenced by surface properties of implants. This review addresses the medical implant-associated pathogens and summarizes the etiology, pathology, and host-impaired immunity in MIAI. Several bacterial and fungal pathogens have been isolated from MIAI; together, they form cross-kingdom species biofilms and support each other in different ways. The adhesion factors initiate the pathogen's adherence on the implant's surface; however, implant-induced impaired immunity promotes the pathogen's colonization and biofilm formation. Depending on the implant's surface properties, immune cell functions get slow or get exaggerated and cause immunity-induced secondary complications resulting in resistant depression and immuno-incompetent fibro-inflammatory zone that compromise implant's performance. Such consequences lead to the unavoidable and straightforward conclusion for the downstream transformation of new ideas, such as the development of multifunctional implant coatings.

A Niclosamide-releasing hot-melt extruded catheter prevents Staphylococcus aureus experimental biomaterial-associated infection

Biomaterial-associated infections are a major healthcare challenge as they are responsible for high disease burden in critically ill patients. In this study, we have developed drug-eluting antibacterial catheters to prevent catheter-related infections. Niclosamide (NIC), originally an antiparasitic drug, was incorporated into the polymeric matrix of thermoplastic polyurethane (TPU) via solvent casting, and catheters were fabricated using hot-melt extrusion technology. The mechanical and physicochemical properties of TPU polymers loaded with NIC were studied. NIC was released in a sustained manner from the catheters and exhibited in vitro antibacterial activity against Staphylococcus aureus and Staphylococcus epidermidis. Moreover, the antibacterial efficacy of NIC-loaded catheters was validated in an in vivo biomaterial-associated infection model using a methicillin-susceptible and methicillin-resistant strain of S. aureus. The released NIC from the produced catheters reduced bacterial colonization of the catheter as well as of the surrounding tissue. In summary, the NIC-releasing hot-melt extruded catheters prevented implant colonization and reduced the bacterial colonization of peri-catheter tissue by methicillin sensitive as well as resistant S. aureus in a biomaterial-associated infection mouse model and has good prospects for preclinical development.

Advances in the Application of Nanomaterials as Treatments for Bacterial Infectious Diseases

Bacteria-targeting nanomaterials have been widely used in the diagnosis and treatment of bacterial infectious diseases. These nanomaterials show great potential as antimicrobial agents due to their broad-spectrum antibacterial capacity and relatively low toxicity. Recently, nanomaterials have improved the accurate detection of pathogens, provided therapeutic strategies against nosocomial infections and facilitated the delivery of antigenic protein vaccines that induce humoral and cellular immunity. Biomaterial implants, which have traditionally been hindered by bacterial colonization, benefit from their ability to prevent bacteria from forming biofilms and spreading into adjacent tissues. Wound repair is improving in terms of both the function and prevention of bacterial infection, as we tailor nanomaterials to their needs, select encapsulation methods and materials, incorporate activation systems and add immune-activating adjuvants. Recent years have produced numerous advances in their antibacterial applications, but even further expansion in the diagnosis and treatment of infectious diseases is expected in the future.

Behavior of Pseudomonas aeruginosa strains on the nanopillar topography of dragonfly (Pantala flavescens) wing under flow conditions

Bacterial associated infection is a threat in the medical field. Pseudomonas aeruginosa, one of the major causative agents for nosocomial infection, has developed resistance to almost all the classes of antibiotics. Recently, nanopillar-like structures were identified on the wings of insects such as cicada and dragonfly. Nanopillars both on natural surfaces and those mimicked on artificial surfaces were reported to possess bactericidal activity against a wide range of bacteria. An earlier study reported strain specific variation in the viability of P. aeruginosa on the nanopillar topography of a dragonfly wing under static condition. Here, we report the behavior of P. aeruginosa strains on a dragonfly wing under hydrodynamic conditions. The results of the study indicated that, under hydrodynamic conditions, P. aeruginosa PAO1 was attached in higher numbers to the wing surface than P. aeruginosa ATCC 9027 but killed in lower numbers. The plausible reason was identified to be the masking of nanopillars by the secreted extracellular polysaccharide. The shear rate of 1.0 s^-1 showed a relatively higher bactericidal effect among the three tested shear rates.

On the function of biosynthesized cellulose as barrier against bacterial colonization of VAD drivelines

Bacterial colonization of drivelines represents a major adverse event in the implantation of left ventricular assist devices (L-VADs) for the treatment of congestive heart failure. From the external driveline interface and through the skin breach, pathogens can ascend to the pump pocket, endangering the device function and the patient’s life. Surface Micro-Engineered Biosynthesized cellulose (BC) is an implantable biomaterial, which minimizes fibrotic tissue deposition and promotes healthy tissue regeneration. The topographic arrangement of cellulose fibers and the typical material porosity support its potential protective function against bacterial permeation; however, this application has not been tested in clinically relevant animal models. Here, a goat model was adopted to evaluate the barrier function of BC membranes. The external silicone mantle of commercial L-VAD drivelines was implanted percutaneously with an intervening layer of BC to separate them from the surrounding soft tissue. End-point evaluation at 6 and 12 weeks of two separate animal groups revealed the local bacterial colonization at the different interfaces in comparison with unprotected driveline mantle controls. The results demonstrate that the BC membranes established an effective barrier against the bacterial colonization of the outer driveline interface. The containment of pathogen infiltration, in combination with the known anti-fibrotic effect of BC, may promote a more efficient immune clearance upon driveline implantation and support the efficacy of local antibiotic treatments, therefore mitigating the risk connected to their percutaneous deployment.

In Situ Coatings of Silver Nanoparticles for Biofilm Treatment in Implant-Retention Surgeries: Antimicrobial Activities in Monoculture and Coculture

Bacterial biofilms are indicated in most medical device-associated infections. Treating these biofilms is challenging yet critically important for applications such as in device-retention surgeries, which can have reinfection rates of up to 80%. This in vitro study centered around our new method of treating biofilm and preventing reinfection. Ionic silver (Ag, in the form of silver nitrate) combined with dopamine and a biofilm-lysing enzyme (α-amylase) were applied to model 4-day-old Staphylococcus aureus biofilms on titanium substrates to degrade the extracellular matrix of the biofilm and kill the biofilm bacteria. In this process, the oxidative self-polymerization of dopamine converted Ag ions into Ag nanoparticles that, together with the resultant self-adhering polydopamine (PDA), formed coatings that strongly bound to the treated substrates. Surprisingly, although these Ag/PDA coatings significantly reduced S. aureus growth in standard bacterial monoculture, they showed much lower antimicrobial activity in coculture of the bacteria and osteoblastic MC3T3-E1 cells in which the bacteria were also found attached to the osteoblasts. This S. aureus- osteoblast interaction was also linked to bacterial survival against gentamicin treatment observed in coculture. Our study thus provided clear evidence suggesting that bacteria's interactions with tissue cells surrounding implants may significantly contribute to their resistance to antimicrobial treatment.

Drug Release from Thermo-Responsive Polymer Brush Coatings to Control Bacterial Colonization and Biofilm Growth on Titanium Implants

Despite decades of biomedical advances, the colonization of implant devices with bacterial biofilms is still a leading cause of implant failure. Clearly, new strategies and materials that suppress both initial and later stage bacterial colonization are required in this context. Ideal would be the implementation of a bactericidal functionality in the implants that is temporally and spatially triggered in an autonomous fashion at the infection site. Herein, the fabrication and validation of functional titanium-based implants with triggered antibiotic release function afforded via an intelligent polymer coating is reported. In particular, thermo-responsive poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) brushes on titanium implants synthesized via a surface-initiated atom transfer radical polymerization with activators regenerated through the electron transfer technique (ARGET ATRP) allows for a controlled and thermally triggered release of the antibiotic levofloxacin at the wound site. Antibiotic loaded brushes are investigated as a function of thickness, loading capacity for antibiotics, and temperature. At temperatures of the infection site >37 °C the lower critical solution temperature behavior of the brushes afforded the triggered release. Hence, in addition to the known antifouling effects, the PDEGMA coating ensured enhanced bactericidal effects, as demonstrated in initial in vivo tests with rodents infected with Staphylococcus aureus.

Biocompatibility Evolves: Phenomenology to Toxicology to Regeneration

The word "biocompatibility," is inconsistent with the observations of healing for so-called biocompatible biomaterials. The vast majority of the millions of medical implants in humans today, presumably "biocompatible," are walled off by a dense, avascular, crosslinked collagen capsule, hardly suggestive of life or compatibility. In contrast, one is now seeing examples of implant biomaterials that lead to a vascularized reconstruction of localized tissue, a biological reaction different from traditional biocompatible materials that generate a foreign body capsule. Both the encapsulated biomaterials and the reconstructive biomaterials qualify as "biocompatible" by present day measurements of biocompatibility. Yet, this new generation of materials would seem to heal "compatibly" with the living organism, where older biomaterials are isolated from the living organism by the dense capsule. This review/perspective article will explore this biocompatibility etymological conundrum by reviewing the history of the concepts around biocompatibility, today's standard methods for assessing biocompatibility, a contemporary view of the foreign body reaction and finally, a compendium of new biomaterials that heal without the foreign body capsule. A new definition of biocompatibility is offered here to address advances in biomaterials design leading to biomaterials that heal into the body in a facile manner.

New developments in anti-biofilm intervention towards effective management of orthopedic device related infections (ODRI's)

Orthopedic device related infections (ODRI's) represent a difficult to treat situation owing to their biofilm based nature. Biofilm infections once established are difficult to eradicate even with an aggressive treatment regimen due to their recalcitrance towards antibiotics and immune attack. The involvement of antibiotic resistant pathogens as the etiological agent further worsens the overall clinical picture, pressing on the need to look into alternative treatment strategies. The present review highlightes the microbiological challenges associated with treatment of ODRI's due to biofilm formation on the implant surface. Further, it details the newer anti-infective modalities that work either by preventing biofilm formation and/or through effective disruption of the mature biofilms formed on the medical implant. The study, therefore aims to provide a comprehensive insight into the newer anti-biofilm interventions (non-antibiotic approaches) and a better understanding of their mechanism of action essential for improved management of orthopedic implant infections.

Role of 4-hydroxybutyrate in increased resistance to surgical site infections associated with surgical meshes

An increased resistance to surgical site infections has been associated with surgical meshes composed of naturally occurring materials, including poly-4-hydroxybutrate (4HB). 4HB is a naturally occurring short-chain fatty acid that has been shown to promote endogenous expression of the Cramp gene coding for the antimicrobial peptide (AMP) cathelicidin LL-37 in murine bone marrow-derived macrophages. The molecular pathways involved in the 4HB-induced cathelicidin LL-37 expression have not yet been identified. The present study showed that transcriptional activation of the Cramp gene by 4HB is independent of inhibition of histone deacetylase (HDAC) activity, and that upregulation of Cramp is modulated by the G-protein coupled receptor GPR109A. Furthermore, an intracellular signaling cascade that promotes the activation of the MAP kinases, p38 and JNK, and a subsequent NF-κB phosphorylation downstream from p38 is essential for the AMP transcriptional response in 4HB-stimulated macrophages. The findings provide a solid scientific basis and rationale for the decreased incidence of surgical site infections with the use of this type of surgical meshes. Further clinical significance is found in the fact that the 4HB activated molecular pathway includes common targets of frequently used nonsteroidal anti-inflammatory drugs (NSAIDs) and other FDA approved drugs recognizing G-protein coupled receptors.

Surface functionalization of PEEK with silicon nitride

Surface roughness, bioactivity, and antibacterial properties are desirable in skeletal implants. We hot-pressed a mix of particulate sodium chloride (NaCl) salt and silicon nitride (β-Si3N4) onto the surface of bulk PEEK. NaCl grains were removed by leaching in water, resulting in a porous PEEK surface embedded with ~15 vol.% β-Si3N4 particles. This functionalized surface showed the osteogenic and antibacterial properties previously reported in bulk silicon nitride implants. Surface enhancement of PEEK with β-Si3N4 could improve the performance of spinal fusion cages, by facilitating arthrodesis and resisting bacteria.

Nitric Oxide Releasing Titanium Surfaces for Antimicrobial Bone-Integrating Orthopedic Implants

Titanium implants in orthopedic applications can fail due to infection and impaired integration into the host. Most research efforts that facilitate osseointegration of the implant have not considered infection, and vice versa. Moreover, most infection control measures involve the use of conventional antibiotics which contributes to the global epidemic of antimicrobial resistance. Nitric oxide (NO) is a promising alternative to antibiotics, and while researchers have investigated NO releasing coatings, there are few reports on the function/robustness or the mechanism of NO release. Our comprehensive mechanistic study has allowed us to design, characterize, and optimize NO releasing coatings to achieve maximum antimicrobial efficacy toward bacteria with minimum cytotoxicity to human primary osteoblasts in vitro. As the antibiotic era is coming to an end and the future of infection control continues to demand new alternatives, the coatings described herein represent a promising therapeutic strategy for use in orthopedic surgeries.

Risk factors for chronic biofilm-related infection associated with implanted medical devices

BACKGROUND

The use of implanted medical devices is associated with a small but clinically important risk of foreign body infection. A key question is: why do some patients develop chronic infection associated with an implanted device, but most do not?

AIMS

The literature on patient-specific risk factors for chronic infections associated with five types of implants was surveyed to glean clues about the etiology of these infections.

SOURCES

Data were collected from 47 articles through calendar year 2017 for five categories of device-related infections: cardiovascular implantable electronic devices (CIEDs), hernia meshes, prosthetic hip and knee joints, prosthetic shoulder joints and breast implants.

CONTENT

Important risk factors include immunomodulation/steroid therapy, diabetes, smoking, and renal disease/haemodialysis-findings that point to a critical role of a compromised innate immune response in determining vulnerable subpopulations.

IMPLICATIONS

A model of biofilm-related device infection is presented that posits defects in the innate immune response both systemically and locally, in the immediate vicinity of an abiotic biomaterial. The limitations of in vitro and animal models of chronic device-related infections are discussed in this context as are implications for research and clinical practice.

Polarization of Macrophages, Cellular Adhesion, and Spreading on Bacterially Contaminated Gold Nanoparticle-Coatings in Vitro

Biomaterial-associated infections often arise from contaminating bacteria adhering to an implant surface that are introduced during surgical implantation and not effectively eradicated by antibiotic treatment. Whether or not infection develops from contaminating bacteria depends on an interplay between bacteria contaminating the biomaterial surface and tissue cells trying to integrate the surface with the aid of immune cells. The biomaterial surface plays a crucial role in defining the outcome of this race for the surface. Tissue integration is considered the best protection of a biomaterial implant against infectious bacteria. This paper aims to determine whether and how macrophages aid osteoblasts and human mesenchymal stem cells to adhere and spread over gold nanoparticle (GNP)-coatings with different hydrophilicity and roughness in the absence or presence of contaminating, adhering bacteria. All GNP-coatings had identical chemical surface composition, and water contact angles decreased with increasing roughness. Upon increasing the roughness of the GNP-coatings, the presence of contaminating Staphylococcus epidermidis in biculture with cells gradually decreased surface coverage by adhering and spreading cells, as in the absence of staphylococci. More virulent Staphylococcus aureus fully impeded cellular adhesion and spreading on smooth gold- or GNP-coatings, while Escherichia coli allowed minor cellular interaction. Murine macrophages in monoculture tended toward their pro-inflammatory "fighting" M1-phenotype on all coatings to combat the biomaterial, but in bicultures with contaminating, adhering bacteria, macrophages demonstrated Ym1 expression, indicative of polarization toward their anti-inflammatory "fix-and-repair" M2-phenotype. Damage repair of cells by macrophages improved cellular interactions on intermediately hydrophilic/rough (water contact angle 30 deg/surface roughness 118 nm) GNP-coatings in the presence of contaminating, adhering Gram-positive staphylococci but provided little aid in the presence of Gram-negative E. coli. Thus, the merits on GNP-coatings to influence the race for the surface and prevent biomaterial-associated infection critically depend on their hydrophilicity/roughness and the bacterial strain involved in contaminating the biomaterial surface.

Concanavalin A-targeted mesoporous silica nanoparticles for infection treatment

The ability of bacteria to form biofilms hinders any conventional treatment for chronic infections and has serious socio-economic implications. For this purpose, a nanocarrier capable of overcoming the barrier of the mucopolysaccharide matrix of the biofilm and releasing its loaded-antibiotic within this matrix would be desirable. Herein, we developed a new nanosystem based on levofloxacin (LEVO)-loaded mesoporous silica nanoparticles (MSN) decorated with the lectin concanavalin A (ConA). The presence of ConA promotes the internalization of this nanosystem into the biofilm matrix, which increases the antimicrobial efficacy of the antibiotic hosted within the mesopores. This nanodevice is envisioned as a promising alternative to conventional treatments for infection by improving the antimicrobial efficacy and reducing side effects. STATEMENT OF SIGNIFICANCE: The present study is focused on finding an adequate therapeutic solution for the treatment of bone infection using nanocarriers that are capable of overcoming the biofilm barrier by increasing the therapeutic efficacy of the loaded antibiotic. For this purpose, we present a nanoantibiotic that increases the effectiveness of levofloxacin to destroy the biofilm formed by the model bacterium E. coli. This work opens new lines of research in the treatment of chronic infections based on nanomedicines.

Recommendations for design and conduct of preclinical in vivo studies of orthopedic device-related infection

Orthopedic device-related infection (ODRI), including both fracture-related infection (FRI) and periprosthetic joint infection (PJI), remain among the most challenging complications in orthopedic and musculoskeletal trauma surgery. ODRI has been convincingly shown to delay healing, worsen functional outcome and incur significant socio-economic costs. To address this clinical problem, ever more sophisticated technologies targeting the prevention and/or treatment of ODRI are being developed and tested in vitro and in vivo. Among the most commonly described innovations are antimicrobial-coated orthopedic devices, antimicrobial-loaded bone cements and void fillers, and dual osteo-inductive/antimicrobial biomaterials. Unfortunately, translation of these technologies to the clinic has been limited, at least partially due to the challenging and still evolving regulatory environment for antimicrobial drug-device combination products, and a lack of clarity in the burden of proof required in preclinical studies. Preclinical in vivo testing (i.e. animal studies) represents a critical phase of the multidisciplinary effort to design, produce and reliably test both safety and efficacy of any new antimicrobial device. Nonetheless, current in vivo testing protocols, procedures, models, and assessments are highly disparate, irregularly conducted and reported, and without standardization and validation. The purpose of the present opinion piece is to discuss best practices in preclinical in vivo testing of antimicrobial interventions targeting ODRI. By sharing these experience-driven views, we aim to aid others in conducting such studies both for fundamental biomedical research, but also for regulatory and clinical evaluation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:271-287, 2019.

Mixed-charge pseudo-zwitterionic mesoporous silica nanoparticles with low-fouling and reduced cell uptake properties

The design of drug delivery systems needs to consider biocompatibility and host body recognition for an adequate actuation. In this work, mesoporous silica nanoparticles (MSNs) surfaces were successfully modified with two silane molecules to provide mixed-charge brushes (-NH3⊕/-PO3⊝) and well evaluated in terms of surface properties, low-fouling capability and cell uptake in comparison to PEGylated MSNs. The modification process consists in the simultaneous direct-grafting of hydrolysable short chain amino (aminopropyl silanetriol, APST) and phosphonate-based (trihydroxy-silyl-propyl-methyl-phosphonate, THSPMP) silane molecules able to provide a pseudo-zwitterionic nature under physiological pH conditions. Results confirmed that both mixed-charge pseudo-zwitterionic MSNs (ZMSN) and PEG-MSN display a significant reduction of serum protein adhesion and macrophages uptake with respect to pristine MSNs. In the case of ZMSNs, this reduction is up to a 70-90% for protein adsorption and c.a. 60% for cellular uptake. This pseudo-zwitterionic modification has been focused on the aim of local treatment of bacterial infections through the synergistic effect between the inherent antimicrobial effect of mixed-charge system and the levofloxacin antibiotic release profile. These findings open promising future expectations for the effective treatment of bacterial infections through the use of mixed-charge pseudo-zwitterionic MSNs furtive to macrophages and with antimicrobial properties. STATEMENT OF SIGNIFICANCE: Herein a novel antimicrobial mixed-charge pseudo-zwitterionic MSNs based system with low-fouling and reduced cell uptake behavior has been developed. This chemical modification has been performed by the simultaneous grafting of short chain organosilanes, containing amino and phosphonate groups, respectively. This nanocarrier has been tested for local infection treatment through the synergy between the antimicrobial effect of mixed-charge brushes and the levofloxacin antibiotic release profile.

Monitoring local delivery of vancomycin from gelatin nanospheres in zebrafish larvae

Background

Infections such as biomaterial-associated infection and osteomyelitis are often associated with intracellular survival of bacteria (eg, Staphylococcus aureus). Treatment of these infections remains a major challenge due to the low intracellular efficacy of many antibiotics. Therefore, local delivery systems are urgently required to improve the therapeutic efficacy of antibiotics by enabling their intracellular delivery.

Purpose

To assess the potential of gelatin nanospheres as carriers for local delivery of vancomycin into macrophages of zebrafish larvae in vivo and into THP-1-derived macrophages in vitro using fluorescence microscopy.

Materials and methods

Fluorescently labeled gelatin nanospheres were prepared and injected into transgenic zebrafish larvae with fluorescent macrophages. Both the biodistribution of gelatin nanospheres in zebrafish larvae and the co-localization of vancomycin-loaded gelatin nanospheres with zebrafish macrophages in vivo and uptake by THP-1-derived macrophages in vitro were studied. In addition, the effect of treatment with vancomycin-loaded gelatin nanospheres on survival of S. aureus-infected zebrafish larvae was investigated.

Results

Internalization of vancomycin-loaded gelatin nanospheres by macrophages was observed qualitatively both in vivo and in vitro. Systemically delivered vancomycin, on the other hand, was hardly internalized by macrophages without the use of gelatin nanospheres. Treatment with a single dose of vancomycin-loaded gelatin nanospheres delayed the mortality of S. aureus-infected zebrafish larvae, indicating the improved therapeutic efficacy of vancomycin against (intracellular) S. aureus infection in vivo.

Conclusion

The present study demonstrates that gelatin nanospheres can be used to facilitate local and intracellular delivery of vancomycin.

Robust Thin Film Surface with a Selective Antibacterial Property Enabled via a Cross-Linked Ionic Polymer Coating for Infection-Resistant Medical Applications

Fabrication of new antibacterial surfaces has become a primary strategy for preventing device-associated infections (DAIs). Although considerable progress has recently been made in reducing DAIs, current antibacterial coating methods are technically complex and do not allow selective bacterial killing. Here, we propose novel anti-infective surfaces made of a cross-linked ionic polymer film that achieve selective bacteria killing while simultaneously favoring the survival of mammalian cells. A one-step polymerization process known as initiated chemical vapor deposition was used to generate a cross-linked ionic polymer film from 4-vinylbenzyl chloride and 2-(dimethylamino) ethyl methacrylate monomers in the vapor phase. In particular, the deposition process produced a polymer network with quaternary ammonium cross-linking sites, which provided the surface with an ionic moiety with an excellent antibacterial contact-killing property. This method confers substrate compatibility, which enables various materials to be coated with ionic polymer films for use in medical implants. Moreover, the ionic polymer-deposited surfaces supported the healthy growth of mammalian cells while selectively inhibiting bacterial growth in coculture models without any detectable cytotoxicity. Thus, the cross-linked ionic polymer-based antibacterial surface developed in this study can serve as an ideal platform for biomedical applications that require a highly sterile environment.

Antimicrobial Peptides in Biomedical Device Manufacturing

Over the past decades the use of medical devices, such as catheters, artificial heart valves, prosthetic joints, and other implants, has grown significantly. Despite continuous improvements in device design, surgical procedures, and wound care, biomaterial-associated infections (BAI) are still a major problem in modern medicine. Conventional antibiotic treatment often fails due to the low levels of antibiotic at the site of infection. The presence of biofilms on the biomaterial and/or the multidrug-resistant phenotype of the bacteria further impair the efficacy of antibiotic treatment. Removal of the biomaterial is then the last option to control the infection. Clearly, there is a pressing need for alternative strategies to prevent and treat BAI. Synthetic antimicrobial peptides (AMPs) are considered promising candidates as they are active against a broad spectrum of (antibiotic-resistant) planktonic bacteria and biofilms. Moreover, bacteria are less likely to develop resistance to these rapidly-acting peptides. In this review we highlight the four main strategies, three of which applying AMPs, in biomedical device manufacturing to prevent BAI. The first involves modification of the physicochemical characteristics of the surface of implants. Immobilization of AMPs on surfaces of medical devices with a variety of chemical techniques is essential in the second strategy. The main disadvantage of these two strategies relates to the limited antibacterial effect in the tissue surrounding the implant. This limitation is addressed by the third strategy that releases AMPs from a coating in a controlled fashion. Lastly, AMPs can be integrated in the design and manufacturing of additively manufactured/3D-printed implants, owing to the physicochemical characteristics of the implant material and the versatile manufacturing technologies compatible with antimicrobials incorporation. These novel technologies utilizing AMPs will contribute to development of novel and safe antimicrobial medical devices, reducing complications and associated costs of device infection.

Mussel-Inspired Multifunctional Hydrogel Coating for Prevention of Infections and Enhanced Osteogenesis

Prevention of postsurgery infection and promotion of biointegration are the key factors to achieve long-term success in orthopedic implants. Localized delivery of antibiotics and bioactive molecules by the implant surface serves as a promising approach toward these goals. However, previously reported methods for surface functionalization of the titanium alloy implants to load bioactive ingredients suffer from time-consuming complex processes and lack of long-term stability. Here, we present the design and characterization of an adhesive, osteoconductive, and antimicrobial hydrogel coating for Ti implants. To form this multifunctional hydrogel, a photo-cross-linkable gelatin-based hydrogel was modified with catechol motifs to enhance adhesion to Ti surfaces and thus promote coating stability. To induce antimicrobial and osteoconductive properties, a short cationic antimicrobial peptide (AMP) and synthetic silicate nanoparticles (SNs) were introduced into the hydrogel formulation. The controlled release of AMP loaded in the hydrogel demonstrated excellent antimicrobial activity to prevent biofilm formation. Moreover, the addition of SNs to the hydrogel formulation enhanced osteogenesis when cultured with human mesenchymal stem cells, suggesting the potential to promote new bone formation in the surrounding tissues. Considering the unique features of our implant hydrogel coating, including high adhesion, antimicrobial capability, and the ability to induce osteogenesis, it is believed that our design provides a useful alternative method for bone implant surface modification and functionalization.

Selective laser melting porous metallic implants with immobilized silver nanoparticles kill and prevent biofilm formation by methicillin-resistant Staphylococcus aureus

Implant-associated infection and limited longevity are two major challenges that orthopedic devices need to simultaneously address. Additively manufactured porous implants have recently shown tremendous promise in improving bone regeneration and osseointegration, but, as any conventional implant, are threatened by infection. In this study, we therefore used rational design and additive manufacturing in the form of selective laser melting (SLM) to fabricate porous titanium implants with interconnected pores, resulting in a 3.75 times larger surface area than corresponding solid implants. The SLM implants were biofunctionalized by embedding silver nanoparticles in an oxide surface layer grown using plasma electrolytic oxidation (PEO) in Ca/P-based electrolytes. The PEO layer of the SLM implants released silver ions for at least 28 days. X-ray diffraction analysis detected hydroxyapatite on the SLM PEO implants but not on the corresponding solid implants. In vitro and ex vivo assays showed strong antimicrobial activity of these novel SLM PEO silver-releasing implants, without any signs of cytotoxicity. The rationally designed SLM porous implants outperformed solid implants with similar dimensions undergoing the same biofunctionalization treatment. This included four times larger amount of released silver ions, two times larger zone of inhibition, and one additional order of magnitude of reduction in numbers of CFU in an ex vivo mouse infection model.

3D scaffold with effective multidrug sequential release against bacteria biofilm

Bone infection is a feared complication following surgery or trauma that remains as an extremely difficult disease to deal with. So far, the outcome of therapy could be improved with the design of 3D implants, which combine the merits of osseous regeneration and local multidrug therapy so as to avoid bacterial growth, drug resistance and the feared side effects. Herein, hierarchical 3D multidrug scaffolds based on nanocomposite bioceramic and polyvinyl alcohol (PVA) prepared by rapid prototyping with an external coating of gelatin-glutaraldehyde (Gel-Glu) have been fabricated. These 3D scaffolds contain three antimicrobial agents (rifampin, levofloxacin and vancomycin), which have been localized in different compartments of the scaffold to obtain different release kinetics and more effective combined therapy. Levofloxacin was loaded into the mesopores of nanocomposite bioceramic part, vancomycin was localized into PVA biopolymer part and rifampin was loaded in the external coating of Gel-Glu. The obtained results show an early and fast release of rifampin followed by sustained and prolonged release of vancomycin and levofloxacin, respectively, which are mainly governed by the progressive in vitro degradability rate of these scaffolds. This combined therapy is able to destroy Gram-positive and Gram-negative bacteria biofilms as well as inhibit the bacteria growth. In addition, these multifunctional scaffolds exhibit excellent bioactivity as well as good biocompatibility with complete cell colonization of preosteoblast in the entire surface, ensuring good bone regeneration. These findings suggest that these hierarchical 3D multidrug scaffolds are promising candidates as platforms for local bone infection therapy.

STATEMENT OF SIGNIFICANCE

The present study is focused in finding an adequate therapeutic solution for the treatment of bone infection based on 3D multifunctional scaffolds, which combines the merits of osseous regeneration and local multidrug delivery. These 3D multidrug scaffolds, containing rifampin, levofloxacin and vancomycin, localized in different compartments to achieve different release kinetics. These 3D multidrug scaffolds displays an early and fast release of rifampin followed by sustained and prolonged release of vancomycin and levofloxacin, which are able to destroy Staphylococcus and Escherichia biofilms as well as inhibit bacteria growth in very short time periods. This new combined therapy approach involving the sequential delivery of antibiofilms with antibiotics constitutes an excellent and promising alternative for bone infection treatment.

Development of Nano-Antimicrobial Biomaterials for Biomedical Applications

Around the globe, there is a great concern about controlling growth of pathogenic microorganisms for the prevention of infectious diseases. Moreover, the greater incidences of cross contamination and overuse of drugs has contributed towards the development of drug resistant microbial strains making conditions even worse. Hospital acquired infections pose one of the leading complications associated with implantation of any biomaterial after surgery and critical care. In this regard, developing non-conventional antimicrobial agents which would prevent the aforementioned causes is under the quest. The rapid development in nanoscience and nanotechnology has shown promising potential for developing novel biocidal agents that would integrate with a biomaterial to prevent bacterial colonization and biofilm formation. Metals with inherent antimicrobial properties such as silver, copper, zinc at nano scale constitute a special class of antimicrobials which have broad spectrum antimicrobial nature and pose minimum toxicity to humans. Hence, novel biomaterials that inhibit microbial growth would be of great significance to eliminate medical device/instruments associated infections. This chapter comprises the state-of-art advancements in the development of nano-antimicrobial biomaterials for biomedical applications. Several strategies have been targeted to satisfy few important concern such as enhanced long term antimicrobial activity and stability, minimize leaching of antimicrobial material and promote reuse. The proposed strategies to develop new hybrid antimicrobial biomaterials would offer a potent antibacterial solution in healthcare sector such as wound healing applications, tissue scaffolds, medical implants, surgical devices and instruments.

Gel-Entrapped Staphylococcus aureus Bacteria as Models of Biofilm Infection Exhibit Growth in Dense Aggregates, Oxygen Limitation, Antibiotic Tolerance, and Heterogeneous Gene Expression

An experimental model that mimicked the structure and characteristics of in vivo biofilm infections, such as those occurring in the lung or in dermal wounds where no biomaterial surface is present, was developed. In these infections, microbial biofilm forms as cell aggregates interspersed in a layer of mucus or host matrix material. This structure was modeled by filling glass capillary tubes with an agarose gel that had been seeded with Staphylococcus aureus bacteria and then incubating the gel biofilm in medium for up to 30 h. Confocal microscopy showed that the bacteria formed in discrete pockets distributed throughout the gel matrix. These aggregates enlarged over time and also developed a size gradient, with the clusters being larger near the nutrient- and oxygen-supplied interface and smaller at greater depths. Bacteria entrapped in gels for 24 h grew slowly (specific growth rate, 0.06 h(-1)) and were much less susceptible to oxacillin, minocycline, or ciprofloxacin than planktonic cells. Microelectrode measurements showed that the oxygen concentration decreased with depth into the gel biofilm, falling to values less than 3% of air saturation at depths of 500 μm. An anaerobiosis-responsive green fluorescent protein reporter gene for lactate dehydrogenase was induced in the region of the gel where the measured oxygen concentrations were low, confirming biologically relevant hypoxia. These results show that the gel biofilm model captures key features of biofilm infection in mucus or compromised tissue: formation of dense, distinct aggregates, reduced specific growth rates, local hypoxia, and antibiotic tolerance.

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.

Antimicrobial peptides and their interaction with biofilms of medically relevant bacteria

Biofilm-associated infections represent one of the major threats of modern medicine. Biofilm-forming bacteria are encased in a complex mixture of extracellular polymeric substances (EPS) and acquire properties that render them highly tolerant to conventional antibiotics and host immune response. Therefore, there is a pressing demand of new drugs active against microbial biofilms. In this regard, antimicrobial peptides (AMPs) represent an option taken increasingly in consideration. After dissecting the peculiar biofilm features that may greatly affect the development of new antibiofilm drugs, the present article provides a general overview of the rationale behind the use of AMPs against biofilms of medically relevant bacteria and on the possible mechanisms of AMP-antibiofilm activity. An analysis of the interactions of AMPs with biofilm components, especially those constituting the EPS, and the obstacles and/or opportunities that may arise from such interactions in the development of new AMP-based antibiofilm strategies is also presented and discussed. This article is part of a Special Issue entitled: Antimicrobial Peptides edited by Karl Lohner and Kai Hilpert.

The role of programmed death ligand 1 pathway in persistent biomaterial-associated infections

Staphylococcus epidermidis is commonly involved in biomaterial-associated infections. Bacterial small colony variants (SCV) seem to be well adapted to persist intracellularly in professional phagocytes evading the host immune response. We studied the expression of PD-L1/L2 on macrophages infected with clinical isolates of S. epidermidis SCV and their parent wild type (WT) strains. The cytokine pattern which is triggered by the examined strains was also analysed. In the study, we infected macrophages with S. epidermidis WT and SCV strains. Persistence and release from macrophages were monitored via lysostaphin protection assays. Moreover, the effect of IFN-γ pre-treatment on bacterial internalisation was investigated. Expression of PD-L1/L2 molecules was analysed with the use of FACS. Inflammatory reaction was measured by IL-10, TNF-α ELISAs, and transcriptional induction of TNF-α. Our study revealed that clinical SCV isolates were able to persist and survive in macrophages for at least 3 days with a low cytotoxic effect and a reduced proinflammatory response as compared to WT strains. Bacteria upregulated PD-L1/L2 expression on macrophages as compared to non-stimulated cells. The results demonstrated that the ability of S. epidermidis SCVs to induce elevated levels of anti-inflammatory cytokine, IL-10, and reduced transcriptional induction of TNF-α, together with expression of PD-L1 on macrophages and the ability to persist intracellularly without damaging the host cell could be the key factor contributing to chronicity of SCV infections.

Biofilms in periprosthetic orthopedic infections

As the number of total joint arthroplasty and internal fixation procedures continues to rise, the threat of infection following surgery has significant clinical implications. These infections may have highly morbid consequences to patients, who often endure additional surgeries and lengthy exposures to systemic antibiotics, neither of which are guaranteed to resolve the infection. Of particular concern is the threat of bacterial biofilm development, since biofilm-mediated infections are difficult to diagnose and effective treatments are lacking. Developing therapeutic strategies have targeted mechanisms of biofilm formation and the means by which these bacteria communicate with each other to take on specialized roles such as persister cells within the biofilm. In addition, prevention of infection through novel coatings for prostheses and the local delivery of high concentrations of antibiotics by absorbable carriers has shown promise in laboratory and animal studies. Biofilm development, especially in an arthoplasty environment, and future diagnostic and treatment options are discussed.

Structural basis of Staphylococcus epidermidis biofilm formation: mechanisms and molecular interactions

Staphylococcus epidermidis is a usually harmless commensal bacterium highly abundant on the human skin. Under defined predisposing conditions, most importantly implantation of a medical device, S. epidermidis, however, can switch from a colonizing to an invasive life style. The emergence of S. epidermidis as an opportunistic pathogen is closely linked to the biofilm forming capability of the species. During the past decades, tremendous advance regarding our understanding of molecular mechanisms contributing to surface colonization has been made, and detailed information is available for several factors active during the primary attachment, accumulative or dispersal phase of biofilm formation. A picture evolved in which distinct factors, though appearing to be redundantly organized, take over specific and exclusive functions during biofilm development. In this review, these mechanisms are described in molecular detail, with a highlight on recent insights into multi-functional S. epidermidis cell surface proteins contributing to surface adherence and intercellular adhesion. The integration of distinct biofilm-promoting factors into regulatory networks is summarized, with an emphasis on mechanism that could allow S. epidermidis to flexibly adapt to changing environmental conditions present during colonizing or invasive life-styles.

Natural Green coating inhibits adhesion of clinically important bacteria

Despite many advances, biomaterial-associated infections continue to be a major clinical problem. In order to minimize bacterial adhesion, material surface modifications are currently being investigated and natural products possess large potential for the design of innovative surface coatings. We report the bioguided phytochemical investigation of Pityrocarpa moniliformis and the characterization of tannins by mass spectrometry. It was demonstrated that B-type linked proanthocyanidins-coated surfaces, here termed Green coatings, reduced Gram-positive bacterial adhesion and supported mammalian cell spreading. The proposed mechanism of bacterial attachment inhibition is based on electrostatic repulsion, high hydrophilicity and the steric hindrance provided by the coating that blocks bacterium-substratum interactions. This work shows the applicability of a prototype Green-coated surface that aims to promote necessary mammalian tissue compatibility, while reducing bacterial colonization.

Analysis of RNAseq datasets from a comparative infectious disease zebrafish model using GeneTiles bioinformatics

We present a RNA deep sequencing (RNAseq) analysis of a comparison of the transcriptome responses to infection of zebrafish larvae with Staphylococcus epidermidis and Mycobacterium marinum bacteria. We show how our developed GeneTiles software can improve RNAseq analysis approaches by more confidently identifying a large set of markers upon infection with these bacteria. For analysis of RNAseq data currently, software programs such as Bowtie2 and Samtools are indispensable. However, these programs that are designed for a LINUX environment require some dedicated programming skills and have no options for visualisation of the resulting mapped sequence reads. Especially with large data sets, this makes the analysis time consuming and difficult for non-expert users. We have applied the GeneTiles software to the analysis of previously published and newly obtained RNAseq datasets of our zebrafish infection model, and we have shown the applicability of this approach also to published RNAseq datasets of other organisms by comparing our data with a published mammalian infection study. In addition, we have implemented the DEXSeq module in the GeneTiles software to identify genes, such as glucagon A, that are differentially spliced under infection conditions. In the analysis of our RNAseq data, this has led to the possibility to improve the size of data sets that could be efficiently compared without using problem-dedicated programs, leading to a quick identification of marker sets. Therefore, this approach will also be highly useful for transcriptome analyses of other organisms for which well-characterised genomes are available.

Staphylococcus epidermidis originating from titanium implants infects surrounding tissue and immune cells

Infection is a major cause of failure of inserted or implanted biomedical devices (biomaterials). During surgery, bacteria may adhere to the implant, initiating biofilm formation. Bacteria are also observed in and recultured from the tissue surrounding implants, and may even reside inside host cells. Whether these bacteria originate from biofilms is not known. Therefore, we investigated the fate of Staphylococcus epidermidis inoculated on the surface of implants as adherent planktonic cells or as a biofilm in mouse experimental biomaterial-associated infection. In order to discriminate the challenge strain from potential contaminating mouse microflora, we constructed a fully virulent green fluorescent S. epidermidis strain. S. epidermidis injected along subcutaneous titanium implants, pre-seeded on the implants or pre-grown as biofilm, were retrieved from the implants as well as the surrounding tissue in all cases after 4days, and in histology bacteria were observed in the tissue co-localizing with macrophages. Thus, bacteria adherent to or in a biofilm on the implant are a potential source of infection of the surrounding tissue, and antimicrobial strategies should prevent both biofilm formation and tissue colonization.

Biomaterial-associated infection: locating the finish line in the race for the surface

Biomaterial-associated infections occur on both permanent implants and temporary devices for restoration or support of human functions. Despite increasing use of biomaterials in an aging society, comparatively few biomaterials have been designed that effectively reduce the incidence of biomaterial-associated infections. This review provides design guidelines for infection-reducing strategies based on the concept that the fate of biomaterial implants or devices is a competition between host tissue cell integration and bacterial colonization at their surfaces.

A zebrafish high throughput screening system used for Staphylococcus epidermidis infection marker discovery

BACKGROUND

Staphylococcus epidermidis bacteria are a major cause of biomaterial-associated infections in modern medicine. Yet there is little known about the host responses against this normally innocent bacterium in the context of infection of biomaterials. In order to better understand the factors involved in this process, a whole animal model with high throughput screening possibilities and markers for studying the host response to S. epidermidis infection are required.

RESULTS

We have used a zebrafish yolk injection system to study bacterial proliferation and the host response in a time course experiment of S. epidermidis infection. By combining an automated microinjection system with complex object parametric analysis and sorting (COPAS) technology we have quantified bacterial proliferation. This system was used together with transcriptome analysis at several time points during the infection period. We show that bacterial colony forming unit (CFU) counting can be replaced by high throughput flow-based fluorescence analysis of embryos enabling high throughput readout. Comparison of the host transcriptome response to S. epidermidis and Mycobacterium marinum infection in the same system showed that M. marinum has a far stronger effect on host gene regulation than S. epidermidis. However, multiple genes responded differently to S. epidermidis infection than to M. marinum, including a cell adhesion gene linked to specific infection by staphylococci in mammals.

CONCLUSIONS

Our zebrafish embryo infection model allowed (i) quantitative assessment of bacterial proliferation, (ii) identification of zebrafish genes serving as markers for infection with the opportunistic pathogen S. epidermidis, and (iii) comparison of the transcriptome response of infection with S. epidermidis and with the pathogen M. marinum. As a result we have identified markers that can be used to distinguish common and specific responses to S. epidermidis. These markers enable the future integration of our high throughput screening technology with functional analyses of immune response genes and immune modulating factors.

Persistence of a bioluminescent Staphylococcus aureus strain on and around degradable and non-degradable surgical meshes in a murine model

Biomaterials are increasingly used for the restoration of human function, but can become infected as a result of peri- or early post-operative bacterial contamination, although biomaterial-associated infections (BAIs) can also initiate at any time from hematogenous spreading of bacteria from an infection elsewhere in the body. Infecting bacteria in BAIs not only seek shelter in their own protective biofilm matrix, but also hide in surrounding tissue. This study compares staphylococcal persistence on and around a degradable and non-degradable surgical mesh through the use of longitudinal bioluminescence imaging in a murine model, including histological evaluation of surrounding tissue after sacrifice. Surgical meshes were first contaminated with bioluminescent Staphylococcus aureus Xen29 and subsequently subcutaneously implanted in mice. Bioluminescent staphylococci persisted on and around non-degradable meshes during the 28-day course of the study, whereas bioluminescence returned to control levels and bacteria disappeared from surrounding tissues once a degradable mesh had fully dissolved. Thus the application of degradable biomaterials yields major advantages with respect to the prevention of BAIs, as dissolution of the implant not only is associated with elimination of the protective biofilm mode of growth of the infecting organisms, but also allows the immune system to clear the surrounding tissue from infecting organisms.

Immobilized antibiotics to prevent orthopaedic implant infections

Many surgical procedures require the placement of an inert or tissue-derived implant deep within the body cavity. While the majority of these implants do not become colonized by bacteria, a small percentage develops a biofilm layer that harbors invasive microorganisms. In orthopaedic surgery, unresolved periprosthetic infections can lead to implant loosening, arthrodeses, amputations and sometimes death. The focus of this review is to describe development of an implant in which an antibiotic tethered to the metal surface is used to prevent bacterial colonization and biofilm formation. Building on well-established chemical syntheses, studies show that antibiotics can be linked to titanium through a self-assembled monolayer of siloxy amines. The stable metal-antibiotic construct resists bacterial colonization and biofilm formation while remaining amenable to osteoblastic cell adhesion and maturation. In an animal model, the antibiotic modified implant resists challenges by bacteria that are commonly present in periprosthetic infections. While the long-term efficacy and stability is still to be established, ongoing studies support the view that this novel type of bioactive surface has a real potential to mitigate or prevent the devastating consequences of orthopaedic infection.