Development, standardization and testing of a bacterial wound infection model based on ex vivo human skin

Fungal quorum sensing molecules as potential drugs in the treatment of chronic wounds and their delivery

INTRODUCTION

Chronic non-healing wounds have emerged as a significant global healthcare challenge. Biofilm induced wound infections has been widely acknowledged. Despite the advanced understanding of biofilm formation, the existing approaches for diagnosing biofilms in wounds remain considerably suboptimal. Chemical signals produced by fungi to sense their environment, known as quorum sensing (QS) molecules are anticipated to cause revolution in non-healing wound antisepsis.

AREAS COVERED

Biofilms render chronic wounds resistant to treatment and impede tissue repair by inducing chronic inflammation. QS is a biochemical signaling pathway that involves certain secreted molecules, namely phenylethanoids, indolyl, and sesquiterpene alcohols that can significantly minimize and obliterate bacterial biofilms if properly applied and released in wound treatments.

EXPERT OPINION

QS molecules (QSMs) possess inhibitory properties that obstruct the formation of microbial biofilms and exhibit synergism with common antimicrobials. They can disrupt biofilms formed by drug-resistant microorganisms. The understanding of the current mechanisms and advancements in the utilization of QSMs within diverse drug delivery systems, and their release dynamics will be crucial in new drug design and delivery. Exploration of co-delivery of drugs alongside QS molecules, and assessing their impact on healing of chronic wounds before moving to clinical trials remain unaddressed.

Diagnostic electron microscopy in human infectious diseases - Methods and applications

Diagnostic electron microscopy (EM) is indispensable in all cases of infectious diseases which deserve or profit from the detection of the entire pathogen (i.e. the infectious unit). The focus of its application has shifted during the last decades from routine diagnostics to diagnostics of special cases, emergencies and the investigation of disease pathogenesis. While the focus of application has changed, the methods remain more or less the same. However, since the number of cases for diagnostic EM has declined as the number of laboratories that are able to perform such investigations, the preservation of the present knowledge is important. The aim of this article is to provide a review of the methods and strategies which are useful for diagnostic EM related to infectious diseases in our days. It also addresses weaknesses as well as useful variants or extensions of established methods. The main techniques, negative staining and thin section EM, are described in detail with links to suitable protocols and more recent improvements, such as thin section EM of small volume suspensions. Sample collection, transport and conservation/inactivation are discussed. Strategies of sample examination and requirements for a proper recognition of structures are outlined. Finally, some examples for the actual application of diagnostic EM related to infectious diseases are presented. The outlook section will discuss recent trends in microscopy, such as automated object recognition by machine learning, regarding their potential in supporting diagnostic EM.

An Injectable Living Hydrogel with Embedded Probiotics as a Novel Strategy for Combating Multifaceted Pathogen Wound Infections

Wound infections pose a significant challenge in healthcare, and traditional antibiotic treatments often result in the development of resistant pathogens. Addressing this gap, ProGel is introduced, a living hydrogel created by entrapping probiotic Lactobacillus plantarum as a therapeutic component within a gelatin matrix. With a double-syringe system, ProGel can be easily mixed and applied, conforming swiftly to any wound shape and forming hydrogel in situ. It also demonstrates robust mechanical and self-healing properties owing to the Schiff-base bonds. ProGel sustains more than 80% viability of the entrapped L. plantarum while restricting their escape from the hydrogel. After a week of storage, more than 70% viability of the entrapped L. plantarum is preserved. Importantly, ProGel exhibits broad-spectrum antimicrobial efficacy against pathogens commonly associated with wound infections, i.e., Pseudomonas aeruginosa (7Log reduction), Staphylococcus aureus (3-7Log reduction), and Candida albicans (40-70% reduction). Moreover, its cytocompatibility is affirmed through coculture with human dermal fibroblasts. The effectiveness of ProGel is further highlighted in more clinically relevant tests on human skin wound models infected with P. aeruginosa and S. aureus, where it successfully prevents the biofilm formation of these pathogens. This study showcases an injectable living hydrogel system for the management of complex wound infections.

Secondary metabolites in topical infectious diseases and nanomedicine applications

Topical infection affects nearly one-third of the world's population; it may result from poor sanitation, hygienic conditions and crowded living and working conditions that accelerate the spread of topical infectious diseases. The problems associated with the anti-infective agents are drug resistance and long-term therapy. Secondary metabolites are obtained from plants, microorganisms and animals, but they are metabolized inside the human body. The integration of nanotechnology into secondary metabolites is gaining attention due to their interaction at the subatomic and skin-tissue levels. Hydrogel, liposomes, lipidic nanoparticles, polymeric nanoparticles and metallic nanoparticles are the most suitable carriers for secondary metabolite delivery. Therefore, the present review article extensively discusses the topical applications of nanomedicines for the effective delivery of secondary metabolites.

A standardized wound infection model for antimicrobial testing of wound dressings in vitro

To investigate the effectiveness of antimicrobial agents against wound infections, experiments using either 2D cultures with planktonic microorganisms or animal infection models are frequently carried out. However, the transferability of the results to human skin is limited by the lack of complexity of the 2D models or by the poor translation of the results from animal models. Hence, there is a need for wound infection models capable of assessing antimicrobial agents. In this study, an easily standardized wound infection model was established. This model consists of a mechanically wounded human skin model on a collagen matrix infected with various clinically relevant bacteria. Infection of the model led to recognition of the pathogens and induction of an inflammatory response. The untreated infection spread over time, causing significant tissue damage. By applying an antimicrobial-releasing wound dressing, the bacterial load could be reduced and the success of the treatment could be further measured by a decrease in the inflammatory reaction. In conclusion, this wound infection model can be used to evaluate new antimicrobial therapeutics as well as to study host-pathogen interactions.

Native human and mouse skin infection models to study Candida auris-host interactions

The World Health Organization (WHO) declared certain fungal pathogens as global health threats for the next decade. Candida auris (C. auris) is a newly emerging skin-tropic multidrug-resistant fungal pathogen that can cause life-threatening infections of high mortality in hospitals and healthcare settings. Here, we address an unmet need and present novel native ex vivo skin models, thus extending previous C. auris-host interaction studies. We exploit histology and immunofluorescence analysis of ex vivo skin biopsies of human adult and fetal, as well as mouse origin infected with C. auris via distinct routes. We demonstrate that an intact skin barrier efficiently protects from C. auris penetration and invasion. Although C. auris readily grows on native human skin, it can reach deeper layers only upon physical disruption of the barrier by needling or through otherwise damaged skin. By contrast, a barrier disruption is not necessary for C. auris penetration of native mouse skin. Importantly, we show that C. auris undergoes morphogenetic changes upon skin penetration, as it acquires pseudohyphal growth phenotypes in deeper human and mouse dermis. Taken together, this new human and mouse skin model toolset yields new insights into C. auris colonization, adhesion, growth and invasion properties of native versus damaged human skin. The results form a crucial basis for future studies on skin immune defense to colonizing pathogens, and offer new options for testing the action and efficacy of topical antimicrobial compound formulations.

Unravelling host-pathogen interactions by biofilm infected human wound models

Approximately 80 % of persistent wound infections are affected by the presence of bacterial biofilms, resulting in a severe clinical challenge associated with prolonged healing periods, increased morbidity, and high healthcare costs. Unfortunately, in vitro models for wound infection research almost exclusively focus on early infection stages with planktonic bacteria. In this study, we present a new approach to emulate biofilm-infected human wounds by three-dimensional human in vitro systems. For this purpose, a matured biofilm consisting of the clinical key wound pathogen Pseudomonas aeruginosa was pre-cultivated on electrospun scaffolds allowing for non-destructive transfer of the matured biofilm to human in vitro wound models. We infected tissue-engineered human in vitro skin models as well as ex vivo human skin explants with the biofilm and analyzed structural tissue characteristics, biofilm growth behavior, and biofilm-tissue interactions. The structural development of biofilms in close proximity to the tissue, resulting in high bacterial burden and in vivo-like morphology, confirmed a manifest wound infection on all tested wound models, validating their applicability for general investigations of biofilm growth and structure. The extent of bacterial colonization of the wound bed, as well as the subsequent changes in molecular composition of skin tissue, were inherently linked to the characteristics of the underlying wound models including their viability and origin. Notably, the immune response observed in viable ex vivo and in vitro models was consistent with previous in vivo reports. While ex vivo models offered greater complexity and closer similarity to the in vivo conditions, in vitro models consistently demonstrated higher reproducibility. As a consequence, when focusing on direct biofilm-skin interactions, the viability of the wound models as well as their advantages and limitations should be aligned to the particular research question of future studies. Altogether, the novel model allows for a systematic investigation of host-pathogen interactions of bacterial biofilms and human wound tissue, also paving the way for development and predictive testing of novel therapeutics to combat biofilm-infected wounds.

Ciprofloxacin-Loaded Polyvinylpyrrolidone Foils for the Topical Treatment of Wound Infections with Methicillin-Resistant Staphylococcus aureus (MRSA)

Bacterial infections are a constant challenge in the management of acute and chronic wounds. Chronic wounds, such as diabetic foot ulcers, have increased significantly in the last few years due to the rise of an aging population. A better understanding of the infectious pathophysiological mechanisms is urgently needed along with new options for the treatment of wound infections and wound-healing disorders. New advances in the preparation of biocompatible dressing materials that can be loaded with antimicrobial drugs may improve the topical treatment of infected wounds. In this study, we investigated the antimicrobial activity of polyvinylpyrrolidone (PVP) foils loaded with ciprofloxacin (Cipro-foils) in the presence of acetic acid as a co-solvent. We used ex vivo human wounds that were infected with two bacterial strains: methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa (PAO1). The effectiveness of the treatment was demonstrated by the quantification of the living bacteria extracted from the wound and the detection of released immunological mediators in skin extracts and in the skin culture media. We found that Cipro-foils effectively treated the infection with both PAO1 and MRSA. Other than PAO1, MRSA had no lytic activity toward skin proteins. MRSA infections increased cytokines' expression and release. Interestingly, treatment with Cipro-foils could partially counteract these effects.

CuS Hybrid Hydrogel for Near-Infrared-Enhanced Infected Wound Healing: A Gelatin-Assisted Synthesis and Direct Incorporation Strategy

Developing antibacterial hydrogels, with good mechanical strength and self-healing ability to resist bacterial invasion and accelerate skin regeneration, is critical for infected full-thickness skin wound treatment. Herein, we report a gelatin-assisted synthesis and direct incorporation strategy to construct a CuS hybrid hydrogel for infected wound healing applications. CuS nanodots (NDs) were synthesized directly inside a gelatin host matrix (Gel-CuS), and these tightly confined and evenly distributed CuS NDs displayed superb dispersibility and stability against oxidation. Gel-CuS was then used to crosslink with oxidized dextran (ODex) to form a Gel-CuS-8/ODex hydrogel (8 stands for the concentration of CuS, in mM) via a facile Schiff-base reaction, which exhibited improved mechanical properties, excellent adhesion and self-healing ability, suitable swelling and degradation behavior, and good biocompatibility. The Gel-CuS-8/ODex hydrogel can act as an efficient antibacterial agent due to its photothermal and photodynamic properties under a 1064 nm laser irradiation. Furthermore, in animal experiments, when being applied as wound dressing, the Gel-CuS-8/ODex hydrogel significantly promoted infected full-thickness cutaneous wound healing through improved epidermis and granulation tissue formation and accelerated generation of new blood vessels, hair follicles, and collagen deposition after proper near-infrared irradiation treatment. This work provides a promising strategy to synthesize functional inorganic nanomaterials tightly and evenly embedded inside modified natural hydrogel networks for wound healing applications.

Epidermal/Dermal Separation Techniques and Analysis of Cell Populations in Human Skin Sheets

Human skin consists of three compartments, each endowed with a particular structure and the presence of several immune and nonimmune cells that together comprise a protective shield and orchestrate multiple processes in the skin. Appropriate processing of human skin samples acquired from healthy volunteers or patients is essential for successful analysis in basic, translational, and clinical research to obtain accurate and reliable results, despite differences between individuals. From the wide range of available assays and methods, it is necessary to select the suitable method for separation of skin compartments, which will provide preservation or high viability of skin cells or whole structures that will be analyzed or further processed. In this paper, we review and discuss skin separation methods and compare their features such as processing time, cell viability, location of the basement membrane after detachment of the epidermis from the dermis, and their application. Furthermore, we visualize different cell populations and structures in epidermal and dermal sheets using confocal microscopy. It is aimed to provide an overview of the optimal processing of human skin samples and their possible application.

Adding a new dimension: Multi-level structure and organization of mixed-species Pseudomonas aeruginosa and Staphylococcus aureus biofilms in a 4-D wound microenvironment

Biofilms in wounds typically consist of aggregates of bacteria, most often Pseudomonas aeruginosa and Staphylococcus aureus, in close association with each other and the host microenvironment. Given this, the interplay across host and microbial elements, including the biochemical and nutrient profile of the microenvironment, likely influences the structure and organization of wound biofilms. While clinical studies, in vivo and ex vivo model systems have provided insights into the distribution of P. aeruginosa and S. aureus in wounds, they are limited in their ability to provide a detailed characterization of biofilm structure and organization across the host-microbial interface. On the other hand, biomimetic in vitro systems, such as host cell surfaces and simulant media conditions, albeit reductionist, have been shown to support the co-existence of P. aeruginosa and S. aureus biofilms, with species-dependent localization patterns and interspecies interactions. Therefore, composite in vitro models that bring together key features of the wound microenvironment could provide unprecedented insights into the structure and organization of mixed-species biofilms. We have built a four-dimensional (4-D) wound microenvironment consisting of a 3-D host cell scaffold of co-cultured human epidermal keratinocytes and dermal fibroblasts, and an in vitro wound milieu (IVWM); the IVWM provides the fourth dimension that represents the biochemical and nutrient profile of the wound infection state. We leveraged this 4-D wound microenvironment, in comparison with biofilms in IVWM alone and standard laboratory media, to probe the structure of mixed-species P. aeruginosa and S. aureus biofilms across multiple levels of organization such as aggregate dimensions and biomass thickness, species co-localization and spatial organization within the biomass, overall biomass composition and interspecies interactions. In doing so, the 4-D wound microenvironment platform provides multi-level insights into the structure of mixed-species biofilms, which we incorporate into the current understanding of P. aeruginosa and S. aureus organization in the wound bed.

β-Lactamase-Responsive Hydrogel Drug Delivery Platform for Bacteria-Triggered Cargo Release

Antibiotic resistance is a growing public health threat that complicates the treatment of infections. β-Lactamase enzymes, which hydrolyze the β-lactam ring present in many common antibiotics, are a major cause of this resistance and are produced by a broad range of bacterial pathogens. Here, we developed hydrogels that degrade specifically in the presence of β-lactamases and β-lactamase-producing bacteria as a platform for bacteria-triggered drug delivery. A maleimide-functionalized β-lactamase-cleavable cephalosporin was used as a crosslinker in the fabrication of hydrogels through end-crosslinked polymerization with multiarm thiol-terminated poly(ethylene glycol) macromers via Michael-type addition. We demonstrated that only hydrogels containing the responsive crosslinker were degraded by β-lactamases and β-lactamase-producing bacteria in vitro and in an ex vivo porcine skin infection model. Fluorescent polystyrene nanoparticles, encapsulated in the hydrogels as model cargo, were released at rates that closely tracked hydrogel wet mass loss, confirming β-lactamase-triggered controlled cargo release. Nonresponsive hydrogels, lacking the β-lactam crosslinker, remained stable in the presence of β-lactamases and β-lactamase-producing bacteria and exhibited no change in mass or nanoparticle release. Furthermore, the responsive hydrogels remained stable in non-β-lactamase enzymes, including collagenases and lipases. These hydrogels have the potential to be used as a bacteria-triggered drug delivery system to control unnecessary exposure to encapsulated antimicrobials, which can provide effective infection treatment without exacerbating resistance.

Effects of Adipose-Derived Stem Cells and Their Conditioned Medium in a Human Ex Vivo Wound Model

Adult stem cells have been extensively investigated for tissue repair therapies. Adipose-derived stem cells (ASCs) were shown to improve wound healing by promoting re-epithelialization and vascularization as well as modulating the inflammatory immune response. In this study, we used ex vivo human skin cultured in a six-well plate with trans-well inserts as a model for superficial wounds. Standardized wounds were created and treated with allogeneic ASCs, ASCs conditioned medium (ASC-CM), or cell culture medium (DMEM) supplemented with fetal calf serum (FCS). Skin viability (XTT test), histology (hematoxylin and eosin, H and E), β-catenin expression as well as inflammatory mediators and growth factors were monitored over 12 days of skin culture. We observed only a moderate time-dependent decrease in skin metabolic activity while skin morphology was preserved, and re-epithelialization occurred at the wound edges. An increase in β-catenin expression was observed in the newly formed epithelia, especially in the samples treated with ASC-CM. In general, increased growth factors and inflammatory mediators, e.g., hepatocytes growth factor (HGF), platelet-derived growth factor subunit AA (PDGF-AA), IL-1α, IL-7, TNF-α, and IL-10, were observed over the incubation time. Interestingly, different expression profiles were observed for the different treatments. Samples treated with ASC-CM significantly increased the levels of inflammatory cytokines and PDGF-AA with respect to control, whereas the treatment with ASCs in DMEM with 10% FCS resulted in significantly increased levels of fibroblast growth factor-basic (FGF-basic) and moderate increases of immunomodulatory cytokines. These results confirm that the wound microenvironment can influence the type of mediators secreted by ASCs and the mode as to how they improve the wound healing process. Comparative investigations with pre-activated ASCs will elucidate further aspects of the wound healing mechanism and improve the protocols of ACS application.

Evaluation of a Multifunctional Polyvinylpyrrolidone/Hyaluronic Acid-Based Bilayer Film Patch with Anti-Inflammatory Properties as an Enhancer of the Wound Healing Process

The management of acute and chronic wounds is still a socioeconomic burden for society due to the lack of suitable tools capable of supporting all the healing phases. The exponential spread of diabetes worldwide and the consequent increase of complicated diabetic ulcers require further efforts to develop scalable, low-cost, and easy-to-use treatments for tackling this emergency. Recently, we explored the fabrication of a polyvinylpyrrolidone/hyaluronic acid-based bilayer wound dressing, characterizing its physicochemical features and detailing its excellent antimicrobial activity. Here, we further demonstrate its biocompatibility on fibroblasts, keratinocytes, and red blood cells. The bilayer shows anti-inflammatory properties, statistically reducing the level of IL-6, IL-1β, and TNF-α, and a capacity to accelerate wound healing in vitro and in healthy and diabetic mice models compared to untreated mice. The outcomes suggest that this bilayer material can be an effective tool for managing different skin injuries.

Nanocarriers for Skin Applications: Where Do We Stand?

Skin penetration of active molecules for treatment of diverse diseases is a major field of research owing to the advantages associated with the skin like easy accessibility, reduced systemic-derived side effects, and increased therapeutic efficacy. Despite these advantages, dermal drug delivery is generally challenging due to the low skin permeability of therapeutics. Although various methods have been developed to improve skin penetration and permeation of therapeutics, they are usually aggressive and could lead to irreversible damage to the stratum corneum. Nanosized carrier systems represent an alternative approach for current technologies, with minimal damage to the natural barrier function of skin. In this Review, the use of nanoparticles to deliver drug molecules, genetic material, and vaccines into the skin is discussed. In addition, nanotoxicology studies and the recent clinical development of nanoparticles are highlighted to shed light on their potential to undergo market translation.

Systems of conductive skin for power transfer in clinical applications

The primary aim of this article is to review the clinical challenges related to the supply of power in implanted left ventricular assist devices (LVADs) by means of transcutaneous drivelines. In effect of that, we present the preventive measures and post-operative protocols that are regularly employed to address the leading problem of driveline infections. Due to the lack of reliable wireless solutions for power transfer in LVADs, the development of new driveline configurations remains at the forefront of different strategies that aim to power LVADs in a less destructive manner. To this end, skin damage and breach formation around transcutaneous LVAD drivelines represent key challenges before improving the current standard of care. For this reason, we assess recent strategies on the surface functionalization of LVAD drivelines, which aim to limit the incidence of driveline infection by directing the responses of the skin tissue. Moreover, we propose a class of power transfer systems that could leverage the ability of skin tissue to effectively heal short diameter wounds. In this direction, we employed a novel method to generate thin conductive wires of controllable surface topography with the potential to minimize skin disruption and eliminate the problem of driveline infections. Our initial results suggest the viability of the small diameter wires for the investigation of new power transfer systems for LVADs. Overall, this review uniquely compiles a diverse number of topics with the aim to instigate new research ventures on the design of power transfer systems for IMDs, and specifically LVADs.

Screening of Surfactants for Improved Delivery of Antimicrobials and Poly-Lactic-co-Glycolic Acid Particles in Wound Tissue

Topical wound management is often a challenge due to the poor penetration of antimicrobials in wound tissue and across the biofilm matrix where bacteria are embedded. Surfactants have been used for decades to improve the stability of formulations, increase drug solubility, and enhance penetration. In this study, we screened different detergents with respect to their cytotoxicity and their ability to improve the penetration of poly-lactic-co-glycolic acid (PLGA) particles in wound tissue. Among the tested surfactants, Kolliphor SLS and Tween 80 increased the penetration of PLGA particles and had a limited cytotoxicity. Then, these surfactants were used to formulate PLGA particles loaded with the poorly water-soluble antibiotic ciprofloxacin. The antimicrobial efficacy of the formulations was tested in a wound infection model based on human ex vivo skin. We found that even though PLGA particles had the same antimicrobial efficiency than the particle-free drug formulation, thanks to their solubilizing and anti-biofilm properties, the surfactants remarkably improved the antimicrobial activity of ciprofloxacin with respect to the drug formulation in water. We conclude that the use of Tween 80 in antimicrobial formulations might be a safe and efficient option to improve the topical antimicrobial management of chronic wound infections.

Vancomycin-Loaded Microneedle Arrays against Methicillin-Resistant Staphylococcus Aureus Skin Infections

Skin and soft tissue infections (SSTIs) caused by methicillin-resistant Staphylococcus aureus (MRSA) are a major healthcare burden, often treated with intravenous injection of the glycopeptide antibiotic vancomycin (VAN). However, low local drug concentration in the skin limits its treatment efficiency, while systemic exposure promotes the development of resistant bacterial strains. Topical administration of VAN on skin is ineffective as its high molecular weight prohibits transdermal penetration. In order to implement a local VAN delivery, microneedle (MN) arrays with a water-insoluble support layer for the controlled administration of VAN into the skin are developed. The utilization of such a support layer results in water-insoluble needle shafts surrounded by drug-loaded water-soluble tips with high drug encapsulation. The developed MN arrays can penetrate the dermal barriers of both porcine and fresh human skin. Permeation studies on porcine skin reveal that the majority of the delivered VAN is retained within the skin. It is shown that the VAN-MN array reduces MRSA growth both in vitro and ex vivo on skin. The developed VAN-MN arrays may be extended to several drugs and may facilitate localized treatment of MRSA-caused skin infections while minimizing adverse systemic effects.

Human Three-Dimensional Models for Studying Skin Pathogens

Skin is the most exposed surface of the human body, separating the microbe-rich external environment, from the sterile inner part. When skin is breached or its homeostasis is perturbed, bacterial, fungal and viral pathogens can cause local infections or use the skin as an entry site to spread to other organs. In the last decades, it has become clear that skin provides niches for permanent microbial colonization, and it actively interacts with microorganisms. This crosstalk promotes skin homeostasis and immune maturation, preventing expansion of harmful organisms. Skin commensals, however, are often found to be skin most prevalent and dangerous pathogens. Despite the medical interest, mechanisms of colonization and invasion for most skin pathogens are poorly understood. This limitation is due to the lack of reliable skin models. Indeed, animal models do not adequately mimic neither the anatomy nor the immune response of human skin. Human 3D skin models overcome these limitations and can provide new insights into the molecular mechanisms of microbial pathogenesis. Herein, we address the strengths and weaknesses of different types of human skin models and we review the main findings obtained using these models to study skin pathogens.

An ex vivo model of medical device-mediated bacterial skin translocation

The skin is a barrier and part of the immune system that protects us from harmful bacteria. Because indwelling medical devices break this barrier, they greatly increase the risk of infection by microbial pathogens. To study how these infections can be prevented through improved clinical practices and medical device technology, it is important to have preclinical models that replicate the early stages of microbial contamination, ingress, and colonization leading up to infection. At present, there are no preclinical ex vivo models specifically developed to simulate conditions for indwelling medical devices. Translocation of pathogens from outside the body across broken skin to normally sterile internal compartments is a rate-limiting step in infectious pathogenesis. In this work, we report a sensitive and reproducible ex vivo porcine skin-catheter model to test how long antimicrobial interventions can delay translocation. Skin preparation was first optimized to minimize tissue damage. The presence of skin dramatically decreased bacterial migration time across the polyurethane catheter interface from > 96 h to 12 h. Using visual colony detection, fluorescence, a luminescent in vitro imaging system, and confocal microscopy, the model was used to quantify time-dependent differences in translocation for eluting and non-eluting antimicrobial catheters. The results show the importance of including tissue in preclinical biofilm models and help to explain current gaps between in vitro testing and clinical outcomes for antimicrobial devices.

Effects of various wound dressings on microbial growth in perfused equine musculocutaneous flaps

OBJECTIVE

To compare the effect of multiple wound dressings on microbial growth in a perfused equine wound model.

SAMPLE

Abdominal musculocutaneous flaps from 16 equine cadavers.

PROCEDURES

8 full-thickness skin wound covered were created in each flap. Tissues were perfused with saline (0.9% NaCl) solution. Wounds were inoculated with methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa (10⁶ CFUs), incubated, and covered with a dressing containing activated charcoal, boric acid, cadexomer iodine, calcium alginate, manuka honey, nanoparticle silver, or polyhexamethylene biguanide or with a control (nonadherent gauze) dressing. Muscle biopsy specimens were obtained at baseline (immediately prior to dressing application) and 6, 12, 18, and 24 hours later for mean bacterial load (MBL) determination. The MBLs at each subsequent time point were compared with that at baseline within dressing types, and MBLs at each time point were compared among dressing types.

RESULTS

MBLs in MRSA-inoculated wounds covered with cadexomer iodine dressings were significantly decreased from baseline at the 6- and 12-hour time points. For P aeruginosa-inoculated wounds, MBLs were significantly increased from baseline in all wounds at various times except for wounds with cadexomer iodine dressings. The MBLs of wounds with cadexomer iodine dressings were lower than all others, although not always significantly different from those for wounds with boric acid, manuka honey, nanoparticle silver, and polyhexamethylene biguanide dressings.

CONCLUSIONS AND CLINICAL RELEVANCE

In this nonviable perfused wound model, growth of MRSA and P aeruginosa was most effectively reduced or inhibited by cadexomer iodine dressings. These results and the effect of the dressings on wound healing should be confirmed with in vivo studies.

Microbiota and cancer: In vitro and in vivo models to evaluate nanomedicines

Nanomedicine implication in cancer treatment and diagnosis studies witness huge attention, especially with the promising results obtained in preclinical studies. Despite this, only few nanomedicines succeeded to pass clinical phase. The human microbiota plays obvious roles in cancer development. Nanoparticles have been successfully used to modulate human microbiota and notably tumor associated microbiota. Taking the microbiota involvement under consideration when testing nanomedicines for cancer treatment might be a way to improve the poor translation from preclinical to clinical trials. Co-culture models of bacteria and cancer cells, as well as animal cancer-microbiota models offer a better representation for the tumor microenvironment and so potentially better platforms to test nanomedicine efficacy in cancer treatment. These models would allow closer representation of human cancer and might smoothen the passage from preclinical to clinical cancer studies for nanomedicine efficacy.

Comparison of Two Hair Removal Methods in Sprague-Dawley Rats (Rattus norvegicus)

Rats commonly undergo surgery for research purposes. However, the effects of different methods of hair removal on wound healing and surgical site infections (SSI) in rats has not been evaluated. The current study evaluated 2 hair removal methods, clipping with an electric clipper and using a depilatory agent, and their effect on wound healing and SSI. Swabs for bacterial culture were obtained on Day 0 just after hair removal, after aseptic skin preparation, and on Days 1 and 3 before conducting skin biopsies to assess bacterial load and recolonization. Full-thickness punch biopsies were taken for histopathologic evaluation on Days 0, 1, 3, 7, and 10. The surgical incisions were assigned an ASEPSIS score on Days 1 and 3. The data revealed that the bacterial load was significantly higher with the depilatory method as compared with the clipper method, but only on Day 1. The histopathologic evaluation found no significant difference in wound healing between the 2 methods. Although the ASEPSIS score was significantly higher for the clipping method than for the depilatory method on Day 1, both techniques were equivalent by Day 3. We conclude that both hair removal methods are safe and efficacious components of aseptic technique in rats.

Polyphosphazenes enable durable, hemocompatible, highly efficient antibacterial coatings

Biocompatible antibacterial coatings are highly desirable to prevent bacterial colonization on a wide range of medical devices from hip implants to skin grafts. Traditional polyelectrolytes are unable to directly form coatings with cationic antibiotics at neutral pH and suffer from high degrees of antibiotic release upon exposure to physiological concentrations of salt. Here, novel inorganic-organic hybrid polymer coatings based on direct layer-by-layer assembly of anionic polyphosphazenes (PPzs) of various degrees of fluorination with cationic antibiotics (polymyxin B, colistin, gentamicin, and neomycin) are reported. The coatings displayed low levels of antibiotic release upon exposure to salt and pH-triggered response of controlled doses of antibiotics. Importantly, coatings remained highly surface active against Escherichia coli and Staphylococcus aureus, even after 30 days of pre-exposure to physiological conditions (bacteria-free) or after repeated bacterial challenge. Moreover, coatings displayed low (<1%) hemolytic activity for both rabbit and porcine blood. Coatings deposited on either hard (Si wafers) or soft (electrospun fiber matrices) materials were non-toxic towards fibroblasts (NIH/3T3) and displayed controllable fibroblast adhesion via PPz fluorination degree. Finally, coatings showed excellent antibacterial activity in ex vivo pig skin studies. Taken together, these results suggest a new avenue to form highly tunable, biocompatible polymer coatings for medical device surfaces.

Ca-Zn-Ag Alginate Aerogels for Wound Healing Applications: Swelling Behavior in Simulated Human Body Fluids and Effect on Macrophages

Chronic non-healing wounds represent a substantial economic burden to healthcare systems and cause a considerable reduction in quality of life for those affected. Approximately 0.5-2% of the population in developed countries are projected to experience a chronic wound in their lifetime, necessitating further developments in the area of wound care materials. The use of aerogels for wound healing applications has increased due to their high exudate absorbency and ability to incorporate therapeutic substances, amongst them trace metals, to promote wound-healing. This study evaluates the swelling behavior of Ca-Zn-Ag-loaded alginate aerogels and their metal release upon incubation in human sweat or wound fluid substitutes. All aerogels show excellent liquid uptake from any of the formulas and high liquid holding capacities. Calcium is only marginally released into the swelling solvents, thus remaining as alginate bridging component aiding the absorption and fast transfer of liquids into the aerogel network. The zinc transfer quota is similar to those observed for common wound dressings in human and animal injury models. With respect to the immune regulatory function of zinc, cell culture studies show a high availability and anti-inflammatory activity of aerogel released Zn-species in RAW 264.7 macrophages. For silver, the balance between antibacterial effectiveness versus cytotoxicity remains a significant challenge for which the alginate aerogels need to be improved in the future. An increased knowledge of the transformations that alginate aerogels undergo in the course of the fabrication as well as during wound fluid exposure is necessary when aiming to create advanced, tissue-compatible aerogel products.

An 'on-demand' photothermal antibiotic release cryogel patch: evaluation of efficacy on an ex vivo model for skin wound infection

A myriad of topical therapies and dressings are available to the clinicians for wound healing skin, but only a very few have shown their effectiveness in promoting wound repair due to challenges in controlling drug release. To address this issue, in this work, a near infrared (NIR)-light activable cryogel based on butyl methacrylate (BuMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) incorporated with reduced graphene oxide (rGO) was fabricated. The obtained cryogel provides the required hydrophilicity beneficial for wound treatment. The excellent photo-thermal properties of rGO allow for heating the cryogel, which results in subsequent swelling of the cryogel (CG) followed by release of the encapsulated drug load, cefepime in our case. Without photothermal activation, no release of payload was observed. The potential of this bandage for wound healing was examined using an ex vivo human skin model infected with Staphylococcus aureus (S. aureus). Apart from the efficacy of the cryogel based wound healing system, our results also suggest that the ex vivo wound model evaluated here provides a rapid and valuable tool to study superficial skin infections in humans and test the efficacy of antimicrobial agents.

A niche in the spotlight: Could external factors critically disturb hair follicle homeostasis and contribute to inflammatory hair follicle diseases?

The anatomy of the hair follicle and the dynamics of its barrier provide a special space for interactions between macromolecules and the underlying tissue. Translocation across the hair follicle epithelium and immune recognition have been confirmed for proteins, nucleic acids, engineered particles, virus particles and others. Tissue responses can be modulated by pro-inflammatory stimuli as demonstrated in penetration and transcutaneous immunization studies. Even under physiological conditions, hair follicle openings are filled with exogenous material ranging from macromolecules, engineered particles to natural particles including diverse communities of microbes. The exposed position of the infundibulum suggests that local inflammatory insults could disturb the finely tuned balance and may trigger downstream responses that initiate or facilitate local outbreaks of inflammatory hair diseases typically occurring in close spatial association with the infundibulum as observed in cicatricial alopecia. The question as to how microbial colonization or deposition of contaminants on the surface of the hair follicle epithelium interact with the barrier status under the influence of individual predisposition, may help us understand local flare-ups of inflammatory hair diseases. Specifically, learning more about skin barrier alterations in the different types of inflammatory hair diseases and cross-talk with exogenous compounds could give new insights in this less explored aspect of hair follicle homeostasis. Such knowledge may not only be used to develop supportive measures to maintain a healthy scalp. It may have wider implications for our understanding on how external factors influence inflammation and immunological responses in the skin.

Ex Vivo Murine Skin Model for B. burgdorferi Biofilm

Borrelia burgdorferi, the causative agent of Lyme disease, has been recently shown to form biofilm structures in vitro and in vivo. Biofilms are tightly clustered microbes characterized as resistant aggregations that allow bacteria to withstand harsh environmental conditions, including the administration of antibiotics. Novel antibiotic combinations have recently been identified for B. burgdorferi in vitro, however, due to prohibiting costs, those agents have not been tested in an environment that can mimic the host tissue. Therefore, researchers cannot evaluate their true effectiveness against B. burgdorferi, especially its biofilm form. A skin ex vivo model system could be ideal for these types of experiments due to its cost effectiveness, reproducibility, and ability to investigate host-microbial interactions. Therefore, the main goal of this study was the establishment of a novel ex vivo murine skin biopsy model for B. burgdorferi biofilm research. Murine skin biopsies were inoculated with B. burgdorferi at various concentrations and cultured in different culture media. Two weeks post-infection, murine skin biopsies were analyzed utilizing immunohistochemical (IHC), reverse transcription PCR (RT-PCR), and various microscopy methods to determine B. burgdorferi presence and forms adopted as well as whether it remained live in the skin tissue explants. Our results showed that murine skin biopsies inoculated with 1 × 107 cells of B. burgdorferi and cultured in BSK-H + 6% rabbit serum media for two weeks yielded not just significant amounts of live B. burgdorferi spirochetes but biofilm forms as well. IHC combined with confocal and atomic force microscopy techniques identified specific biofilm markers and spatial distribution of B. burgdorferi aggregates in the infected skin tissues, confirming that they are indeed biofilms. In the future, this ex vivo skin model can be used to study development and antibiotic susceptibility of B. burgdorferi biofilms in efforts to treat Lyme disease effectively.

Manuka honey microneedles for enhanced wound healing and the prevention and/or treatment of Methicillin-resistant Staphylococcus aureus (MRSA) surgical site infection

Manuka honey (MH) is currently used as a wound treatment and suggested to be effective in Methicillin-resistant Staphylococcus aureus (MRSA) elimination. We sought to optimize the synthesis of MH microneedles (MHMs) while maintaining the MH therapeutic effects. MHMs were synthesized using multiple methods and evaluated with in vitro assays. MHMs demonstrated excellent bactericidal activity against MRSA at concentrations ≥ 10% of honey, with vacuum-prepared honey appearing to be the most bactericidal, killing bacterial concentrations as high as 8 × 10⁷ CFU/mL. The wound-healing assay demonstrated that, at concentrations of 0.1%, while the cooked honey had incomplete wound closure, the vacuum-treated honey trended towards faster wound closure. In this study, we demonstrate that the method of MHM synthesis is crucial to maintaining MH properties. We optimized the synthesis of MHMs and demonstrated their potential utility in the treatment of MRSA infections as well as in wound healing. This is the first report of using MH as a substrate for the formation of dissolvable microneedles. This data supports the need for further exploration of this new approach in a wound-healing model and opens the door for the future use of MH as a component of microneedle scaffolds.

C-Type Lectin Receptors in Host Defense Against Bacterial Pathogens

Antigen-presenting cells (APCs) are present throughout the human body—in tissues, at barrier sites and in the circulation. They are critical for processing external signals to instruct both local and systemic responses toward immune tolerance or immune defense. APCs express an extensive repertoire of pattern-recognition receptors (PRRs) to detect and transduce these signals. C-type lectin receptors (CLRs) comprise a subfamily of PRRs dedicated to sensing glycans, including those expressed by commensal and pathogenic bacteria. This review summarizes recent findings on the recognition of and responses to bacteria by membrane-expressed CLRs on different APC subsets, which are discussed according to the primary site of infection. Many CLR-bacterial interactions promote bacterial clearance, whereas other interactions are exploited by bacteria to enhance their pathogenic potential. The discrimination between protective and virulence-enhancing interactions is essential to understand which interactions to target with new prophylactic or treatment strategies. CLRs are also densely concentrated at APC dendrites that sample the environment across intact barrier sites. This suggests an–as yet–underappreciated role for CLR-mediated recognition of microbiota-produced glycans in maintaining tolerance at barrier sites. In addition to providing a concise overview of identified CLR-bacteria interactions, we discuss the main challenges and potential solutions for the identification of new CLR-bacterial interactions, including those with commensal bacteria, and for in-depth structure-function studies on CLR-bacterial glycan interactions. Finally, we highlight the necessity for more relevant tissue-specific in vitro, in vivo and ex vivo models to develop therapeutic applications in this area.

Biodistribution and intracellular localization of hyaluronan and its nanogels. A strategy to target intracellular S. aureus in persistent skin infections

Intracellular pathogens are a critical challenge for antimicrobial therapies. Staphylococcus aureus (S. aureus) causes approximately 85% of all skin and soft tissue infections in humans worldwide and more than 30% of patients develop chronic or recurrent infections within three months, even after appropriate antibacterial therapies. S. aureus is also one of the most common bacteria found in chronic wounds. Recent evidences suggest that S. aureus is able to persist within phagolysosomes of skin cells (i.e. keratinocytes, phagocytic cells), being protected from both the immune system and a number of antimicrobials. To overcome these limits, nano-formulations that enable targeted therapies against intracellular S. aureus might be developed. Herein, the biodistribution and intracellular localisation of hyaluronan (HA) and HA-based nanoparticles (nanogels, NHs) are investigated, both after intravenous (i.v.) injections (in mice) and topical administrations (in ex vivo human skin). Results indicate HA and NHs accumulate especially in skin and liver of mice after i.v. injection. After topical application on human skin explants, no penetration of both HA and NHs was detected in skin with intact stratum corneum. By contrast, in barrier-disrupted human skin (with partial removal and loosening of stratum corneum), HA and NHs penetrate to the viable epidermis and are taken up by keratinocytes. In mechanically produced wounds (skin without epidermis) they accumulate in wound tissue and are taken up by dermis cells, e.g. fibroblasts and phagocytic cells. Interestingly, in all cases, the cellular uptake is CD44-mediated. In vitro studies confirmed that after CD44-mediated uptake, both HA and NHs accumulate in lysosomes of dermal fibroblasts and macrophages, as previously reported for keratinocytes. Finally, the colocalisation between intracellular S. aureus and HA or NHs is demonstrated, in macrophages. Altogether, for the first time, these results strongly suggest that HA and HA-based NHs can provide a targeted therapy to intracellular S. aureus, in persistent skin or wound infections.

An Engineered Infected Epidermis Model for In Vitro Study of the Skin's Pro-Inflammatory Response

Wound infection is a major clinical challenge that can significantly delay the healing process, can create pain, and requires prolonged hospital stays. Pre-clinical research to evaluate new drugs normally involves animals. However, ethical concerns, cost, and the challenges associated with interspecies variation remain major obstacles. Tissue engineering enables the development of in vitro human skin models for drug testing. However, existing engineered skin models are representative of healthy human skin and its normal functions. This paper presents a functional infected epidermis model that consists of a multilayer epidermis structure formed at an air-liquid interface on a hydrogel matrix and a three-dimensionally (3D) printed vascular-like network. The function of the engineered epidermis is evaluated by the expression of the terminal differentiation marker, filaggrin, and the barrier function of the epidermis model using the electrical resistance and permeability across the epidermal layer. The results showed that the multilayer structure enhances the electrical resistance by 40% and decreased the drug permeation by 16.9% in the epidermis model compared to the monolayer cell culture on gelatin. We infect the model with Escherichia coli to study the inflammatory response of keratinocytes by measuring the expression level of pro-inflammatory cytokines (interleukin 1 beta and tumor necrosis factor alpha). After 24 h of exposure to Escherichia coli, the level of IL-1β and TNF-α in control samples were 125 ± 78 and 920 ± 187 pg/mL respectively, while in infected samples, they were 1429 ± 101 and 2155.5 ± 279 pg/mL respectively. However, in ciprofloxacin-treated samples the levels of IL-1β and TNF-α without significant difference with respect to the control reached to 246 ± 87 and 1141.5 ± 97 pg/mL respectively. The robust fabrication procedure and functionality of this model suggest that the model has great potential for modeling wound infections and drug testing.

Bioengineered Platforms for Chronic Wound Infection Studies: How Can We Make Them More Human-Relevant?

Chronic wound infections are an important cause of delayed wound healing, posing a significant healthcare burden with consequences that include hospitalization, amputation, and death. These infections most often take the form of three-dimensional biofilm communities, which are notoriously recalcitrant to antibiotics and immune clearance, contributing to the chronic wound state. In the chronic wound microenvironment, microbial biofilms interact closely with other key components, including host cellular and matrix elements, immune cells, inflammatory factors, signaling components, and mechanical cues. Intricate relationships between these contributing factors not only orchestrate the development and progression of wound infections but also influence the therapeutic outcome. Current medical treatment for chronic wound infections relies heavily on long-term usage of antibiotics; however, their efficacy and reasons for failure remain uncertain. To develop effective therapeutic approaches, it is essential to better understand the complex pathophysiology of the chronic wound infection microenvironment, including dynamic interactions between various key factors. For this, it is critical to develop bioengineered platforms or model systems that not only include key components of the chronic wound infection microenvironment but also recapitulate interactions between these factors, thereby simulating the infection state. In doing so, these platforms will enable the testing of novel therapeutics, alone and in combinations, providing insights toward composite treatment strategies. In the first section of this review, we discuss the key components and interactions in the chronic wound infection microenvironment, which would be critical to recapitulate in a bioengineered platform. In the next section, we summarize the key features and relevance of current bioengineered chronic wound infection platforms. These are categorized and discussed based on the microenvironmental components included and their ability to recapitulate the architecture, interactions, and outcomes of the infection microenvironment. While these platforms have advanced our understanding of the underlying pathophysiology of chronic wound infections and provided insights into therapeutics, they possess certain insufficiencies that limit their clinical relevance. In the final section, we propose approaches that can be incorporated into these existing model systems or developed into future platforms developed, thus enhancing their biomimetic and translational capabilities, and thereby their human-relevance.

Evaluation of Drug Delivery and Efficacy of Ciprofloxacin-Loaded Povidone Foils and Nanofiber Mats in a Wound-Infection Model Based on Ex Vivo Human Skin

Topical treatment of wound infections is often a challenge due to limited drug availability at the site of infection. Topical drug delivery is an attractive option for reducing systemic side effects, provided that a more selective and sustained local drug delivery is achieved. In this study, a poorly water-soluble antibiotic, ciprofloxacin, was loaded on polyvinylpyrrolidone (PVP)-based foils and nanofiber mats using acetic acid as a solubilizer. Drug delivery kinetics, local toxicity, and antimicrobial activity were tested on an ex vivo wound model based on full-thickness human skin. Wounds of 5 mm in diameter were created on 1.5 × 1.5 cm skin blocks and treated with the investigated materials. While nanofiber mats reached the highest amount of delivered drug after 6 h, foils rapidly achieved a maximum drug concentration and maintained it over 24 h. The treatment had no effect on the overall skin metabolic activity but influenced the wound healing process, as observed using histological analysis. Both delivery systems were efficient in preventing the growth of Pseudomonas aeruginosa biofilms in ex vivo human skin. Interestingly, foils loaded with 500 µg of ciprofloxacin accomplished the complete eradication of biofilm infections with 1 × 109 bacteria/wound. We conclude that antimicrobial-loaded resorbable PVP foils and nanofiber mats are promising delivery systems for the prevention or topical treatment of infected wounds.

Chronic wound microbiome colonization on mouse model following cryogenic preservation

Chronic wound infections are increasingly recognized to be dynamic and polymicrobial in nature, necessitating the development of wound models which reflect the complexities of infection in a non-healing wound. Wound slough isolated from human chronic wounds and transferred to mice was recently shown to create polymicrobial infection in mice, and there is potential this tool may be improved by cryogenic preservation. The purpose of this study was to investigate the application of cryogenic preservation to transferring polymicrobial communities, specifically by quantifying the effects of cryopreservation and wound microbiome transplantation. Slough from an established murine polymicrobial surgical excision model and five patients were subjected to three preservation strategies: refrigeration until infection, freezing in liquid nitrogen, or freezing in liquid nitrogen with glycerol solution prior to infection in individual mice. Four days following inoculation onto mice, wound microbiota were quantified using either culture isolation or by 16s rRNA gene community profiling and quantitative PCR. Cryogenic preservation did not significantly reduce bacterial viability. Reestablished microbial communities were significantly associated with patient of origin as well as host context (i.e., originally preserved from a patient versus mouse infection). Whereas preservation treatment did not significantly shape community composition, the transfers of microbiomes from human to mouse were characterized by reduced diversity and compositional changes. These findings indicated that changes should be expected to occur to community structure after colonization, and that compositional change is likely due to the rapid change in infection context as opposed to preservation strategy. Furthermore, species that were present in higher relative abundance in wound inoculate were more likely to colonize subsequent wounds, and wound inoculate with higher bacterial load established wound communities that were more compositionally similar. Results inform expectations for the complementation of chronic wound in vivo modeling with cryogenic preservation archives.

A tractable, simplified ex vivo human skin model of wound infection

The prevalence of infection in chronic wounds is well documented in the literature but not optimally studied due to the drawbacks of current methodologies. Here, we describe a tractable and simplified ex vivo human skin model of infection that addresses the critical drawbacks of high costs and limited translatability. Wounds were generated from excised abdominal skin from cosmetic procedures and cultured, inoculated with Staphylococcus aureus strain UAMS-1, or under aseptic conditions. After three days, the infected wounds exhibited biofilm formation and significantly impaired reepithelialization compared to the control. Additionally, promigratory and proreparative genes were significantly downregulated, while proinflammatory genes were significantly upregulated, demonstrating molecular characterizations of impaired healing as in chronic wounds. This model allows for a simplified and versatile tool for the study of wound infection and subsequent development of novel therapies.