Biomimetic 3D in vitro model of biofilm triggered osteomyelitis for investigating hematopoiesis during bone marrow infections

In Vitro Models of Bacterial Biofilms: Innovative Tools to Improve Understanding and Treatment of Infections

Bacterial infections are a growing concern to the health care systems. Bacteria in the human body are often found embedded in a dense 3D structure, the biofilm, which makes their eradication even more challenging. Indeed, bacteria in biofilm are protected from external hazards and are more prone to develop antibiotic resistance. Moreover, biofilms are highly heterogeneous, with properties dependent on the bacteria species, the anatomic localization, and the nutrient/flow conditions. Therefore, antibiotic screening and testing would strongly benefit from reliable in vitro models of bacterial biofilms. This review article summarizes the main features of biofilms, with particular focus on parameters affecting biofilm composition and mechanical properties. Moreover, a thorough overview of the in vitro biofilm models recently developed is presented, focusing on both traditional and advanced approaches. Static, dynamic, and microcosm models are described, and their main features, advantages, and disadvantages are compared and discussed.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Rebuilding the hematopoietic stem cell niche: Recent developments and future prospects

Hematopoietic stem cells (HSCs) have proven their clinical relevance in stem cell transplantation to cure patients with hematological disorders. Key to their regenerative potential is their natural microenvironment - their niche - in the bone marrow (BM). Developments in the field of biomaterials enable the recreation of such environments with increasing preciseness in the laboratory. Such artificial niches help to gain a fundamental understanding of the biophysical and biochemical processes underlying the interaction of HSCs with the materials in their environment and the disturbance of this interplay during diseases affecting the BM. Artificial niches also have the potential to multiply HSCs in vitro, to enable the targeted differentiation of HSCs into mature blood cells or to serve as drug-testing platforms. In this review, we will introduce the importance of artificial niches followed by the biology and biophysics of the natural archetype. We will outline how 2D biomaterials can be used to dissect the complexity of the natural niche into individual parameters for fundamental research and how 3D systems evolved from them. We will present commonly used biomaterials for HSC research and their applications. Finally, we will highlight two areas in the field of HSC research, which just started to unlock the possibilities provided by novel biomaterials, in vitro blood production and studying the pathophysiology of the niche in vitro. With these contents, the review aims to give a broad overview of the different biomaterials applied for HSC research and to discuss their potentials, challenges and future directions in the field. STATEMENT OF SIGNIFICANCE: Hematopoietic stem cells (HSCs) are multipotent cells responsible for maintaining the turnover of all blood cells. They are routinely applied to treat patients with hematological diseases. This high clinical relevance explains the necessity of multiplication or differentiation of HSCs in the laboratory, which is hampered by the missing natural microenvironment - the so called niche. Biomaterials offer the possibility to mimic the niche and thus overcome this hurdle. The review introduces the HSC niche in the bone marrow and discusses the utility of biomaterials in creating artificial niches. It outlines how 2D systems evolved into sophisticated 3D platforms, which opened the gateway to applications such as, expansion of clinically relevant HSCs, in vitro blood production, studying niche pathologies and drug testing.

3D Cocultures of Osteoblasts and Staphylococcus aureus on Biomimetic Bone Scaffolds as a Tool to Investigate the Host-Pathogen Interface in Osteomyelitis

Osteomyelitis (OM) is an infectious disease of the bone primarily caused by the opportunistic pathogen Staphylococcus aureus (SA). This Gram-positive bacterium has evolved a number of strategies to evade the immune response and subvert bone homeostasis, yet the underlying mechanisms remain poorly understood. OM has been modeled in vitro to challenge pathogenetic hypotheses in controlled conditions, thus providing guidance and support to animal experimentation. In this regard, traditional 2D models of OM inherently lack the spatial complexity of bone architecture. Three-dimensional models of the disease overcome this limitation; however, they poorly reproduce composition and texture of the natural bone. Here, we developed a new 3D model of OM based on cocultures of SA and murine osteoblastic MC3T3-E1 cells on magnesium-doped hydroxyapatite/collagen I (MgHA/Col) scaffolds that closely recapitulate the bone extracellular matrix. In this model, matrix-dependent effects were observed in proliferation, gene transcription, protein expression, and cell–matrix interactions both of the osteoblastic cell line and of bacterium. Additionally, these had distinct metabolic and gene expression profiles, compared to conventional 2D settings, when grown on MgHA/Col scaffolds in separate monocultures. Our study points to MgHA/Col scaffolds as biocompatible and bioactive matrices and provides a novel and close-to-physiology tool to address the pathogenetic mechanisms of OM at the host–pathogen interface.

Staphylococcus aureus Internalization in Osteoblast Cells: Mechanisms, Interactions and Biochemical Processes. What Did We Learn from Experimental Models?

Bacterial internalization is a strategy that non-intracellular microorganisms use to escape the host immune system and survive inside the human body. Among bacterial species, Staphylococcus aureus showed the ability to interact with and infect osteoblasts, causing osteomyelitis as well as bone and joint infection, while also becoming increasingly resistant to antibiotic therapy and a reservoir of bacteria that can make the infection difficult to cure. Despite being a serious issue in orthopedic surgery, little is known about the mechanisms that allow bacteria to enter and survive inside the osteoblasts, due to the lack of consistent experimental models. In this review, we describe the current knowledge about S. aureus internalization mechanisms and various aspects of the interaction between bacteria and osteoblasts (e.g., best experimental conditions, bacteria-induced damages and immune system response), focusing on studies performed using the MG-63 osteoblastic cell line, the best traditional (2D) model for the study of this phenomenon to date. At the same time, as it has been widely demonstrated that 2D culture systems are not completely indicative of the dynamic environment in vivo, and more recent 3D models—representative of bone infection—have also been investigated.

Best served small: nano battles in the war against wound biofilm infections

The global challenge of antimicrobial resistance is of increasing concern, and alternatives to currently used antibiotics or methods to improve their stewardship are sought worldwide. Microbial biofilms, complex 3D communities of bacteria and/or fungi, are difficult to treat with antibiotics for several reasons. These include their protective coats of extracellular matrix proteins which are difficult for antibiotics to penetrate. Nanoparticles (NP) are one way to rise to this challenge; whilst they exist in many forms naturally there has been a profusion in synthesis of these small (<100 nm) particles for biomedical applications. Their small size allows them to penetrate the biofilm matrix, and as well as some NP being inherently antimicrobial, they also can be modified by doping with antimicrobial payloads or coated to increase their effectiveness. This mini-review examines the current role of NP in treating wound biofilms and the rise in multifunctionality of NP.

Enhanced eradication of intracellular and biofilm-residing methicillin-resistant Staphylococcus aureus (MRSA) reservoirs with hybrid nanoparticles delivering rifampicin

Osteomyelitis carries a high risk of recurrence even after extended, aggressive antibiotic therapy. One of the key challenges is to eradicate the reservoirs of methicillin-resistant Staphylococcus aureus (MRSA) inside the host bone cells and their biofilms. Our goal is to develop rifampicin loaded lipid-polymer hybrid nanocarriers (Rf-LPN) and evaluate if they can achieve enhanced rifampicin delivery to eradicate these intracellular and biofilm-residing MRSA. After optimization of the composition, Rf-LPN demonstrated size around 110 nm in diameter that remained stable in serum-supplemented medium, drug payload up to 11.7% and sustained rifampicin release for 2 weeks. When comparing Rf-LPN with free rifampicin, moderate but significant (p < 0.05) improvement of the activities against three osteomyelitis-causing bacteria (USA300-0114, CDC-587, RP-62A) in planktonic form were observed. In comparison, the enhancements in the activities against the biofilms and intracellular MRSA by Rf-LPN were even more substantial. The MBEC50 values against USA300-0114, CDC-587, and RP-62A were 42 vs 155, 70 vs 388, and 265 ng/ml vs over 400 ng/ml, respectively, and up to 18.5-fold reduction in the intracellular MRSA counts in osteoblasts was obtained. Confocal microscope images confirmed extensive accumulation of Rf-LPN inside the biofilm matrix and MRSA-infected osteoblasts. Overall, in this proof-of-concept study we have developed and validated the strategy to exploit the nanoparticle-cell and nanoparticle-biofilm interactions with a new rifampicin nanoformulation for prevention of osteomyelitis recurrence and chronicity caused by the elusive MRSA.

Current Concepts of Osteomyelitis: From Pathologic Mechanisms to Advanced Research Methods

Osteomyelitis is an inflammation of the bone and bone marrow that is most commonly caused by a Staphylococcus aureus infection. Much of our understanding of the underlying pathophysiology of osteomyelitis, from the perspective of both host and pathogen, has been revised in recent years, with notable discoveries including the role played by osteocytes in the recruitment of immune cells, the invasion and persistence of S. aureus in submicron channels of cortical bone, and the diagnostic role of polymorphonuclear cells in implant-associated osteomyelitis. Advanced in vitro cell culture models, such as ex vivo culture models or organoids, have also been developed over the past decade, and have become widespread in many fields, including infectious diseases. These models better mimic the in vivo environment, allow the use of human cells, and can reduce our reliance on animals in osteomyelitis research. In this review, we provide an overview of the main pathologic concepts in osteomyelitis, with a focus on the new discoveries in recent years. Furthermore, we outline the value of modern in vitro cell culture techniques, with a focus on their current application to infectious diseases and osteomyelitis in particular.

Preparation, in vitro release and antibacterial activity evaluation of rifampicin and moxifloxacin-loaded poly(D,L-lactide-co-glycolide) microspheres

Osteomyelitis is difficult to treat because infective bone is poorly accessible for intravenously administering antibiotics and biofilm formation increases bacterial resistance. In this study, microspheres prepared using poly(lactide-co-glycolide) (PLGA) and embedded with moxifloxacin (MOX-PLGA microspheres) and rifampicin/moxifloxacin (RIF/MOX-PLGA microspheres) using the water-in-oil-in-water double emulsion solvent evaporation technique were used for local delivery. Shape of MOX-PLGA microspheres and RIF/MOX-PLGA microspheres were spherical, mean particle size of them were 20.52 μm and 16.62 μm, respectively. Encapsulation efficiency of the MOX-PLGA microspheres was 17.35% ± 2.42%. However, the encapsulation efficiency for MOX and RIF in RIF/MOX-PLGA microspheres was 33.25% ± 7.51% and 49.0% ± 11.25%, respectively. Moxifloxacin and rifampicin were released slowly from microspheres. Both microspheres can efficiently release antibiotics in vitro. Antibacterial and bacterial biofilm-inhibition properties of the released solution were investigated from RIF/MOX-PLGA, MOX-PLGA, and blank PLGA microspheres at varying time points in vitro. RIF/MOX-PLGA microspheres demonstrated the strongest antibacterial activity and bacterial biofilm-inhibition property than the other two microspheres (p < .05). This study suggests that the novel RIF/MOX-PLGA microspheres can be used as a promising carrier for osteomyelitis treatment.