Osteomyelitis is a devastating disease caused by microbial infection of bone. While the frequency of infection following elective orthopedic surgery is low, rates of reinfection are disturbingly high. Staphylococcus aureus is responsible for the majority of chronic osteomyelitis cases and is often considered to be incurable due to bacterial persistence deep within bone. Unfortunately, there is no consensus on clinical classifications of osteomyelitis and the ensuing treatment algorithm. Given the high patient morbidity, mortality, and economic burden caused by osteomyelitis, it is important to elucidate mechanisms of bone infection to inform novel strategies for prevention and curative treatment. Recent discoveries in this field have identified three distinct reservoirs of bacterial biofilm including: Staphylococcal abscess communities in the local soft tissue and bone marrow, glycocalyx formation on implant hardware and necrotic tissue, and colonization of the osteocyte-lacuno canalicular network (OLCN) of cortical bone. In contrast, S. aureus intracellular persistence in bone cells has not been substantiated in vivo, which challenges this mode of chronic osteomyelitis. There have also been major advances in our understanding of the immune proteome against S. aureus, from clinical studies of serum antibodies and media enriched for newly synthesized antibodies (MENSA), which may provide new opportunities for osteomyelitis diagnosis, prognosis, and vaccine development. Finally, novel therapies such as antimicrobial implant coatings and antibiotic impregnated 3D-printed scaffolds represent promising strategies for preventing and managing this devastating disease. Here, we review these recent advances and highlight translational opportunities towards a cure.

While Staphylococcus aureus osteomyelitis is considered to be incurable, the major bacterial reservoir in live cortical bone has remained unknown. In addition to biofilm bacteria on necrotic tissue and implants, studies have implicated intracellular infection of osteoblasts and osteocytes as a mechanism of chronic osteomyelitis. Thus, we performed the first systematic transmission electron microscopy (TEM) studies to formally define major reservoirs of S. aureus in chronically infected mouse (Balb/c J) long bone tissue. Although rare, evidence of colonized osteoblasts was found. In contrast, we readily observed S. aureus within canaliculi of live cortical bone, which existed as chains of individual cocci and submicron rod-shaped bacteria leading to biofilm formation in osteocyte lacunae. As these observations do not conform to the expectations of S. aureus as non-motile cocci 1.0–1.5 µm in diameter, we also performed immunoelectron microscopy (IEM) following in vivo BrdU labeling to assess the role of bacterial proliferation in canalicular invasion. The results suggest that the deformed bacteria: 1) enter canaliculi via asymmetric binary fission; and 2) migrate toward osteocyte lacunae via proliferation at the leading edge. Additional in vitro studies confirmed S. aureus migration through a 0.5 µm porous membrane. Collectively, these findings define a novel mechanism of bone infection, and provide possible new insight as to why S. aureus implant related infections of bone tissue are so challenging to treat.

Additive manufacturing, also known as 3D printing, has emerged over the past 3 decades as a disruptive technology for rapid prototyping and manufacturing. Vat polymerization, powder bed fusion, material extrusion, and binder jetting are distinct technologies of additive manufacturing, which have been used in a wide variety of fields, including biomedical research and tissue engineering. The ability to print biocompatible, patient-specific geometries with controlled macro- and micropores, and to incorporate cells, drugs and proteins has made 3D-printing ideal for orthopaedic applications, such as bone grafting. Herein, we performed a systematic review examining the fabrication of calcium phosphate (CaP) ceramics by 3D printing, their biocompatibility in vitro, and their bone regenerative potential in vivo, as well as their use in localized delivery of bioactive molecules or cells. Understanding the advantages and limitations of the different 3D printing approaches, CaP materials, and bioactive additives through critical evaluation of in vitro and in vivo evidence of efficacy is essential for developing new classes of bone graft substitutes that can perform as well as autografts and allografts or even surpass the performance of these clinical standards.

Conventional antibiotics fail to successfully treat chronic osteomyelitis, endocarditis, and device-related and airway infections. These recurring infections are associated with the emergence of SCV, which are recalcitrant to conventional antibiotics. Studies have investigated antibiotic therapies to treat SCV-related infections but have had little success, emphasizing the need to identify novel antimicrobial drugs. However, drug discovery is a costly and time-consuming process. An alternative strategy is drug repurposing, which could identify FDA-approved and well-characterized drugs that could have off-label utility in treating SCV. In this study, we adapted a high-throughput AK-based assay to identify 4 FDA-approved drugs, daunorubicin, ketoconazole, rifapentine, and sitafloxacin, which display antimicrobial activity against S. aureus SCV, suggesting an avenue for drug repurposing in order to effectively treat SCV-related infections. Additionally, this screening paradigm can easily be adapted for other drug/chemical libraries to identify compounds bactericidal against SCV.