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.

Osteomyelitis is an infection of bone that can result from contiguous spread from surrounding tissue, direct bone trauma due to surgery or injury, or haematogenous spread from systemic bacteraemia. It remains a significant health-care burden with a prevalence of ~22 cases per 100,000 person-years in the United States, and its incidence has been rising over time, especially in the elderly and individuals with diabetes 1 . Although it is a heterogeneous disease, subset classifications include implant-associated osteomyelitis (including peri-prosthetic joint infection (PJI) and instrumented spinal infections), fracture-related infection, acute haematogenous osteomyelitis, diabetic foot infection, septic arthritis and native spinal osteomyelitis.Crucial to expanding our understanding of osteomyelitis and advancing treatment algorithms has been the application of animal models, which illustrate the interaction between the pathogen and cells of both the immune and skeletal systems in a manner that in vitro models cannot yet replicate. Animal models are available to study virtually all aspects of skeletal infection, and typically involve inoculation of bacteria at the time of implant placement (Fig. 1). They can vary in complexity from simple models where metal implants are placed under the skin (for example, tissue cage 2 ) or into cortical bone (for example, metal wire 3 ) versus more complex models that mimic functional orthopaedic devices 4 . Additionally, approaches have been developed to induce non-implant infections by haematogenous inoculation into the tail vein 5 , direct inoculation into vertebral bodies or intervertebral discs 6 to induce vertebral osteomyelitis, or inoculation into the foot pad of diabetic obese rodents to induce diabetic foot infection 7 .As disease pathogenesis differs across different infection classes, so does microbial aetiology. Many different microorganisms have been implicated in skeletal infection, and the most common, along with their incidence and tropism, are shown in Table 1. In general, Staphylococcus aureus and coagulase-negative staphylococci (CoNS), such as Staphylococcus epidermidis and Staphylococcus lugdunensis, are responsible for up to two-thirds of all skeletal infections, with S. aureus being the most prevalent single pathogen. Additionally, antimicrobial resistance remains a challenge in osteomyelitis treatment with up to 50% of cases of S. aureus osteomyelitis caused by methicillin-resistant S. aureus (MRSA) strains 8 . Other less commonly identified pathogens include Enterococcus spp., Pseudomonas aeruginosa, Escherichia coli and Cutibacterium acnes (Table 1). Most cases of osteomyelitis are monomicrobial; however,

S. aureus biofilm creates a favorable environment that increases antibiotic resistance, impairs host immunity, and increases tolerance to nutritional deprivation. Secreted proteins from bacterial cells within the biofilm and the quorum-sensing agr system contribute to immune evasion. Additional immunoevasive properties of S. aureus include the formation of staphylococcal abscess communities (SACs) and canalicular invasion. Novel approaches to target biofilm and increase resistance to implant colonization include novel antibiotic therapy, immunotherapy, and local implant treatments. Challenges remain given the diverse mechanisms developed by S. aureus to alter the host immune responses. Further understanding of these processes should provide novel therapeutic mechanisms to enhance eradication after PJI.

PURPOSE OF REVIEW:Staphylococcus aureus is the primary pathogen responsible for osteomyelitis, which remains a major healthcare burden. To understand its dominance, here we review the unique pathogenic mechanisms utilized by S. aureus, that enable it to cause incurable osteomyelitis.RECENT FINDINGS: Using an arsenal of toxins and virulence proteins, S. aureus kills and usurps immune cells during infection, to produce non-neutralizing pathogenic antibodies that thwart adaptive immunity. S. aureus also has specific mechanisms for distinct biofilm formation on implants, necrotic bone tissue, bone marrow and within the osteocyte-lacuno canicular networks (OLCN) of live bone. In vitro studies have also demonstrated potential for intracellular colonization of osteocytes, osteoblasts and osteoclasts. SUMMARY: S. aureus has evolved a multitude of virulence mechanisms to achieve life-long infection of bone, most notably colonization of OLCN. Targeting S. aureus proteins involved in these pathways could provide new targets for antibiotics and immunotherapies.

Staphylococcus aureus infection of bone is challenging to treat because it colonizes the osteocyte lacuno-canalicular network (OLCN) of cortical bone. To elucidate factors involved in OLCN invasion and identify novel drug targets, we completed a hypothesis-driven screen of 24 S. aureus transposon insertion mutant strains for their ability to propagate through 0.5 μm-sized pores in the Microfluidic Silicon Membrane Canalicular Arrays (μSiM-CA), developed to model S. aureus invasion of the OLCN. This screen identified the uncanonical S. aureus transpeptidase, penicillin binding protein 4 (PBP4), as a necessary gene for S. aureus deformation and propagation through nanopores. In vivo studies revealed that Δpbp4 infected tibiae treated with vancomycin showed a significant 12-fold reduction in bacterial load compared to WT infected tibiae treated with vancomycin (p<0.05). Additionally, Δpbp4 infected tibiae displayed a remarkable decrease in pathogenic bone-loss at the implant site with and without vancomycin therapy. Most importantly, Δpbp4 S. aureus failed to invade and colonize the OLCN despite high bacterial loads on the implant and in adjacent tissues. Together, these results demonstrate that PBP4 is required for S. aureus colonization of the OLCN and suggest that inhibitors may be synergistic with standard of care antibiotics ineffective against bacteria within the OLCN.