Elucidating the Mechanisms of S. aureus Motility in Bone and Developing Interventions

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Staphylococcus aureus is involved in 80% of all musculoskeletal infections (MSKI) costing $17,000–$150,000 per patient. Approximately 50% of these infections are caused by methicillin-resistant S. aureus (MRSA) acquired in both hospital and community. With >1.5 million total joint replacements (TJR) performed each year, the most rigorous prophylaxis and aseptic surgical techniques cannot reduce osteomyelitis (OM) rates below 0.5%–2%. Treating established MSKI remains extremely challenging, with current rates of recurrent or persistent infection following revision surgery still as high as 33%. The persistence of S. aureus infection is attributed to its arsenal of immune evasion and antimicrobial resistance mechanisms. Despite great efforts to develop solutions, treatment paradigms have not improved the poor clinical outcomes for OM patients over the last four decades. However, our CoRTOBI paradigm-shifting discovery of S. aureus colonization of the osteocyte lacuno-canalicular network (OLCN) of live cortical bone during OM in mice and patients may explain why previous approaches for treating recurring bone infections have failed, and provide a new therapeutic strategy for eliminating chronic OM. It also begs important questions about the mechanisms that: 1) enable spherical S. aureus to deform into submicron-rod shaped bacteria to invade the OLCN, and 2) render susceptible S. aureus strains refractory to antibiotics after OLCN invasion. Over the past four years we developed a novel bone infection-on-chip utilizing silicon nanomembrane with submicron (~500 nm) array of pores to simulate OLCN orifices (µSiM-CA). By targeted deletion of candidate genes, we identified cell wall transpeptidase proteins, penicillin binding protein 4 (Pbp4), as essential for S. aureus propagation through submicron channels of the µSiM-CA chips in vitro and then demonstrated that they inhibit OLCN colonization in vivo. Moreover, we developed and performed a high throughput screening campaign to identify PBP4 inhibitors (iPBP4). In this renewal, we will first demonstrate the efficacy of PBP4 small molecule inhibitors (iPBP4) in abrogating the OLCN invasion in mouse models of osteomyelitis. We will then identify targets for OM therapy based on gene expression changes that affords S. aureus adaptive tolerance to antibiotics in a novel µSiM- OLCN Chip platform. Finally, we will test the premise that OLCN colonization likely involves many additional factors other than PBP4, and that other chemical classes of OLCN colonization inhibitors can be identified by empirically defining the genetic determinants. These potential targets can then be used to identify corresponding putative therapeutics in a single screening approach. At the completion of this renewal program, CoRTOBI will have: 1) validated recently discovered iPBP4 candidates and potentially new PBP-independent hits against OLCN colonization, 2) a molecular genetic understanding of S. aureus refractory response to antibiotics following OLCN colonization, and 3) translational methods for iPBP4 impregnated 3D-printed scaffolds in one-stage revision surgery for bone infections.