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Osteomyelitis is an inflammatory bone disease caused by an infecting microorganism leading to a gradual bone loss. Due to the difficulty in studying osteomyelitis directly in patients, animal models allow researchers to investigate the pathogenesis of the infection and the development of novel prophylactic, anti-inflammatory and antimicrobial treatment strategies. This review is specifically focused on the in vivo mouse osteomyelitis studies available in literature. Thus, a systematic search on Web of Science and PubMed was conducted using the query “(infection) AND (mice OR mouse OR murine) AND (model OR models) AND (arthroplasty OR fracture OR (internal fixator) OR (internal fixation OR prosthesis OR implant OR osteomyelitis)”. After critical assessment of the studies according to the inclusion and exclusion criteria, 135 studies were included in the detailed analysis. Based on the model characteristics, the studies were classified into five subject groups: haematogenous osteomyelitis, post-traumatic osteomyelitis, bone-implant-related infection, peri-prosthetic joint infection, fracture-related infection. In addition, the characteristics of the mice used, such as inbred strain, age or gender, the characteristics of the pathogens used, the inoculation methods, the type of anaesthesia and analgesia used during surgery and the procedures for evaluating the pathogenicity of the infecting micro-organism were described. Overall, the mouse is an excellent first step in vivo model to study the pathogenesis, inflammation and healing process of osteomyelitis and to evaluate novel prophylaxis and treatment strategies.

A fracture-related infection (FRI) is a serious complication that can occur after surgical fixation of bone fractures. Affected patients may encounter delayed healing and functional limitations. Although it is well established that Staphylococcus aureus (S. aureus) is the main causative pathogen of an FRI, the pathophysiology of an S. aureus-induced FRI is not well characterised over time. Therefore, an experimental study in mice comparing S. aureus-inoculated and non-inoculated groups was performed that particularly focused on staphylococcal abscess communities (SACs) and host cellular response. C57Bl/6N female mice received a double osteotomy of the femur, which was stabilised using a titanium 6-hole MouseFix locking plate and four screws. Animals were either S. aureus-inoculated or non-inoculated and euthanised between 1 and 28 d post-surgery. Histopathological evaluation showed normal bone healing for non-inoculated mice, whereas inoculated mice had no fracture consolidation and severe osteolysis. Within the bone marrow of inoculated mice, SACs were observed from 7 d, which increased in size and number over time. A fibrin pseudocapsule enclosed the SACs, which were surrounded by many Ly6G+ neutrophils with some Ly6C+ monocytes and F4/80+ macrophages, the majority of which were viable. The abscesses were encapsulated by fibrin(ogen), collagen and myofibroblasts, with regulatory T cells and M2 macrophages at the periphery. Only bone marrow monocytes and neutrophils of inoculated mice displayed functional suppression of T cells, indicative of myeloid-derived suppressor cells. The present study revealed that an FRI in mice is persistent over time and associated with osteolysis, SAC formation and an immunosuppressive environment.

Biomaterial implants and devices increase the risk of microbial infections due to the biofilm mode of growth of infecting bacteria on implant materials, in which bacteria are protected against antibiotic treatment and the local immune system. Matrix-metalloproteinases (MMPs) and cell surface integrin receptors facilitate transmigration of inflammatory cells toward infected or inflamed tissue. This study investigates the relationship between MMP- and integrin-expression and the clearance of infecting Staphylococcus aureus around implanted biomaterials in a murine model.MMP- and integrin αvβ3-expression were monitored in mice, with and without subcutaneously implanted biomaterial samples, in the absence and presence of bioluminescent S. aureus Xen36. Staphylococcal persistence was imaged longitudinally over time using bioluminescence imaging. The activatable MMPSense®680 and integrin-targeted IntegriSense®750 probes were injected on different days after implantation and their signal intensity and localisation monitored using fluorescence imaging. After sacrifice 7 or 16 days post-implantation, staphylococci from biomaterial samples and surrounding tissues were cultured on agar-plates and presence of host inflammatory cells was histologically evaluated.MMP- and integrin-expression were equally enhanced in presence of staphylococci or biomaterials up to 7 days post-implantation, but their localisation along the biomaterial samples differed. Bacterial clearance from tissue was higher in the absence of biomaterials. It is of clinical relevance that MMP- and integrin-expression were enhanced in presence of both staphylococci and biomaterials, although the immune system in the presence of biomaterials remained hampered in eradicating bacteria during the first 7 days post-implantation.