BackgroundPost-arthroplasty infections represent a devastating complication of total joint replacement surgery, resulting in multiple reoperations, prolonged antibiotic use, extended disability and worse clinical outcomes. As the number of arthroplasties in the U.S. will exceed 3.8 million surgeries per year by 2030, the number of post-arthroplasty infections is projected to increase to over 266,000 infections annually. The treatment of these infections will exhaust healthcare resources and dramatically increase medical costs.Methodology/Principal FindingsTo evaluate novel preventative therapeutic strategies against post-arthroplasty infections, a mouse model was developed in which a bioluminescent Staphylococcus aureus strain was inoculated into a knee joint containing an orthopaedic implant and advanced in vivo imaging was used to measure the bacterial burden in real-time. Mice inoculated with 5×103 and 5×104 CFUs developed increased bacterial counts with marked swelling of the affected leg, consistent with an acute joint infection. In contrast, mice inoculated with 5×102 CFUs developed a low-grade infection, resembling a more chronic infection. Ex vivo bacterial counts highly correlated with in vivo bioluminescence signals and EGFP-neutrophil fluorescence of LysEGFP mice was used to measure the infection-induced inflammation. Furthermore, biofilm formation on the implants was visualized at 7 and 14 postoperative days by variable-pressure scanning electron microscopy (VP-SEM). Using this model, a minocycline/rifampin-impregnated bioresorbable polymer implant coating was effective in reducing the infection, decreasing inflammation and preventing biofilm formation.Conclusions/SignificanceTaken together, this mouse model may represent an alternative pre-clinical screening tool to evaluate novel in vivo therapeutic strategies before studies in larger animals and in human subjects. Furthermore, the antibiotic-polymer implant coating evaluated in this study was clinically effective, suggesting the potential for this strategy as a therapeutic intervention to combat post-arthroplasty infections.

Post-arthroplasty infections are a devastating problem in orthopaedic surgery. While acute infections can be treated with a single stage washout and liner exchange, chronic infections lead to multiple reoperations, prolonged antibiotic courses, extended disability and worse clinical outcomes. Unlike previous mouse models that studied an acute infection, this work aimed to develop a model of a chronic post-arthroplasty infection. To achieve this, a stainless steel implant in the knee joints of mice was inoculated with a bioluminescent S. aureus strain (1×102–1×104 CFUs) and in vivo imaging was used to monitor the bacterial burden for 42 days. Four different S. aureus stains were compared in which the bioluminescent construct was integrated in an antibiotic selection plasmid (ALC2906), the bacterial chromosome (Xen29 and Xen40) or a stable plasmid (Xen36). ALC2906 had increased bioluminescent signals through day 10, after which the signals became undetectable. In contrast, Xen29, Xen40 and Xen36 had increased bioluminescent signals through 42 days with the highest signals observed with Xen36. ALC2906, Xen29 and Xen40 induced significantly more inflammation than Xen36 as measured by in vivo EGFP-neutrophil florescence of LysEGFP mice. All four strains induced comparable biofilm formation as determined by variable-pressure scanning electron microscopy. Using a titanium implant, Xen36 had higher in vivo bioluminescence signals than Xen40 but had similar biofilm formation and adherent bacteria. In conclusion, Xen29, Xen40 and especially Xen36, which had stable bioluminescence constructs, are feasible for long-term in vivo monitoring of bacterial burden and biofilm formation to study chronic post-arthroplasty infections and potential antimicrobial interventions.

MyD88 is an adapter molecule that is used by both IL-1R and TLR family members to initiate downstream signaling and promote immune responses. Given that IL-1β is induced after S. aureus infections and TLR2 is activated by S. aureus lipopeptides, we hypothesized that IL-1β and TLR2 contribute to MyD88-dependent protective immune responses against post-arthroplasty S. aureus infections. To test this hypothesis, we used a mouse model of a post-arthroplasty S. aureus infection to compare the bacterial burden, biofilm formation and neutrophil recruitment in IL-1β-deficient, TLR2-deficient and wildtype mice. By using in vivo bioluminescence imaging, we found that the bacterial burden in IL-1β-deficient mice was 26-fold higher at 1 day after infection and remained 3- to 10-fold greater than wildtype mice through day 42. In contrast, the bacterial burden in TLR2-deficient mice did not differ from wildtype mice. In addition, implants harvested from IL-1β-deficient mice had more biofilm formation and 14-fold higher adherent bacteria compared with those from wildtype mice. Finally, IL-1β-deficient mice had ~50% decreased neutrophil recruitment to the infected postoperative joints than wildtype mice. Taken together, these findings suggest a mechanism by which IL-1β induces neutrophil recruitment to help control the bacterial burden and the ensuing biofilm formation in a post-surgical joint.