Assessing the efficacy of systemic antibiotics for biofilm-associated infection in an ovine model of simulated fracture-related infection.

Introduction: Infection remains a major complication of open fractures, with rates reaching up to 70 % after severe injury. Systemic antibiotics often fail to achieve the therapeutic levels needed to disrupt biofilm at the wound site due to compromised blood flow and systemic dilution. This study investigates the efficacy of systemic antibiotics against Staphylococcus aureus and Pseudomonas aeruginosa monomicrobial biofilms in an ovine model of simulated fracture-related infection (FRI). Methods: An established model of long-bone FRI in the right hind limb of mature Rambouillet sheep was adapted. Local soft tissue trauma was induced, the periosteum was stripped from the tibial surface, and a simulated fracture was created on the bone surface. The site was inoculated with mature biofilm grown on fracture fixation plates. Sheep were assigned to a treatment group receiving 10 d of systemic antibiotic therapy or a positive control group that received no treatment. All animals were sacrificed at 21 d, and microbiological and histological analysis was performed. Results: Systemic antibiotics failed to produce a statistically significant reduction in S. aureus biofilm compared to the positive control. Systemic therapy significantly reduced P. aeruginosa bioburden compared to the positive control, but levels remained above the clinical threshold for infection. The histological analysis revealed moderate improvement from systemic treatment. Conclusions: This investigation established the limitations of systemic antibiotic therapy in this model of long-bone FRI against S. aureus and P. aeruginosa biofilms. Microbiological and histological analyses revealed hallmark features of recalcitrance to systemic treatment, validating the utility of this model to study anti-infective therapies. These findings highlight the need for new antibiotic delivery strategies to manage biofilm-associated infections.