Antibiotic Selection for Sustained Local Therapy May Be Important to Reduce Pseudomonas aeruginosa Dominance Against Staphylococcus aureus and Bone Response Modulation in an Ovine Polymicrobial Fracture-Related Infection Model

Polymicrobial fracture-related infections (FRIs) underpinned by bacterial biofilms pose therapeutic challenges distinct from those of monomicrobial infection, as interspecies dynamics may alter both antimicrobial response and host tissue injury. This study evaluated systemic and local antibiotic strategies in an ovine FRI model co-inoculated with a polymicrobial Staphylococcus aureus and Pseudomonas aeruginosa biofilm. At 21 days, untreated positive controls retained substantial total bioburden yet paradoxically preserved bone volume and exhibited low osteoclast activity, contrasting with the marked osteolysis previously observed in monomicrobial infection controls using this model. Although untreated controls remained S. aureus predominant, their S. aureus:P. aeruginosa ratio declined from approximately 200:1 at inoculation to 15:1, indicating a progressive ecological shift over time. A systemic levofloxacin plus rifampin regimen more effectively reduced S. aureus than P. aeruginosa. Likewise, all antibiotic-treated groups shifted from an S. aureus-dominant inoculum toward P. aeruginosa predominance, consistent with unequal antimicrobial suppression altering polymicrobial pathogen balance. Single-dose local antibiotic powder and calcium sulfate bead strategies provided inconsistent microbiological benefit and failed to preserve bone. Reloadable local antibiotic delivery reduced S. aureus tissue bioburden and preserved the greatest bone volume among antibiotic-treated groups but the antibiotics selected for delivery did not prevent persistent P. aeruginosa colonization or the shift in pathogen balance. These findings indicate that microbiological burden reduction, residual pathogen composition, and bone preservation may result in distinct treatment outcomes in polymicrobial FRI. This study highlights the need for appropriate antibiotic selection with ecologically informed, dual targeted local and systemic strategies to manage polymicrobial, biofilm-complicated FRIs.