BackgroundBiofilm formation is a major clinical challenge contributing to treatment failure of periprosthetic joint infection (PJI). Lytic bacteriophages (phages) can target biofilm associated bacteria at localized sites of infection. The aim of this study is to investigate whether combination therapy of phage and vancomycin is capable of clearingStaphylococcus aureusbiofilm-like aggregates formed in human synovial fluid.MethodsIn this study,S. aureusBP043, a PJI clinical isolate was utilized. This strain is a methicillin-resistantS. aureus(MRSA) biofilm-former. Phage Remus, known to infectS. aureus, was selected for the treatment protocol. BP043 was grown as aggregates in human synovial fluid. The characterization ofS. aureusaggregates was assessed for structure and size using scanning electron microscopy (SEM) and flow cytometry, respectively. Moreover, the formed aggregates were subsequently treatedin vitrowith: (a) phage Remus [∼108plaque-forming units (PFU)/ml], (b) vancomycin (500 μg/ml), or (c) phage Remus (∼108PFU/ml) followed by vancomycin (500 μg/ml), for 48 h. Bacterial survival was quantified by enumeration [colony-forming units (CFU)/ml]. The efficacy of phage and vancomycin against BP043 aggregates was assessedin vivoas individual treatments and in combination. Thein vivomodel utilizedGalleria mellonellalarvae which were infected with BP043 aggregates pre-formed in synovial fluid.ResultsScanning electron microscopy (SEM) images and flow cytometry data demonstrated the ability of human synovial fluid to promote formation ofS. aureusaggregates. Treatment with Remus resulted in significant reduction in viableS. aureusresiding within the synovial fluid aggregates compared to the aggregates that did not receive Remus (p< 0.0001). Remus was more efficient in eliminating viable bacteria within the aggregates compared to vancomycin (p< 0.0001). Combination treatment of Remus followed by vancomycin was more efficacious in reducing bacterial load compared to using either Remus or vancomycin alone (p= 0.0023,p< 0.0001, respectively). When testedin vivo, this combination treatment also resulted in the highest survival rate (37%) 96 h post-treatment, compared to untreated larvae (3%;p< 0.0001).ConclusionWe demonstrate that combining phage Remus and vancomycin led to synergistic interaction against MRSA biofilm-like aggregatesin vitroandin vivo.