(319a) Mechanisms Contributing to the Formation of "Floating Biofilms" in Staphylococcus Aureus Orthopedic Infections (Invited Talk)

Despite the wide use of antibiotics, breakthrough infections frequently occur after orthopedic surgery. The major pathogen involved with such infections is Staphylococcus aureus. Here, we investigated the mechanisms that contribute to the exceptional recalcitrance of S. aureus to antibiotic treatment during joint infections by an ex-vivo approach using human synovial fluid. We found that even with antibiotic concentrations that far exceed the expected bactericidal levels, S. aureus is not eradicated in synovial fluid (SF).We show that this is due to the fact that in SF, S. aureus forms exceptionally strong biofilm-like aggregates that significantly increase resistance to antibiotic treatment. Screening a transposon bank identified S. aureus fibronectin- and fibrinogen-binding proteins as important for the formation of those macroscopic clumps, suggesting an important role of fibrin-containing clots in the formation of bacterial aggregates during joint infection. While these S. aureus surface proteins are a prerequisite for agglomeration, low activity of the Agr quorum-sensing regulatory system and resulting low production of the phenol-soluble modulin (PSM) surfactant peptides, which function by disrupting interactions of biofilm matrix molecules with the bacterial cell surface, cause agglomerates to grow to exceptional dimensions. Together, our findings support a two-step model of staphylococcal prosthetic joint infection: Interaction of S. aureus surface proteins with host matrix proteins such as fibrin initiates agglomeration; thereafter, the bacterial agglomerates grow to extremely large sizes owing to the specific conditions present in joints. Our findings provide a mechanistic explanation for the reported extreme resistance of joint infection to antibiotic treatment and have important implications for anti-staphylococcal therapeutic strategies.