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Considerable progress has been made toward elucidating the mechanism of Staphylococcus aureus aggregation in synovial fluid. In this study, aggregate morphology was assessed following incubation under several simulated postsurgical joint conditions. Using fluorescently labeled synovial fluid polymers, we show that aggregation occurs through two distinct mechanisms: (i) direct bridging between S. aureus cells and host fibrinogen and (ii) an entropy-driven depletion mechanism facilitated by hyaluronic acid and albumin. By screening surface adhesin-deficient mutants ( clfA , clfB , fnbB , and fnbA ), we identified the primary genetic determinant of aggregation in synovial fluid to be clumping factor A. To characterize this bridging interaction, we employed an atomic force microscopy-based approach to quantify the binding affinity of either wild-type S. aureus or the adhesin mutant to immobilized fibrinogen. Surprisingly, we found there to be cell-to-cell variability in the binding strength of the bacteria for immobilized fibrinogen. Superhigh-resolution microscopy imaging revealed that fibrinogen binding to the cell wall is heterogeneously distributed at both the single cell and population levels. Finally, we assessed the antibiotic tolerance of various aggregate morphologies arising from newly deciphered mechanisms of polymer-mediated synovial fluid-induced aggregation. The formation of macroscopic aggregates under shear was highly tolerant of gentamicin, while smaller aggregates, formed under static conditions, were susceptible. We hypothesize that aggregate formation in the joint cavity, in combination with shear, is mediated by both polymer-mediated aggregation mechanisms, with depletion forces enhancing the stability of essential bridging interactions. IMPORTANCE The formation of a bacterial biofilm in the postsurgical joint environment significantly complicates the resolution of an infection. To form a resilient biofilm, incoming bacteria must first survive the initial invasion of the joint space. We previously found that synovial fluid induces the formation of Staphylococcus aureus aggregates, which may provide rapid protection during the early stages of infection. The state of the host joint environment, including the presence of fluid flow and fluctuating abundance of synovial fluid polymers, determines the rate and size of aggregate formation. By expanding on our knowledge of the mechanism and pathogenic implications of synovial fluid-induced aggregation, we hope to contribute insights for the development of novel methods of prevention and therapeutic intervention.

Early bacterial survival in the postsurgical joint is still a mystery. Recently, synovial fluid-induced aggregation was proposed as a potential mechanism of bacterial protection upon entry into the joint. As synovial fluid is secreted back into the joint cavity following surgery, rapid fluctuations in synovial fluid concentrations, composition, and viscosity occur. These changes, along with fluid movement resulting from postoperative joint motion, modify the environment and potentially affect the kinetics of aggregate formation. Through this work, we sought to evaluate the influence of exposure time, synovial fluid concentration, viscosity, and fluid dynamics on aggregation. Furthermore, we aimed to elucidate the primary mechanism of aggregate formation by assessing the interaction of bacterial adhesins with the synovial fluid polymer fibrinogen. Following incubation under each simulated postoperative joint condition, the aggregates were imaged using confocal microscopy. Our analysis revealed the formation of two distinct aggregate phenotypes, depending on whether the incubation was conducted under static or dynamic conditions. Using a surface adhesin mutant, we have narrowed down the genetic determinants for synovial fluid aggregate formation and identified essential host polymers. We report here that synovial fluid-induced aggregation is influenced by various changes specific to the postsurgical joint environment. While we now have evidence that select synovial fluid polymers facilitate bridging aggregation through essential bacterial adhesins, we suspect that their utility is limited by the increasing viscosity under static conditions. Furthermore, dynamic fluid movement recovers the ability of the bacteria with surface proteins present to aggregate under high-viscosity conditions, yielding large, globular aggregates. IMPORTANCE Infection is a major complication of knee and hip joint replacement surgery, which is used to treat arthritis or joint damage. We have shown that Staphylococcus aureus, a common bacterial pathogen, aggregates upon contact with synovial fluid. Within seconds, the bacterial cells interact with synovial fluid polymers in the joint fluid through their cell wall adhesins. The rapid formation of these aggregates likely aids in early bacterial survival in the joint, potentially contributing to the likelihood of developing an infection. By strengthening our basic understanding of the mechanics of synovial fluid aggregate formation under clinically relevant conditions, we hope to expand the knowledge of how to prevent or disrupt aggregation and reduce and more successfully treat these joint infections.

Rapid synovial fluid-induced aggregation of Staphylococcus aureus is currently being investigated as an important factor in the establishment of periprosthetic joint infections (PJIs). Pathogenic advantages of aggregate formation have been well documented in vitro , including recalcitrance to antibiotics and protection from host immune defenses. The objective of the present work was to determine the strain dependency of synovial fluid-induced aggregation by measuring the degree of aggregation of 21 clinical S. aureus isolates cultured from either PJI or bloodstream infections using imaging and flow cytometry. Furthermore, by measuring attached bacterial biomass using a conventional crystal violet assay, we assessed whether there is a correlation between the aggregative phenotype and surface-associated biofilm formation. While all of the isolates were stimulated to aggregate upon exposure to bovine synovial fluid (BSF) and human serum (HS), the extent of aggregation was highly variable between individual strains. Interestingly, the PJI isolates aggregated significantly more upon BSF exposure than those isolated from bloodstream infections. While we were able to stimulate biofilm formation with all of the isolates in growth medium, supplementation with either synovial fluid or human serum inhibited bacterial surface attachment over a 24 h incubation. Surprisingly, there was no correlation between the degree of synovial fluid-induced aggregation and quantity of surface-associated biofilm as measured by a conventional biofilm assay without host fluid supplementation. Taken together, our findings suggest that synovial fluid-induced aggregation appears to be widespread among S. aureus strains and mechanistically independent of biofilm formation. IMPORTANCE Bacterial infections of hip and knee implants are rare but devastating complications of orthopedic surgery. Despite a widespread appreciation of the considerable financial, physical, and emotional burden associated with the development of a prosthetic joint infection, the establishment of bacteria in the synovial joint remains poorly understood. It has been shown that immediately upon exposure to synovial fluid, the viscous fluid in the joint, Staphylococcus aureus rapidly forms aggregates which are resistant to antibiotics and host immune cell clearance. The bacterial virulence associated with aggregate formation is likely a step in the establishment of prosthetic joint infection, and as such, it has the potential to be a potent target of prevention. We hope that this work contributes to the future development of therapeutics targeting synovial fluid-induced aggregation to better prevent and treat these infections.