Rapid aggregation of Staphylococcus aureus in synovial fluid is influenced by synovial fluid concentration, viscosity, and fluid dynamics-with evidence of polymer bridging

Early bacterial survival in the post-surgical 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 concentration, composition, and viscosity occur. These changes, along with fluid movement from post-operative joint motion, will 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 synovial fluid polymer, fibrinogen. Following incubation in each simulated post-operative joint condition, the aggregates were imaged using confocal microscopy. Our analysis revealed the formation of two distinct aggregate phenotypes dependent 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 required. We report here that synovial fluid-induced aggregation is influenced by various changes specific to the post-surgical 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 present surface proteins to aggregate under high viscosity conditions, yielding large, globular aggregates

In vitro staphylococcal aggregate morphology and protection from antibiotics is dependent on distinct mechanisms arising from postsurgical joint components and fluid motion

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 in several simulated postsurgical joint conditions. Using fluorescently labeled synovial fluid polymers, we show that aggregation occurs through two distinct mechanisms: direct bridging between S. aureus cells and host fibrinogen, and 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 to immobilized fibrinogen. Super high resolution microscopy imaging revealed that fibrinogen binding to the cell wall is heterogeneously distributed at both the single cell and population level. 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, were 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. <br/

Synovial fluid-induced aggregation occurs across Staphylococcus aureus clinical isolates and is mechanistically independent of attached biofilm formation

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 media, supplementation with either synovial fluid or human serum inhibited bacterial surface attachment over a 24-hour 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, synovial fluid-induced aggregation appears to be widespread among S. aureus strains and mechanistically independent of biofilm formation