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. 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 will 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.