Staats, Amelia M., Burback, Peter W., Casillas-Ituarte, Nadia N., Li, Daniel, Hostetler, Michaela R., Sullivan, Anne, Horswill, Alexander R., Lower, Steven K. and Stoodley, Paul (2023) In vitro staphylococcal aggregate morphology and protection from antibiotics is dependent on distinct mechanisms arising from postsurgical joint components and fluid motion. Journal of Bacteriology. (In Press)
Abstract
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.
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