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 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.