Synovial Fluid has Antibacterial Effects

Author(s):
Samy Gabriel; Reuben Judd; Michael Bubb

Background:

Septic arthritis is one of the most dreaded and challenging complications of total joint arthroplasty. While external factors are thought to play a large role in post-operative infections, there is a possibility that inherent patient susceptibilities may play a role as well. There are a few reports of the antimicrobial properties of synovial fluid, however the exact mechanism of this property has not been identified.

Hypothesis:

The purpose of this report is to demonstrate the bactericidal effects of human synovial fluid, and attempt to characterize it. We hypothesize that antimicrobial peptides may be responsible for the bactericidal effects of synovial fluid.

Methods:

After IRB approval, synovial fluid was collected from patients who underwent knee aspiration. This fluid was then ultra-centrifuged and cell free supernatant was plated with bacterial suspension and broth in 96 well microtiter plates. Growth was measured by optical density at 30 minute intervals and compared to controls. Growth curves were created to evaluate rate of growth in each solution. This test was repeated using different doses of synovial fluid in the bacterial suspensions to assess the strength of the antibacterial effect. Synovial fluid supernatant was then separated by size exclusion and ion exchange chromatography and the process was repeated to assess the activity of components of synovial fluid. Mass spectrometry was then performed on the fraction that maintained activity.

Results:

All four collected samples of cell free synovial fluid showed decreased optical density (mean=0.117) at 5 hours of growth compare to that of controls (mean= .413, p>0.0001). Subsequent fractionation has led to two peaks of UV absorption, with only one fraction maintaining anti-microbial activity (mean optical density =0.291 vs control mean=0.411, p=0.007). In the two patient samples that were tested using different amounts of synovial fluid in the bacterial suspension, doses of 60, 40, 20, and 10 uL of synovial fluid showed significant antibacterial effect with slight increases of bacterial proliferation as the dose of synovial fluid was decreased from 60 to 10 uL.

Conclusions:

Cell free synovial fluid displays a dose dependent bactericidal effect. After purification, this effect was limited to a single fraction. Based on UV absorption, it is likely that a small peptide present in this fraction is responsible for the observed anti-microbial activity. Mass spectrometry identified dermcidin as a candidate peptide present in the active fraction. Ongoing studies are being performed to further elucidate and confirm the identity of active peptides.