Summary

Tobramycin was demonstrated to be equally effective as vancomycin in bacterial bioburden reduction on allograft tendon and may aid in antibiotic stewardship efforts by avoiding selective pressures imposed by overreliance on vancomycin.

Abstract

Objectives
Anterior cruciate ligament (ACL) reconstruction is one of the most common sports medicine procedures in the United States, with around 100,000 occurring annually. Infection rates are relatively low, but it can be devastating, leading to graft failure or septic joints and subsequent repeat surgery. Vancomycin wrapping was recently introduced to combat infection, which involves wrapping the tendon graft in sterile gauze saturated with vancomycin solution. This has been shown to reduce surgical site infection, though growing concerns surrounding vancomycin resistance indicate value in alternative antibiotic choices for graft wrapping. We hypothesize that tobramycin will be as effective as vancomycin as an antibiotic wrap medication at reducing Staphylococcus epidermidis bacteria on tendon allograft specimens.

Methods

Staphylococcus epidermidis ATC12228, which does not form biofilm, was inoculated onto human gracilis cadaveric tendon. Tendons were submerged in 5 mL of inoculum at 1x106 cfu/mL for 30 minutes. The tendons were then transferred to sterile petri dishes with sterile gauze saturated with 5 mL of either tobramycin or vancomycin at concentration of 1, 2.5, 5, or 10 mg/mL for either 10 or 20 minutes. They were then transferred into 2 mL of sterile Brain Heart Infusion media. This media was vortexed then diluted and plated for colony counting.

Results

ANOVA analysis revealed that both vancomycin (p = 0.0002) and tobramycin (p < 0.0001) reduce bacterial concentration with statistical significance. Longer vancomycin exposure led to decreased bacterial concentration (p = 0.0013), but tobramycin did not demonstrate statistically significant changes with increased treatment time (p = 0.6598). Tobramycin produced a statistically significant reduction in bacterial concentration at concentrations as low as 0.1 mg/mL, and vancomycin produced a statistically significant reduction in bacterial bioburden concentration at 2.5 mg/mL. This data is visualized in figures 1-3.

Conclusion

Tobramycin was demonstrated to be equally effective as vancomycin in bacterial bioburden reduction on allograft tendon. Further studies clarifying biomechanical and cytotoxic effects of tobramycin on tendon and surrounding tissues will solidify it as a useful clinical alternative to vancomycin in ACL reconstruction. While some cytotoxic effects have been reported in the use of tobramycin, achieving effects equivalent to vancomycin with much lower doses of tobramycin may circumvent issues arising from cytotoxicity or biomechanical destabilization. This will aid in antibiotic stewardship efforts by avoiding selective pressures imposed by overreliance on vancomycin.