Summary
tobramycin is as effective as vancomycin in bacterial concentration reduction but is able to achieve this level of reduction at lower doses.
Abstract
Background
Approximately 100,000 Anterior Cruciate Ligament (ACL) reconstructions occur annually in the United States. Post-operative surgical site infection is relatively rare but devastating complication often leading to graft failure or septic arthritis of the knee necessitating repeat surgery. Staphylococcus epidermidis is a common pathogen in allograft contamination. Wrapping allografts in vancomycin-soaked gauze has been adopted as a common sterilization technique in the operating room to reduce surgical site infection. However, there has been an emergence of vancomycin resistant strains of Staphylococcus aureus and Staphylococcus epidermidis that has made finding effective alternatives to vancomycin very important. We hypothesize that tobramycin will be as effective as vancomycin in reducing the concentrations of Staphylococcus epidermidis bacteria on tendon allografts.
Methods
Staphylococcus epidermidis strain ATC12228 was inoculated onto human gracilis cadaveric tendon at a concentration of ~1x106 CFU/mL for 30 minutes. The tendons were transferred to sterile petri dishes and wrapped in sterile gauze saturated with 5 mL of tobramycin or vancomycin at concentrations of 1, 2.5, 5, or 10 mg/mL for either 10 or 20 minutes. Tobramycin was further tested at concentration of 0.1 mg/mL. Inoculated allografts were then transferred into sterile Brain Heart Infusion (BHI) media and vortexed for 1 minute, serially diluted, and plated for colony counting. Statistical analysis was performed utilizing 2-way ANOVA testing. Results were considered statistically significant when p < 0.05.
Results
Vancomycin (p = 0.0002) and tobramycin (p < 0.0001) reduce bacterial concentration. Longer exposure led to decreased bacterial concentration with vancomycin (p = 0.0013). Tobramycin was found to produce a statistically significant reduction in bacterial concentration at concentrations as low as 0.1 mg/mL (p < 0.0001 and p = 0.0029 at 10 and 20 minutes), while vancomycin produced a statistically significant reduction at a concentration as low as 2.5 mg/mL (p < 0.0001 at both 10 and 20 minutes).
Conclusion
This study demonstrates that tobramycin is as effective as vancomycin in bacterial concentration reduction but is able to achieve this level of reduction at lower doses. Further studies clarifying biomechanical and cytotoxic effects of tobramycin on the allograft tendon are indicated to solidify its use as a clinical alternative to vancomycin in ACL reconstruction. This will aid in increasing antibiotic stewardship efforts by avoiding selective pressures imposed by overuse of one antibiotic.