Effect of the Addition of Vancomycin on the Performance of an Automated Nonradioactive System for Detection of Mycobacteria

 A recently developed automated, nonradioactive system for the detection of mycobacteria (MB/BacT; Organon Teknika, Belgium) has provided good results, but the contamination rate was found to be higher than that obtained with the radiometric Bactec 460 system (Becton Dickinson, USA). In the present study, the effects of adding vancomycin (1 μg/ml) to the antibiotic mixture of the nonradioactive system were evaluated, and the performance of the system with versus without vancomycin was compared. Three hundred sputum samples were tested, using the radiometric system as the reference method. Mycobacteria were isolated from 47 (15.7%) samples. The nonradioactive system with and without vancomycin detected 42 and 43 strains, respectively; the time to detection was 1 day shorter with the medium without vancomycin (15.7 days vs. 14.3 days). The radiometric system detected 42 strains of mycobacteria in a mean detection time of 13.6 days. Contamination rates with the nonradioactive system were 6.7% in the medium without vancomycin and 2.7% in the medium with vancomycin. The latter figure was approximately the same as the contamination rate found with the radiometric system (2.3%). Our data suggest that the addition of vancomycin considerably reduces the number of contaminants in the MB/BacT medium without affecting the performance of the system.