Dr. Alcaide and colleagues have recently reported an evaluation of the BACTEC MGIT 960 (Becton Dickinson) and MB/BacT (Organon Teknika) systems for recovery of mycobacteria from clinical specimens (1). We have recently completed a similar evaluation at the University College Hospital Galway.
Our evaluation was conducted on 493 specimens submitted between November 1999 and February 2000. There were 365 respiratory tract specimens and 128 nonrespiratory tract specimens (including pleural fluid, urine, and cerebrospinal fluid). Sputum samples were predigested with an equal volume of Sputasol (Oxoid) following centrifugation at 3,000 × g for 15 min. A slide was prepared from the sediment for microscopy, and the remaining sediment was decontaminated with 4% aqueous NaOH for 20 min followed by neutralization with 14% KH2PO4. Following centrifugation, the sediment was resuspended in approximately 1 ml of neutralization buffer and 0.5 ml of material was inoculated into each of the two systems. Bronchoalveolar lavage specimens were treated in a similar manner. Urine specimens were concentrated by centrifugation, and the deposit was decontaminated with 2.5 ml of 5% sulfuric acid for 30 min. Other contaminated specimens were decontaminated with 4% NaOH followed by neutralization. Slides for microscopy were stained with Auramine O. Antimicrobials were added to the culture vials prior to inoculation (PANTA for BACTEC MGIT 960 and MAS [MB/BACT antibiotic supplement] for MB/BacT). Cultures were incubated for 6 weeks. From positive cultures, a smear was stained by the Ziehl-Neelsen method. Chocolate agar media were inoculated to check for contamination with bacteria other than mycobacteria.
Isolates of mycobacteria were identified by conventional methods (2, 3). In total, 18 isolates (18 in MB/BacT; 16 in BACTEC MGIT 960) of M. tuberculosis were obtained. The 16 specimens positive in both systems were from the respiratory tract and were positive on microscopy. The two isolates detected only in the MB/BacT system after 28 days of incubation were pleural biopsy and pleural fluid specimens (same patient) and were negative on microscopy. These results are consistent with those of Alcaide et al., indicating no significant difference in the isolation rate of M. tuberculosis between the BACTEC MGIT 960 and the MB/BacT systems (1).
Four MOTT (mycobacteria other than M. tuberculosis) isolates (one M. kansasii and three Mycobacterium avium-M. intracellulare) were detected. The M. kansasii isolate was detected only in the BACTEC MGIT 960 system; the M. avium-M. intracellulare isolates were detected in both systems. The numbers of MOTT isolates are small, and we note that Alcaide et al. found that the MB/BacT was significantly better than the BACTEC MGIT 960 at isolating M. kansasii.
The mean times to detection (TTD) of a positive culture of M. tuberculosis were 8.5 days (range, 4 to 23 days) in the BACTEC MGIT 960 system and 13.4 days (range, 2 to 39 days) in the MB/BacT system. This is consistent with the finding of Alcaide et al. that the mean TTD is significantly shorter in the BACTEC MGIT 960 than in the MB/BacT culture system. In this study, the contamination rate was 8.5% in the BACTEC MGIT 960 system, similar to the results described in previous reports (5). The contamination rate of 25% in the MB/BacT culture system is high relative to the results of Alcaide et al. and others (1, 4) and resulted in contamination with staphylococci or streptococci in 5 of 18 (28%) cultures positive for M. tuberculosis. In our experience, both systems are effective in isolating mycobacteria; however, the BACTEC MGIT 960 system is preferable to the MB/BacT system in particular because of lower contamination rates and also because of a superior mean TTD for M. tuberculosis. Since much of the contamination was due to staphylococci and streptococci, it is possible that the routine addition of vancomycin to the MB/BacT system may reduce the contamination rate.
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