Drug susceptibility testing of Mycobacterium tuberculosis against second-line drugs using the Bactec MGIT 960 System

OBJECTIVE: To establish the critical test concentrations for seven second-line anti-tuberculosis drugs in the Bactec Mycobacterial Growth Indicator Tube (MGIT) 960 TB system and to evaluate its efficacy compared to the Bactec 460 TB system. DESIGN: This study was carried out in three phases. In Phase I, pan-susceptible strains were tested to establish the minimum Inhibitory concentration; in Phase II, mostly resistant strains were tested to determine a critical test concentration; and in Phase III, actual clinical isolates were tested to validate the optimal critical concentrations established ill Phases I and II. RESULTS: The critical concentrations established for seven second-line drugs with the Bactec MGIT 960 system arc amikacin 1.0 mu g/ml, capreomycin 2.5 mu g/ml, kanamycin 2.5 mu g/ml, ofloxacin 2.0 mu g/ml, moxifloxacin 1.0 mu g/ml, ethionamide 5.0 mu g/ml and para-amino salicylic acid 4.0 mu g/ml. CONCLUSION: The Bactec MGIT 960 System is an accurate and reliable method for rapid drug susceptibility testing (DST) of Mycobacterium tuberculosis against second-line drugs. In the present study, few of the strains were resistant to fluoroquinolones and further DST for this group Is required.