Second-line drug susceptibility testing of Mycobacterium tuberculosis by MGIT960 system, the microplate colorimetric-based method and the proportion method

NORA MORCILLO; BELÉN IMPERIALE; BEATRÍZ DI GIULIO

Porto, Portugal

Congreso; 30 Congreso Anual de la Sociedad Europea de Micobacteriología; 2009

Sociedad Europea de Micobacteriología (ESM)

The accurate treatment of tuberculosis (TB) cases due to multidrug-resistant and extensively drug-resistant Mycobacterium tuberculosis emphasizes the necessity of new tools for rapid detection of these strains in clinical laboratories. Minimal inhibitory concentrations (MICs) by MGIT960 and the colorimetric microplate method using dyes as MTT or resarzurin (CMM) were determined for the following drugs (µg/ml): amikacin (AMK): 2.0, 4.0, 8.0; kanamycin (KM), capreomycin (CPM), ethionamide (ETH): 2.5, 5.0, 10.0; cycloserine (CS): 15.0; ofloxacin (OFX) and linezolide (LZ): 0.5, 1.0, 2.0; and moxifloxacin (MOX) 0.25, 0.5, 1.0. MICs were performed on 94 clinical isolates. The proportion method on Middlebrook 7H11 (PM) was used as gold standard. Inoculated MGITs were incubated in the instrument for no longer than 21 days. A strain tested by MGIT960 was considered resistant if a positive signal flagged from the drug-containing tube within 5 days of the positive control tube. Microplates of the CMM were incubated for an average of 8 days. Statistical methods were applied to define drug-resistant strains on the basis of the comparison between results obtained by MGIT960 and CMM with the PM. The following critical concentrations were identified (µg/ml): AMK: 4.0; CPM, ETH and KM: 5.0; CS: 30.0; LZ: 1.0; MOX: 0.5; OFX: 2.0. Accuracy of MGIT960 and M-MTT was 100% for AMK, CPM, OFX, MOX and LZ. In this study tubes incubation and positivity detection was manually obtained from the MGIT960 instrument which actually can be adapted to automatically detect both susceptible and resistant strains to each one of the second-line drugs. Results were obtained in less than 10 days for both MGIT960 and CMM. On the other hand CMM, as a complete homemade method, was cheaper but more laborious than MGIT960. Our results showed that both methods could be promissory implemented as a rapid diagnosis tools to detect MDR and XDR-TB cases in clinical practice.