European Society of Mycobacteriology - Instituto Nacional de Saúde ...

OP-25
SECOND-LINE DRUG SUSCEPTIBILITY TESTING OF Mycobacterium
tuberculosis BY MGIT 960 SYSTEM, THE MICROPLATE
COLORIMETRIC-BASED METHOD AND THE PROPORTION METHOD
Nora Morcillo 1 , Belén Imperiale, 1, 2 , Beatriz Di Giulio 3
1 - Reference Laboratory of Tuberculosis Control Program of Buenos Aires Province, Dr. Cetrángolo Hospital Buenos
Aires, Argentina.
2 - National Council of Scientific and Technological Research, Buenos Aires City.
3 - P. V. de Cordero Hospital, San Fernando, Buenos Aires, Argentina.
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.
68 ESM 2009