Evaluation of the BBL MGIT (Mycobacterial Growth Indicator Tube) AST SIRE System for Antimycobacterial Susceptibility Testing of Mycobacterium tuberculosis to 4 Primary Antituberculous Drugs

Abstract

Objective.—To evaluate the performance of the BBL MGIT (Mycobacterial Growth Indicator Tube) AST SIRE system for the antimycobacterial susceptibility testing of Mycobacterium tuberculosis to isoniazid (at a concentration equivalent to the lower concentration used for testing by the method of proportion), rifampin, ethambutol, and streptomycin. Design.—Thirty-one clinical isolates and 30 challenge strains provided by the Centers for Disease Control and Prevention (CDC) were tested by MGIT AST SIRE using 2 methods of inoculum preparation, and results were compared with those of the method of proportion, which was considered the reference method. Clinical isolates for which the results of the 2 methods were discordant also were tested at 2 reference laboratories. Results.—Based on data from our site and the reference laboratories, agreement rates between initial MGIT AST SIRE results and the method of proportion for the clinical isolates with the inoculum prepared from a McFarland equivalent and from a positive MGIT tube, respectively, were 100% and 96.8% for isoniazid, 100% and 100% for rifampin, 96.8% and 100% for ethambutol, and 100% and 100% for streptomycin, excluding the isolate for which the discordant streptomycin result could not be resolved. For the 30 challenge isolates, agreement rates between MGIT AST SIRE and expected results and between method of proportion and expected results, respectively, were 96.7% and 93.3% for isoniazid, 93.3% and 100% for rifampin, 83.3% and 100% for ethambutol, and 93.3% and 100% for streptomycin. For the clinical isolates, the mean time to an MGIT AST SIRE result of susceptible was 6.15 ± 0.13 days (range, 5–8 days). For a result of resistant, the mean time overall was 5.00 ± 0.24 days (range, 3–8 days). Conclusion.—These data suggest that the MGIT AST SIRE system, using either method of inoculum preparation, is an acceptable alternative to the BACTEC 460 TB method of susceptibility testing of clinical isolates of M tuberculosis to isoniazid, rifampin, ethambutol, and streptomycin. Reasons for the lower agreement with the CDC challenge isolates should be investigated. Further evaluation of the MGIT AST SIRE system using a concentration of isoniazid equivalent to the higher concentration tested by the method of proportion would be useful, because the decision concerning use of this agent generally is based on the susceptibility test result at the higher concentration.

Performance of WHO-Endorsed Rapid Tests for Detection of Susceptibility to First-Line Drugs in Patients with Pulmonary Tuberculosis in Bangladesh

“…A liquid culture-based system such as the BACTEC MGIT 960 (MGIT 960) (BD Biosciences, Sparks, NV, USA) is rapid when compared with the solid culture that has been recommended by WHO for TB cultures and DST in low and medium income countries [ 11 ]. Cultured isolates obtained from the MGIT 960 method are tested for drug susceptibility using BACTEC MGIT 960 SIRE kits (MGIT SIRE) [ 12 ]. The Xpert MTB/RIF (Xpert) assay (Cepheid, Sunnyvale, CA, USA) is an automated semi-nested, real-time quantitative PCR assay that simultaneously detects the M. tuberculosis complex and resistance to rifampicin (RIF) [ 13 , 14 ].…”

Agreement between methods for antimicrobial susceptibility to Mycobacterium tuberculosis strains isolated in Montería, Córdoba: mycobacteria growth indicator tube vs. proportion method.

“…El MGIT manual ha presentado resultados discrepantes en EMB y STR pero especialmente EMB. La sensibilidad obtenida para EMB y STR en el presente estudio fue 66.7% y 71.4%, respectivamente, otros estudios informan discrepancias en EMB y STR 4,6,7 . Estas discrepancias quizá se deben a diversos factores como heterorresistencia, naturaleza bacteriostática, la reducida actividad de la droga en medios de cultivo, los diferentes períodos de incubación en el MGIT manual y el método de las proporciones, que se puede asociar con las distintas fases de degradación del antibiótico y el estrecho margen entre la concentración mínima inhibitoria (MIC) de las cepas susceptibles y resistentes, es decir que el punto de corte está cerca de la MIC entre susceptibilidad y resistencia 18,21 Si bien el BACTEC 460 es el método mejor validado para pruebas de susceptibilidad de M. tuberculosis y el que se toma como referencia a nivel mundial 7 , requiere equipos costosos que no se encuentran disponibles en ciudades como Montería.…”

“…Los microorganismos al respirar consumen el oxígeno disuelto en el medio lo que permite observar la fluorescencia que es proporcional al número de bacterias presentes. La fluorescencia se puede descubrir manualmente con lámpara de Wood a 365 nm de luz ultravioleta [4][5][6][7] .…”

“…Estudios preliminares informan que el método MGIT puede servir para evaluar la susceptibilidad antimicrobiana [4][5][6][7][8][9] , permite descubrir cepas MDR en un menor tiempo y a un menor costo, convirtiéndose en una alternativa para países en vías de desarrollo. El objetivo de este estudio fue analizar la concordancia del método manual MGIT AST SIRE en cepas de M. tuberculosis aisladas en la ciudad de Montería, frente al método de proporciones múltiples.…”