Multicenter Evaluation of Fully Automated BACTEC Mycobacteria Growth Indicator Tube 960 System for Susceptibility Testing of Mycobacterium tuberculosis

Abstract

Drug-resistant strains of Mycobacterium tuberculosis, though not a novel phenomenon, are emerging worldwide. According to the latest figures of the World Health Organization and the International Union against Tuberculosis and Lung Diseases, drug resistance, in particular acquired resistance, has poured additional fuel on the fire of global tuberculosis (TB) (18). Several outbreaks of multidrug-resistant TB (7) were characterized by delayed diagnoses, inadequate treatment regimens, high rates of mortality, and significant rates of transmission and have taught us two lessons: first, the days are definitely gone where full susceptibility of TB bacilli to front-line drugs can be taken for granted. Second, rapid detection of drug resistance is paramount, not only for effective treatment of TB patients but also for initiating adequate public health measures.In the quest for new nonradiometric, culture-based strategies which allow both rapid detection of acid-fast bacilli and testing of susceptibility to antimicrobial agents, new liquid medium-based systems, such as the MB/BacT (Organon-Teknika, Durham, N.C.), ESP Culture System II (AccuMed International, Westlake, Ohio), MB Redox (Biotest, Dreieich, Germany), and the Mycobacteria Growth Indicator Tube 960 (MGIT 960, Becton Dickinson Microbiology Systems, Sparks, Md.), have become available. They all aim not only at recovering mycobacteria from clinical specimens but also at generating antimicrobial susceptibility testing (AST) data with a shorter turnaround time than that observed with the current "gold standard," the agar proportion method (11). The performance of a new system should be comparable with that of the BACTEC 460 TB system, with elimination of the two core problems associated with the old BACTEC 460 TB technology, i.e., the risk of needle punctures and disposal of radioactive waste. Preliminary studies utilizing those new systems report good overall agreement of AST results with those generated with established methods (1, 3-5, 8, 10, 12-13, 15).Recent automation of the MGIT 960 technology was another step forward, as it allows continuous monitoring of positive fluorescence, which is based on bacterial growth. It is noninvasive and eliminates potential reading difficulties during visual judging of the tubes, apart from saving labor. The threshold algorithms help in determining the susceptibility automatically.In this multicenter study we have evaluated the reproducibility and reliability of the BACTEC MGIT 960 instrument for testing of M. tuberculosis susceptibility to isoniazid (INH), rifampin (RIF) ethambutol (EMB), and streptomycin (STR) and have compared the results to those obtained by the radiometric procedure. Discordant results were resolved by testing the strains with the agar proportion method using Löwenstein-Jensen (LJ) medium (6). This was done by an additional site which thus acted as an independent arbiter. Last, in order to address safety, we performed drug susceptibility testing in plastic MGITs, in addition to the glass tubes.

Tuberculosis ethambutol resistance: Concordance between phenotypic and genotypic test results

“…Re-testing of strains with discordant results was performed both on LJ medium (as before), and the Bactec MGIT 960 system (5.0 mg/ml for EMB) as described previously. 4 For MIC determination, suspensions were made from fresh cultures grown on LJ. The suspensions were adjusted to the optical density of McFarland 1.…”

A systematic review of rapid diagnostic tests for the detection of tuberculosis infection

“…[95][96][97][98] Reductions in turnaround times from 21 days for LJ-based tests to 6-12 days for automated liquid culture systems for susceptibility testing have been reported. 98,99 The liquid culture methods are expensive, however, and require elaborate technology. 100 In addition, skilled and experienced staff are crucial as contamination rates have been high when inexperienced and untrained staff have used these systems.…”

Tuberculosis: A Literature Review

“…The growth of bacteria enhances the fluorescence (emission of light). The growth of bacteria is visually observed in the presence of nonhomogeneous light turbidity or appearance of small granular in the medium 38,40 .…”