The Bactec MGIT 960 system for testing susceptibility to second-line drugs was evaluated with 117 clinical strains in a multicenter study. The four drugs studied were levofloxacin, amikacin, capreomycin, and ethionamide. The critical concentration established for levofloxacin and amikacin was 1.5 g/ml, that established for capreomycin was 3.0 g/ml, and that established for ethionamide was 5.0 g/ml. The overall level of agreement between the agar proportion method and the MGIT 960 system was 96.4%, and the levels of agreement for the individuals drugs were 99.1% for levofloxacin, 100% for amikacin, 97.4% for capreomycin, and 88.9% for ethionamide. The rate of reproducibility of the drug susceptibility testing results between the participating laboratories was 99.5%.The increase in the incidence of multidrug-resistant tuberculosis (MDR TB) and the emergence of extensively drugresistant tuberculosis present tremendous challenges to the global efforts to combat tuberculosis (1,5,16,21). Rapid methods enabling accurate susceptibility testing of first-line and second-line drugs are critical for the early diagnosis of MDR TB and extensively drug-resistant tuberculosis and the initiation of effective regimens. Various drug susceptibility testing (DST) methods that use solid media, including the agar proportion method (AP) and other methods, have the drawback of prolonged turnaround times (TATs). The World Health Organization and the U.S. Centers for Disease Control and Prevention have recommended the use of liquid culture systems for the diagnosis of tuberculosis and DST to improve TATs (22,25). The Bactec 460 (Becton Dickinson Diagnostic Systems, Sparks, MD), a radiometric liquid system, provided an excellent alternative for testing of the susceptibilities of Mycobacterium tuberculosis complex (MTBC) isolates to streptomycin, isoniazid, rifampin (rifampicin), and ethambutol (SIRE) and to pyrazinamide (PZA) with improved TATs. The MGIT 960 liquid, nonradiometric SIRE DST (Becton Dickinson Diagnostic Systems), whose performance is comparable to that of the Bactec 460 system, has been commercially available since April 2002 (4, 20, 23). The Microbial Diseases Laboratory (MDL) of the California Department of Public Health implemented SIRE DST with the MGIT 960 system in 2004. With confidence in the SIRE DST with the MGIT 960 system, a study that used the same platform to test the susceptibilities of MTBC isolates to four classes of second-line drugs, levofloxacin (LVX), amikacin (AMK), capreomycin (CAP), and ethionamide (ETH), was initiated in November 2004. The study was conducted at two laboratories: MDL and the TB Reference Laboratory of the Veteran Affairs Medical Center (VA) in West Haven, CT. Here we report the results of the multicenter study, in which the critical concentrations of the test drugs were established, the performance of the MGIT 960 system was compared to that of AP, and the interlaboratory reproducibility of the method was evaluated.(Part of this work was presented at the 46th Interscience Conference...
In a large multicenter study involving six major study sites in the United States, Canada, and Europe, the susceptibilities of 272Mycobacterium tuberculosis strains to classical second-line antituberculosis (anti-TB) drugs (capreomycin, cycloserine, ethionamide, and kanamycin) and newer compounds (amikacin, clofazimine, ofloxacin, and rifabutin) were determined by the radiometric BACTEC 460 procedure and the conventional proportion method on Middlebrook 7H10 agar. Previously established critical concentrations for classical second-line anti-TB drugs were compared with several concentrations in liquid medium to establish equivalence. MICs of newer compounds determined in liquid medium were either the same or up to four times lower than those determined in agar medium. After establishing critical concentrations (breakpoints) in the extended testing of clinical isolates, we obtained an excellent overall correlation between the two systems, with no errors with amikacin, kanamycin, and ofloxacin and very few major or very major errors with the other drugs; however, for cycloserine, no breakpoint concentration could be recommended due to repeatedly inconsistent results by both methods. Based on these data we conclude that the BACTEC 460 procedure is a simple and rapid method requiring 4 to 8 days on average to generate accurate antimicrobial susceptibility testing (AST) results for eight anti-TB drugs other than those considered primary ones. These data not only fill a major gap of knowledge regarding the critical test concentrations of secondary anti-TB drugs but also provide a baseline for future evaluations ofM. tuberculosis AST with the more recently developed, nonradiometric broth-based culture systems.
A previously uncharacterized, slowly growing, scotochromogenic Mycobacterium species was detected by HPLC analysis of the cell-wall-bound mycolic acids. The mycolic acid pattern standard was shown to be a lateeluting, contiguous peak cluster occurring at approximately 8-9 min. The mycolic acid pattern was noted to be most similar in number of peaks and range of elution to that reported previously for Mycobacterium asiaticum.
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