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Reevaluation of the Critical Concentration for Drug Susceptibility Testing of Mycobacterium tuberculosis against Pyrazinamide Using Wild-Type MIC Distributions and pncA Gene Sequencing

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Author

Summary, in English

Pyrazinamide (PZA) is a potent first-line agent for the treatment of tuberculosis (TB) with activity also against a significant part of drug-resistant Mycobacterium tuberculosis strains. Since PZA is active only at acid pH, testing for susceptibility to PZA is difficult and insufficiently reproducible. The recommended critical concentration for PZA susceptibility (MIC, 100 mg/liter) used in the Bactec systems (460 and MGIT 960) has not been critically evaluated against wild-type MIC distributions in clinical isolates of Mycobacterium tuberculosis. Using the Bactec MGIT 960 system, we determined the PZA MICs for 46 clinical M. tuberculosis isolates and compared the results to pncA sequencing and previously obtained Bactec 460 data. For consecutive clinical isolates (n = 15), the epidemiological wild-type cutoff (ECOFF) for PZA was 64 mg/liter (MIC distribution range, <= 8 to 64 mg/liter), and no pncA gene mutations were detected. In strains resistant in both Bactec systems (n = 18), the PZA MICs ranged from 256 to >= 1,024 mg/liter. The discordances between pncA sequencing, susceptibility results in Bactec 460, and MIC determinations in Bactec MGIT 960 were mainly observed in strains with MICs close to or at the ECOFF. We conclude that in general, wild-type and resistant strains were clearly separated and correlated to pncA mutations, although some isolates with MICs close to the ECOFF cause reproducibility problems within and between methods. To solve this issue, we suggest that isolates with MICs of <= 64 mg/liter be classified susceptible, that an intermediary category be introduced at 128 mg/liter, and that strains with MICs of >= 128 mg/liter be classified resistant.

Publishing year

2012

Language

English

Pages

1253-1257

Publication/Series

Antimicrobial Agents and Chemotherapy

Volume

56

Issue

3

Document type

Journal article

Publisher

American Society for Microbiology

Topic

  • Microbiology in the medical area

Status

Published

Research group

  • Clinical Microbiology, Malmö

ISBN/ISSN/Other

  • ISSN: 1098-6596