Rapid radiometric method for pyrazinamide susceptibility testing of Mycobacterium tuberculosis.

Pyrazinamide (PZA) is one of the most important drugs in modern chemotherapy of tuberculosis. Since PZA is active only at an acid pH, testing the susceptibility of Mycobacterium tuberculosis to PZA is difficult and timeconsuming. Therefore, we evaluated the BACTEC system for rapid testing of PZA susceptibility at pH 6. A total of 91 M. tuberculosis strains and 2 different strains of M. bovis BCG were screened for susceptibility to PZA. Each strain was tested in special 7H12 broth supplemented with polyoxyethylene stearate containing 25, 50 and 100 micrograms PZA/ml. Strains resistant to 100 micrograms/ml were retested against 25-100 micrograms/ml and at an extended range of PZA concentrations from 200-6,400 micrograms/ml. The MIC was determined with all strains within 4-20 (mean 7) days. Of the 77 susceptible strains, based on the pyrazinamidase test, MIC were less than or equal to 25 micrograms/ml for 34 strains, 50 for 38 and 100 for 2 strains. Three pyrazinamidase-positive strains had still higher MIC, 1 at 800 and 2 at 3,200 micrograms/ml. PZA-resistant strains had MIC of 800 or greater. Monoresistance to PZA has not been detected to date. The clear bimodal distribution of MIC in this method could enable the routine clinical microbiology laboratory to perform PZA susceptibility testing as easily as the 4 drugs now tested in the BACTEC system.