Radiometric method for testing susceptibility of mycobacteria to pyrazinamide in 7H12 broth

The test of susceptibility to pyrazinamide requires an acid environment (pH less than or equal to 5.5). This, however, is not favorable to the growth of Mycobacterium tuberculosis, especially in solid agar media. To obviate this difficulty, we developed a testing method with 7H12 broth medium and based on radiometric readings of the growth. The radiometric method employed in this study (BACTEC system) provides an opportunity to detect the dynamics of growth by daily recording of the growth index, which reflects the metabolic activity of the multiplying bacteria. In our technique, M. tuberculosis isolates were initially cultivated at pH 6.8. After logarithmic growth had begun, phosphoric acid solution was added to obtain pH 5.5. When pyrazinamide was added simultaneously with the acid, the growth index of susceptible cultures decreased, whereas it continued to increase in pH 5.5 control vials and in tests with pyrazinamide-resistant strains.