Pyrazinamide susceptibility testing in Mycobacterium tuberculosis using the fast resazurin microtiter assay plate

Department of Clinical Analysis and Biomedicine, State University of Maringa, Maringa, PR, Brazil.

To evaluate the performance of the resazurin microtiter assay (REMA) plate at pH 5.5 in detecting Mycobacterium tuberculosis susceptibility to pyrazinamide (PZA).

The minimal inhibitory concentration (MIC) of PZA in M. tuberculosis H₃₇Rv and M. bovis AN5 reference strains and in 34 clinical M. tuberculosis isolates (26 PZA-susceptible and eight PZA-resistant) was determined using REMA at pH 5.5 and compared to REMA at pH 6.0.

REMA at pH 5.5 was helpful in discriminating PZA-susceptible from resistant M. tuberculosis isolates when ⩿50 μg/ml PZA was considered as the cut-off for PZA susceptibility. Furthermore, it provided results in 8 days. However, two PZA-resistant isolates failed to grow at pH 5.5.

As the REMA method is rapid, inexpensive, easy to perform and read, it would be of great usefulness in low-income countries for detecting PZA-resistant M. tuberculosis. REMA at pH 5.6-5.9 should be evaluated on an extended panel of clinical M. tuberculosis isolates with a greater range of MIC values in different laboratories for a better understanding of its utility in differentiating PZA-resistant from PZA-susceptible isolates.