Systematic Analysis of Pyrazinamide-Resistant Spontaneous Mutants and Clinical Isolates of Mycobacterium tuberculosis

Pyrazinamide (PZA) is a first-line antitubercular drug known for its activity against persistent Mycobacterium tuberculosis bacilli. We set out to systematically determine the PZA susceptibility profiles and mutations in the pyrazinamidase (pncA) gene of a collection of multidrug-resistant tuberculosis (MDR-TB) clinical isolates and PZA-resistant (PZAr) spontaneous mutants. The frequency of acquired resistance to PZA was determined to be 10−5 bacilli in vitro. Selection at a lower concentration of PZA yielded a significantly larger number of spontaneous mutants. The methodical approach employed allowed for determination of the frequency of the PZAr phenotype correlated with mutations in the pncA gene, which was 87.5% for the laboratory-selected spontaneous mutants examined in this study. As elucidated by structural analysis, most of the identified mutations were foreseen to affect protein activity through either alteration of an active site residue or destabilization of protein structure, indicating some preferential mutation site rather than random scattering. Twelve percent of the PZAr mutants did not have a pncA mutation, strongly indicating the presence of at least one other mechanism(s) of PZAr.