Mutually exclusive genotypes for pyrazinamide and 5-chloropyrazinamide resistance reveal a potential resistance-proofing strategy.

The pyrazinamide (PZA) analog 5-chloropyrazinamide (5-Cl PZA) is active against mycobacterial species, including PZA-resistant strains of Mycobacterium tuberculosis. In M. smegmatis, overexpression of the type 1 fatty acid synthase (FAS I) confers resistance to 5-Cl PZA, a potent FAS I inhibitor. Since M. tuberculosis and M. bovis cannot tolerate FAS I overexpression, 5-Cl PZA resistance mutations have yet to be described for tubercle bacilli. In an attempt to identify other factors that govern the activity of 5-Cl PZA, we selected for 5-Cl PZA-resistant isolates from a library of transposon-mutagenized M. smegmatis isolates. Here, we report that increased expression of the M. smegmatis pyrazinamidase PzaA confers resistance to 5-Cl PZA and susceptibility to PZA in M. smegmatis, M. tuberculosis, and M. bovis. In contrast, while ectopic overexpression of the M. tuberculosis pyrazinamidase PncA increases PZA susceptibility, this amidase does not mediate resistance to 5-Cl PZA. We conclude that PncA-independent turnover of 5-Cl PZA represents a potential mechanism of resistance to this compound for M. tuberculosis, which will likely translate into enhanced PZA susceptibility. Thus, countersusceptibility can be manipulated as a resistance-proofing strategy for PZA-based compounds when these agents are used simultaneously.