Role of Acid pH and Deficient Efflux of Pyrazinoic Acid in Unique Susceptibility of Mycobacterium tuberculosis to Pyrazinamide

Abstract

Pyrazinamide (PZA) is an important antituberculosis drug. Unlike most antibacterial agents, PZA, despite its remarkable in vivo activity, has no activity against Mycobacterium tuberculosis in vitro except at an acidic pH. M. tuberculosis is uniquely susceptible to PZA, but other mycobacteria as well as nonmycobacteria are intrinsically resistant. The role of acidic pH in PZA action and the basis for the unique PZA susceptibility of M. tuberculosisare unknown. We found that in M. tuberculosis, acidic pH enhanced the intracellular accumulation of pyrazinoic acid (POA), the active derivative of PZA, after conversion of PZA by pyrazinamidase. In contrast, at neutral or alkaline pH, POA was mainly found outside M. tuberculosis cells. PZA-resistantM. tuberculosis complex organisms did not convert PZA into POA. Unlike M. tuberculosis, intrinsically PZA-resistant M. smegmatis converted PZA into POA, but it did not accumulate POA even at an acidic pH, due to a very active POA efflux mechanism. We propose that a deficient POA efflux mechanism underlies the unique susceptibility of M. tuberculosisto PZA and that the natural PZA resistance of M. smegmatis is due to a highly active efflux pump. These findings may have implications with regard to the design of new antimycobacterial drugs.

Drug resistance mechanisms and drug susceptibility testing for tuberculosis

“…PZA is a prodrug in which the antimycobacterial activity requires hydrolysis by MTB pyrazinamidase (PZase)/nicotinamidase encoded by pncA to convert into its active form pyrazinoic acid (POA) causing cytoplasmic acidification and depletion of membrane potential. 21,48 The activity of PZA is generally thought to be dependent on acidic pH 49,50 ; however, it has recently been reported that the PZA activity can be independent of acidic pH and intrabacterial acidification. 51,52 The latter finding can be confusing as it does not mean that PZA works at neutral pH in general, irrespective of the metabolic status of the TB bacteria.…”

Role of mycobacterial efflux transporters in drug resistance: an unresolved question

“…PZA is one of the most important drugs used to treat TB. PZA is a pro-drug that enters M. tuberculosis by passive diffusion and is converted to the active compound pyrazi-noic acid (POA) by a nicotinamidase/pyrazinamidase enzyme encoded by the pncA gene (Zhang et al, 1999). Hirano et al (1997) reported that 72%-97% of PZA-resistant M. tuberculosis clinical isolates carry a mutation in the coding region of the pncA gene or in its putative promoter region.…”

Exploring the potential of endophytes from medicinal plants as sources of antimycobacterial compounds

“…Mechanism not fully understood, though it is thought to be inhibiting vital enzyme activities and disrupting membrane transport (Heifets and Lindholm-Levy, 1992;Zhang et al, 1999;Zhang and Mitchison, 2003) Mutation in the gene pncA, preventing drug activation (Scorpio et al, 1997) Rifamycins: rifampin, rifabutin, rifalazil, rifapentine Inhibition of protein synthesis, particularly in transcriptional initiation (Kunin, 1996) and induction of programmed cell death (Engelberg-Kulka et al, 2004) Mutation in the target structural gene rpoB (Telenti et al, 1993) Riminophenazines: clofazimine, B746, B4157…”