Reconsidering some approved antimicrobial agents for tuberculosis

The Pup-Proteasome System Protects Mycobacteria from Antimicrobial Antifolates

Protein turnover via the Pup-proteasome system (PPS) is essential for nitric oxide resistance and virulence of Mycobacterium tuberculosis, the causative agent of tuberculosis. Our study revealed components of PPS as novel determinants of intrinsic antifolate resistance in both M. tuberculosis and nonpathogenic M. smegmatis The lack of expression of the prokaryotic ubiquitin-like protein (Pup) or the ligase, PafA, responsible for ligating Pup to its protein targets, enhanced antifolate susceptibility in M. smegmatis Cross-species expression of M. tuberculosis homologs restored wild-type resistance to M. smegmatis proteasomal mutants. Targeted deletion of prcA and prcB, encoding the structural components of the PPS proteolytic core, similarly resulted in reduced antifolate resistance. Furthermore, sulfonamides were synergistic with acidified nitrite, and the synergy against mycobacteria was enhanced in the absence of proteasomal activity. In M. tuberculosis, targeted mutagenesis followed by genetic complementation of mpa, encoding the regulatory subunit responsible for translocating pupylated proteins to the proteolytic core, demonstrated a similar function of PPS in antifolate resistance. The overexpression of dihydrofolate reductase, responsible for the reduction of dihydrofolate to tetrahydrofolate, or disruption of the Lonely Guy gene, responsible for PPS-controlled production of cytokinins, abolished PPS-mediated antifolate sensitivity. Together, our results show that PPS protects mycobacteria from antimicrobial antifolates via regulating both folate reduction and cytokinin production.

The potential role of trimethoprim-sulfamethoxazole in the treatment of drug-resistant tuberculosis

Tuberculosis (TB) remains a serious public health threat worsened by emerging drug resistance. Mycobacterium tuberculosis has become resistant not only to front-line drugs but also to second-line antimicrobials directed at drug-resistant TB. Renewed efforts are devoted for the development of new antibiotics active against TB. Also, repurposing of other antibiotics is being explored to shorten the time to develop new drugs against M. tuberculosis. As a result, trimethoprim-sulfamethoxazole (SXT) has emerged as a potential new option to treat drug-resistant TB. SXT has been found to be surprisingly active against drug-resistant M. tuberculosis, not only in vitro but also in vivo. The potential role of SXT for the treatment of multidrug resistant/extensively drug resistant TB might be explored in further clinical evaluations.

Importance of the genetic diversity within the Mycobacterium tuberculosis complex for the development of novel antibiotics and diagnostic tests of drug resistance

Despite being genetically monomorphic, the limited genetic diversity within the Mycobacterium tuberculosis complex (MTBC) has practical consequences for molecular methods for drug susceptibility testing and for the use of current antibiotics and those in clinical trials. It renders some representatives of MTBC intrinsically resistant against one or multiple antibiotics and affects the spectrum and consequences of resistance mutations selected for during treatment. Moreover, neutral or silent changes within genes responsible for drug resistance can cause false-positive results with hybridization-based assays, which have been recently introduced to replace slower phenotypic methods. We discuss the consequences of these findings and propose concrete steps to rigorously assess the genetic diversity of MTBC to support ongoing clinical trials.

Strategies for potentiation of ethionamide and folate antagonists against Mycobacterium tuberculosis

Antifolates inhibit de novo folate biosynthesis, whereas ethionamide targets the mycolate synthetic pathway in Mycobacterium tuberculosis. These antibiotics are effective against M. tuberculosis but their use has been hampered by concerns over toxicity and low therapeutic indexes. With the increasing spread of drug-resistant forms, interest in using old drugs for tuberculosis treatment has been renewed. Specific inhibitors targeting resistance mechanisms could sensitize M. tuberculosis to these available, clinically approved drugs. This review discusses recently developed strategies to boost the antituberculous activity of ethionamide and antifolates. These approaches might help broaden the currently limited chemotherapeutic options of not only drug-resistant but also drug-susceptible tuberculosis, which still remains one of the most common infectious diseases in the developing world.

Bacterial conversion of folinic acid is required for antifolate resistance

Antifolates, which are among the first antimicrobial agents invented, inhibit cell growth by creating an intracellular state of folate deficiency. Clinical resistance to antifolates has been mainly attributed to mutations that alter structure or expression of enzymes involved in de novo folate synthesis. We identified a Mycobacterium smegmatis mutant, named FUEL (which stands for folate utilization enzyme for leucovorin), that is hypersusceptible to antifolates. Chemical complementation indicated that FUEL is unable to metabolize folinic acid (also known as leucovorin or 5-formyltetrahydrofolate), whose metabolic function remains unknown. Targeted mutagenesis, genetic complementation, and biochemical studies showed that FUEL lacks 5,10-methenyltetrahydrofolate synthase (MTHFS; also called 5-formyltetrahydrofolate cyclo-ligase; EC 6.3.3.2) activity responsible for the only ATP-dependent, irreversible conversion of folinic acid to 5,10-methenyltetrahydrofolate. In trans expression of active MTHFS proteins from bacteria or human restored both antifolate resistance and folinic acid utilization to FUEL. Absence of MTHFS resulted in marked cellular accumulation of polyglutamylated species of folinic acid. Importantly, MTHFS also affected M. smegmatis utilization of monoglutamylated 5-methyltetrahydrofolate exogenously added to the medium. Likewise, Escherichia coli mutants lacking MTHFS became susceptible to antifolates. These results indicate that folinic acid conversion by MTHFS is required for bacterial intrinsic antifolate resistance and folate homeostatic control. This novel mechanism of antimicrobial antifolate resistance might be targeted to sensitize bacterial pathogens to classical antifolates.