Relationship between the uptake of isoniazid and its action on in vivo mycolic acid synthesis in Mycobacterium tuberculosis

Isoniazid Killing of Mycobacterium smegmatis NADH Pyrophosphatase Mutant at Single-Cell Level using Microfluidics and Time-Lapse Microscopy

We introduce single-cell analysis for isoniazid-treated Mycobacterium smegmatis mutant, msm1946-NADH pyrophosphatase, using microfluidics and automated time-lapse microscopy. Mycobacterial NADH pyrophosphatase isoforms play an important role for the mechanism of isoniazid and ethionamide activation. Our single-cell analysis revealed important insights on isoniazid killing mechanism that was masked by traditional killing assays, raised significant questions related to viable but non-culturable subpopulation of cells, and existing methods that defines minimum inhibitory concentration of drugs. The major goal of this study was quantitatively analyze bacterial cell parameters to obtain high-resolution data for the time evolution of antibiotic killing at the single-cell level. The presented tools and methods could be applied to the closely related organisms to provide more detailed information for the design and employment of antibiotic treatments.

Chemotherapy of Tuberculosis

The management of tuberculosis (TB) can be a challenging process that has implications both for the affected patient and public health. Effective anti-TB chemotherapy both cures and renders the patient noncontagious. Biological factors specific to M. tuberculosis necessitate the use of multiple drugs for prolonged durations to adequately eradicate infection. Recommended regimens address the complexities of eliminating organisms from diverse reservoirs while preventing the emergence of drug resistance. First-line anti-TB therapy for drug susceptible disease effectively cures almost all patients within 6-9 months. The loss of first-line agents, due to resistance or intolerance, necessitates lengthy treatment courses, frequently 12-18 months or longer. Due to the long treatment times and the implications of missed doses, directly-observed therapy (DOT) is considered the standard of care. Drugs used for the treatment of TB have serious potential toxicities that require close monitoring and prompt response. A strong public health infrastructure and robust social supports are important elements to assure successful treatment. These numerous factors compel public health entities to take a lead role in the management of TB, either through the direct management of TB treatment or by assuring the activities of partner organizations.

Tuberculosis Drug Development: History and Evolution of the Mechanism-Based Paradigm

Modern tuberculosis (TB) chemotherapy is widely viewed as a crowning triumph of anti-infectives research. However, only one new TB drug has entered clinical practice in the past 40 years while drug resistance threatens to further destabilize the pandemic. Here, we review a brief history of TB drug development, focusing on the evolution of mechanism(s)-of-action studies and key conceptual barriers to rational, mechanism-based drugs.

Dielectrophoresis-based purification of antibiotic-treated bacterial subpopulations

Persistence of bacteria during antibiotic therapy is a widespread phenomenon, of particular importance in refractory mycobacterial infections such as leprosy and tuberculosis. Persistence is characterized by the phenotypic tolerance of a subpopulation of bacterial cells to antibiotics. Characterization of these "persister" cells is often difficult due to the transient, non-heritable nature of the phenotype and due to the presence of contaminating material from non-persisting cells, which usually comprise the larger fraction. In this study, we use 3D carbon-electrode arrays for dielectrophoresis-based separation of intact cells from damaged cells, revealed by differential staining with propidium iodide, and we use this procedure to purify intact cells from cultures of Mycobacterium smegmatis treated with isoniazid, a frontline anti-tuberculosis drug. The method presented in this study could be used for rapid label-free enrichment of intact persister cells from antibiotic-treated cultures while preserving the metastable persister phenotype. This approach would facilitate the downstream analysis of low-frequency subpopulations of cells using conventional omics techniques, such as transcriptomic and proteomic analysis.

Arylamine N-acetyltransferase is required for synthesis of mycolic acids and complex lipids in Mycobacterium bovis BCG and represents a novel drug target

Mycolic acids represent a major component of the unique cell wall of mycobacteria. Mycolic acid biosynthesis is inhibited by isoniazid, a key frontline antitubercular drug that is inactivated by mycobacterial and human arylamine N-acetyltransferase (NAT). We show that an in-frame deletion of Mycobacterium bovis BCG nat results in delayed entry into log phase, altered morphology, altered cell wall lipid composition, and increased intracellular killing by macrophages. In particular, deletion of nat perturbs biosynthesis of mycolic acids and their derivatives and increases susceptibility of M. bovis BCG to antibiotics that permeate the cell wall. Phenotypic traits are fully complemented by introduction of Mycobacterium tuberculosis nat. We infer from our findings that NAT is critical to normal mycolic acid synthesis and hence other derivative cell wall components and represents a novel target for antituberculosis therapy. In addition, this is the first report of an endogenous role for NAT in mycobacteria.

Agonodepsides a and B: two new depsides from a filamentous fungus F7524

Two new compounds, agonodepsides A (1) and B (2), were isolated from a nonsporulating filamentous fungus, F7524. The compounds were purified via reversed-phase chromatography and their structures determined by spectroscopic methods. Agonodepside A (1) was found to inhibit the mycobacterial InhA enzyme with an IC50 value of 75 microM, while 2 was inactive at 100 microM.

Activation of the pro-drug ethionamide is regulated in mycobacteria

The anti-tuberculosis drug ethionamide (ETH), which is a structural analog of isoniazid (INH), is known to strongly inhibit mycolic acid synthesis in Mycobacterium tuberculosis. Although several targets have been identified for INH, only speculative information is available concerning ETH. Mutations within the promoter and the coding region of enoyl-acyl carrier protein reductase (InhA) were found to confer resistance to both drugs, thus leading to the impression that INH and ETH may share a common mode of action. However, a notable distinction between the two drugs lies in the lack of cross-resistance in clinical isolates. This may be attributed in part to the fact that the pro-drug INH must be activated via KatG, and no activation step for ETH has yet been described. Here we report the identification of an activator for ETH. The ETH activator (Rv3854c), which we have termed EthA, was found to be homologous to various monooxygenases and induced ETH sensitivity when overexpressed in mycobacteria. Interestingly, the neighboring open reading frame (Rv3855), which was found homologous to transcriptional repressors of the tetR family, led to ETH resistance when overexpressed. In addition, chromosomal inactivation of this gene by transposition led to ETH hypersensitivity. These data strongly suggest that Rv3855, which we have termed EthR, regulates the production of EthA, which subsequently activates the pro-drug ETH. This study opens up new avenues of research relating to ETH activation in mycobacteria, possibly leading to an improved efficacy of ETH and to the generation of new anti-mycobacterial agents.

Antimycobacterial activities of isoxyl and new derivatives through the inhibition of mycolic acid synthesis

Isoxyl (ISO), a thiourea (thiocarlide; 4, 4'-diisoamyloxythiocarbanilide), demonstrated potent activity against Mycobacterium tuberculosis H37Rv (MIC, 2.5 micrograms/ml), Mycobacterium bovis BCG (MIC, 0.5 microgram/ml), Mycobacterium avium (MIC, 2.0 microgram/ml), and Mycobacterium aurum A+ (MIC, 2.0 microgram/ml), resulting in complete inhibition of mycobacteria grown on solid media. Importantly, a panel of clinical isolates of M. tuberculosis from different geographical areas with various drug resistance patterns were all sensitive to ISO in the range of 1 to 10 microgram/ml. In a murine macrophage model, ISO exhibited bactericidal killing of viable intracellular M. tuberculosis in a dose-dependent manner (0.05 to 2.50 microgram/ml). The selective action of ISO on mycolic acid synthesis was studied through the use of [1, 2-14C]acetate labeling of M. tuberculosis H37Rv, M. bovis BCG, and M. aurum A+. At its MIC for M. tuberculosis, ISO inhibited the synthesis of both fatty acids and mycolic acids (alpha-mycolates by 91.6%, methoxymycolates by 94.3%, and ketomycolates by 91.1%); at its MIC in M. bovis BCG, ISO inhibited the synthesis of alpha-mycolates by 87.2% and that of ketomycolates by 88.5%; and the corresponding inhibitions for M. aurum A+ were 87.1% for alpha-mycolates, 87.2% for ketomycolates, and 86.5% for the wax-ester mycolates. A comparison with isoniazid (INH) and ethionamide (ETH) demonstrated marked similarity in action, i.e., inhibition of the synthesis of all kinds of mycolic acids. However, unlike INH and ETH, ISO also inhibited the synthesis of shorter-chain fatty acids. ISO showed no acute toxicity against primary macrophage cell cultures as demonstrated by diminution of redox activity. A homologous series of ISO derivatives were synthesized. Most derivatives were as effective or more effective than the parent compound in the agar proportion assay. Thus, these thioureas, like INH and ETH, specifically inhibit mycolic acid synthesis and show promise in counteracting a wide variety of drug-sensitive and -resistant strains of M. tuberculosis.

Mechanism of isoniazid uptake in Mycobacterium tuberculosis

Initial transport kinetics of isoniazid (INH) and its uptake at the plateau were studied in Mycobacterium tuberculosis H37Rv under various experimental conditions. The initial uptake velocity increased linearly with INH concentration from 2 x 10(-6) M to 10(-2) M. It was modified neither by addition of a protonophore that abolished proline transport, nor following ATP depletion by arsenate, which inhibited glycerol uptake, two transport processes taken as controls for secondary active transport and facilitated diffusion, respectively. Microaerobiosis or low temperature (4 degrees C) were without effect on initial uptake. It is thus likely that INH transport in M. tuberculosis proceeds by a passive diffusion mechanism, and that catalase-peroxidase (KatG) is not involved in the actual transport. However, conditions inhibiting KatG activity (high INH concentration, microaerobiosis, low temperature) decrease cell radioactivity at the uptake plateau. It is proposed that INH transport occurs by passive diffusion. KatG is involved only in the intracellular accumulation of oxidized derivatives of INH, especially of isonicotinic acid, which is trapped inside cells in its ionized form. This model explains observed and previously known characteristics of the accumulation of radioactivity in the presence of [14C]INH for various species and strains of mycobacteria.

Mass-spectrometric identification of trehalose 6-monomycolate synthesized by the cell-free system of Bacterionema matruchotii

The fluffy layer fraction prepared from Bacterionema matruchotii was found to possess high activity for the biosynthesis of mycolic acids which were bound to an unknown compound by an alkali-labile linkage [T. Shimakata, M. Iwaki, and T. Kusaka (1984) Arch. Biochem. Biophys. 229, 329-339]. To determine the structure of the mycolate-containing compound, it was purified and analyzed by field desorption (FD) and secondary ion mass spectrometry (SI-MS). When non-labelled palmitic acid was used as a precursor in the in vitro biosynthetic system, the underivatized product had a cationized molecular ion, [M + Na]+, at m/z 843 in FD-MS and a protonated ion, [M + H]+, at m/z 821 in SI-MS, corresponding to the quasimolecular ion of trehalose monomycolate (C32:0). In SI-MS, characteristic fragment ions due to cleavage of glycosidic linkages were clearly detected in addition to the molecular ion. If [1-13C]palmitic acid was the precursor, 2 mass unit increases in both the quasimolecular and fragment ions were observed, indicating that two molecules of palmitate were incorporated into the product. alpha-Trehalose was found in the aqueous phase after saponification of the product. By the electron impact mass spectrometry of the trimethylsilylated product, the mycolate was found to be esterified with an hydroxyl group at position 6 of the trehalose molecule. These results clearly demonstrated that the predominant product synthesized by the fluffy layer fraction with palmitate as substrate was 6-monomycolate (C32:0) of alpha-D-trehalose. Because newly synthesized mycolic acid was mainly in the form of trehalose monomycolate instead of free mycolate or trehalose dimycolate, the role of trehalose in the biosynthesis of mycolic acid is discussed.

In vitro synthesis of mycolic acids by the fluffy layer fraction of Bacterionema matruchotii

Biosynthetic activity for mycolic acid occurred in the fluffy layer fraction but not in the 5000g supernatant of Bacterionema matruchotii. With [1-14C]palmitic acid as precursor for the in vitro system, the predominant product was identified as C32:0 mycolic acid by radio-gas-liquid chromatographic (radio-GLC) and gas chromatographic/mass spectroscopic analyses; if [1-14C]stearic acid was used, two major radioactive peaks appeared on GLC: one corresponding to the peak of (C34:0 + C34:1) mycolic acids and the other to (C36:0 + C36:1) mycolic acids. By pyrolysis/radio-GLC analysis, C32:0 mycolic acid synthesized by [1-14C]palmitic acid was pyrolyzed at 300 degrees C to form palmitaldehyde (the mero moiety) and methyl palmitate (the branch moiety). The pH optimum for the incorporation of [1-14C]palmitate into bacterionema mycolic acids was 6.4 and the reaction required a divalent cation. The in vitro system utilized myristic, palmitic, stearic and oleic acids (probably via their activated forms) well as precursors, among which myristic and palmitic acids were more effective than the rest. Avidin showed no effect on the biosynthesis of mycolic acid from 14C-palmitate whereas cerulenin, a specific inhibitor of beta-ketoacyl synthetase in de novo fatty acid synthesis, inhibited the reaction at a relatively higher concentration. Thin-layer chromatographic analysis of lipids extracted from the reacting mixture without alkaline hydrolysis showed that both exogenous [1-14C]fatty acid and synthesized mycolic acids were bound to an unknown compound by an alkali-labile linkage and this association seemed to occur prior to the condensation of two molecules of fatty acid.

Identification of a nicotinamide adenine dinucleotide glycohydrolase and an associated inhibitor in isoniazid-susceptible and -resistant Mycobacterium phlei

Nicotinamide adenine dinucleotide glycohydrolase (NADase) activity was demonstrated in the catalases fraction of Sephadex G-200-chromatographed sonic extracts of isoniazid (INH)-susceptible (Inhs) and -resistant (Inhr) Mycobacterium phlei. Since crude extracts had no demonstrable activity even after heating, active fractions of the NADase were purified chromatographically by removing the inhibitor with Sephadex G-200. Assays for oxidized nicotinamide adenine dinucleotide (NAD+) hydrolytic activity were done by following the disappearance of NAD+ by the methods of alcohol dehydrogenase or cyanide addition. The NADase activity was linear with respect to time as well as concentration of enzyme and was inhibited in the presence of 0.04 M NADP, benzoic acid hydrazide, or nicotinamide. Crude extracts or pooled concentrated Sephadex G-200 fractions eluting after the catalase inhibited NADase activity by at least 70%. Inhibitor activity was present in both the Inhs and Inhr strains of M. phlei. The activity of the partially purified inhibitors was reversible by INH or nicotinic acid hydrazide at levels between 10 and 100 mM. These findings indicate that an NADase inhibitor system which is sensitive to reversal by INH functions in both the Inhs and Inhr strains; however, unlike previous studies with other mycobacterial species, the enzyme is sensitive to inhibition by nicotinamide. Furthermore, the inhibitors are heat stable and sensitive to reversal by nicotinic acid hydrazide as well as INH.

Metabolic role of free mycolic acids in Mycobacterium tuberculosis

Small amounts of free mycolic acids and trehalose dimycolate that are rapidly formed by Mycobacterium tuberculosis H37Ra are probably derived from mycolyl acetyl trehalose and transferred to the cell wall. However, the transfer of mycolic acids from mycolyl acetyl trehalose to the cell wall still appears to be the more prominent route.

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Enumeration of tubercle bacilli in sputum of patients with pulmonary tuberculosis

A method for precise microscopic enumeration of acid-fast bacilli in the sputum of patients with pulmonary tuberculosis is described. The total number of acid-fast bacilli and the number of culturable cell units of tubercle bacilli per milliliter of sputum were determined in 269 specimens from 28 patients collected prior to and/or during chemotherapy, to establish the usefulness of the procedure for evaluation of the effects of chemotherapy. Prior to or very early in the course of therapy, there was good agreement between microscopically and culturally measurable bacilli in 27 of 28 patients. Thereafter, there were systematic divergences in five patients and occasional divergences in most others. Yet, in general, in 20 of the 28 patients, the microscopically determined population provided a useful measure of the number of culturable tubercle bacilli present. Although the microscopic procedure does not allow species identification of mycobacteria or differentiation between viable and nonviable cells, it often provided, within hours rather than days or weeks, useful information on the residual burden of acid-fast bacilli in sputum during chemotherapy. The nature of the disease in those patients in whom microscopy failed to provide useful information is discussed.

Scanning electron microscopy of the H37Ra strain of Mycobacterium tuberculosis exposed to isoniazid

The morphology of cells of the H37Ra strain of Mycobacterium tuberculosis exposed to 0.5 mug of isonicotinic acid hydrazide (isoniazid) per ml was examined by scanning electron microscopy (SEM). Cells that were exposed to isoniazid for 3 h showed no detectable change, whereas cells exposed to the drug for 24 h exhibited diverse morphological features. From our examination of these SEM pictures, we have reconstructed the probable sequence of morphological changes to be as follows: (i) the wrinkling of the cell surface was ascribed as the earliest observable change, (ii) the cell surface then became very rough and ragged, (iii) eventually the cytoplasmic material was extruded from the cell, (iv) this event produced a collapsed cell, (v) the cells began to fragment, (vi) the fragmented cells then coalesced to form an amorphous mass of cell debris.