Altered expression profile of mycobacterial surface glycopeptidolipids following treatment with the antifungal azole inhibitors econazole and clotrimazole

Ferritin from Mycobacterium abscessus is involved in resistance to antibiotics and oxidative stress

Mycobacterium abscessus subsp. massiliense (Mycma) is a rapidly growing Mycobacterium belonging to the M. abscessus complex that is often associated with lung and soft tissue infection outbreaks. Mycma is resistant to many antimicrobials, including those used for treating tuberculosis. Therefore, Mycma infections are difficult to treat and may lead to high infectious complication rates. Iron is essential for bacterial growth and establishment of infection. During infection, the host reduces iron concentrations as a defense mechanism. To counteract the host-induced iron deficiency, Mycma produces siderophores to capture iron. Mycma has two ferritins (encoded by mycma_0076 and mycma_0077) modulated by different iron concentrations, which allow the survival of this pathogen during iron scarcity. In this study, we constructed knockout (Mycma 0076KO) and complemented (Mycma 0076KOc) gene strains for mycma_0076 to understand the function of 0076 ferritin. Deletion of mycma_0076 in Mycma led to the transition in colony morphology from smooth to rough, alteration of the glycopeptidolipids spectra, increased permeability of the envelope, reduction in biofilm formation, increased susceptibility to antimicrobials and hydrogen peroxide-induced oxidative stress, and decreased internalization by macrophages. This study shows that Mycma_0076 ferritin in Mycma is involved in resistance to oxidative stress and antimicrobials, and alteration of cell envelope architecture. KEY POINTS: • Deletion of the mycma_0076 gene altered colony morphology to rough; • Mycma 0076KO changed GPL profile; • Absence of Mycma_0076 ferritin results in increased susceptibility to antimicrobials and oxidative stress in Mycma. Legend: a In wild-type M. abscessus subsp. massiliense strain, iron is captured from the environment by carboxymycobactins and mycobactins (1). Iron-dependent regulator (IdeR) proteins bind to ferrous iron (Fe⁺²) in the bacterial cytoplasm leading to the activation of the IdeR-Fe⁺² complex (2). The activated complex binds to the promoter regions of iron-dependent genes, called iron box, which in turn help in the recruitment of RNA polymerase to promote transcription of genes such as mycma_0076 and mycma_0077 ferritin genes (3). Mycma_0076 and Mycma_0077 ferritins bind to excess iron in the medium and promote Fe²⁺ oxidation into ferric iron (Fe³⁺) and store iron molecules to be released under iron scarcity conditions. (4) Genes related to biosynthesis and transport of glycopeptidolipids (GPL) are expressed normally and the cell envelope is composed of different GPL species (colored squares represented on the cell surface (GPLs). Consequently, WT Mycma present smooth colony phenotype (5). b In Mycma 0076KO strain, the lack of ferritin 0076 causes overexpression of mycma_0077 (6), but does not restore wild-type iron homeostasis and thus may result in free intracellular iron, even in the presence of miniferritins (MaDps). The excess iron potentiates oxidative stress (7) by generating hydroxyl radicals through Fenton Reaction. During this process, through an unknown mechanism, that could involve Lsr2 (8), the expression of GPL synthesis locus is regulated positively and/or negatively, resulting in alteration of GPL composition in the membrane (as represented by different colors of squares on the cell surface), resulting in a rough colony phenotype (9). The changes of GPL can increase cell wall permeability, contributing to antimicrobial susceptibility (10).

Mutations in the anti-sigma H factor RshA confer resistance to econazole and clotrimazole in Mycobacterium smegmatis

Azole drugs such as econazole, are active on Mycobacterium tuberculosis and Mycobacterium smegmatis; however, the identification of their target(s) is still pending. It has been reported that mutations in the non-essential system mmpL5-mmpS5 conferred resistance to econazole in M. tuberculosis. We herein report that an azole-resistant mutant screen in M. smegmatis rendered mutations in rshA, encoding a non-essential anti-sigma H protein.

Weirdo19ES is a novel singleton mycobacteriophage that selects for glycolipid deficient phage-resistant M. smegmatis mutants

The sequencing and bioinformatics analysis of bacteriophages infecting mycobacteria has yielded a large amount of information on their evolution, including that on their environmental propagation on other genera such as Gordonia, closely related to Mycobacterium. However, little is known on mycobacteriophages cell biology such as the nature of their receptor(s) or their replication cycle. As part of our on-going screening for novel mycobacteriophages, we herein report the isolation and genome bioinformatics analysis of Weirdo19ES, a singleton Siphoviridae temperate mycobacteriophage with a 70.19% GC content. Nucleotide and protein sequence comparison to actinobacteriophage databases revealed that Weirdo19ES shows low homology to Gordonia phage Ruthy and mycobacteriophages falling in clusters Q and G and to singleton DS6A.Weirdo19ES also displays uncommon features such as a very short Lysin A gene (with only one enzymatic domain) and two putative HNH endonucleases. Mycobacterium smegmatis mutants resistant to Weirdo19ES are cross- resistant to I3. In agreement with that phenotype, analysis of cell envelope of those mutants showed that Weirdo19ES shares receptors with the transducing mycobacteriophage I3.This singleton mycobacteriophage adds up to the uncommonness of local mycobacteriophages previously isolated by our group and helps understanding the nature of mycobacteriophage receptors.

Inhibitors of Mycobacterium marinum virulence identified in a Dictyostelium discoideum host model

Tuberculosis remains one of the major threats to public health worldwide. Given the prevalence of multi drug resistance (MDR) in Mycobacterium tuberculosis strains, there is a strong need to develop new anti-mycobacterial drugs with modes of action distinct from classical antibiotics. Inhibitors of mycobacterial virulence might target new molecular processes and may represent a potential new therapeutic alternative. In this study, we used a Dictyostelium discoideum host model to assess virulence of Mycobacterium marinum and to identify compounds inhibiting mycobacterial virulence. Among 9995 chemical compounds, we selected 12 inhibitors of mycobacterial virulence that do not inhibit mycobacterial growth in synthetic medium. Further analyses revealed that 8 of them perturbed functions requiring an intact mycobacterial cell wall such as sliding motility, bacterial aggregation or cell wall permeability. Chemical analogs of two compounds were analyzed. Chemical modifications altered concomitantly their effect on sliding motility and on mycobacterial virulence, suggesting that the alteration of the mycobacterial cell wall caused the loss of virulence. We characterized further one of the selected compounds and found that it inhibited the ability of mycobacteria to replicate in infected cells. Together these results identify new antimycobacterial compounds that represent new tools to unravel the molecular mechanisms controlling mycobacterial pathogenicity. The isolation of compounds with anti-virulence activity is the first step towards developing new antibacterial treatments.

Efficacy of moxifloxacin & econazole against multidrug resistant (MDR) Mycobacterium tuberculosis in murine model

Background & objectives:

Studies have shown the bactericidal potential of econazole and clotrimazole against Mycobacterium tuberculosis under in vitro and ex vivo conditions along with their synergism with conventional antituberculosis drugs. These molecules were also found to be effective against different multidrug resistant (MDR) M. tuberculosis isolates in vitro. Hence the present study was designed to evaluate the in vivo antimycobacterial potential of moxifloxacin and econazole alone and in combination against multidrug resistant tuberculosis (MDR-TB) in a mice model.

Methods:

Mice were infected with 2.5×10⁷ bacilli of MDR strain of M. tuberculosis by aerosol route of infection. After four weeks of infection, chemotherapy was started orally by moxifloxacin 8.0 mg/kg body wt and econazole 3.3 mg/kg alone and in combination, as well as with four first line anti-tuberculosis drugs as a positive control. The animals were sacrificed and the lungs and spleen were excised under aspetic conditions. The tissues were homogenized with sterile normal saline, an aliquot of the homogenate was plated on Middlebrook 7H11 agar supplemented with oleate albumin dextrose catalase (OADC) and incubated at 37°C for four weeks. The number of visible and individual colonies were counted.

Results:

The first line anti-tuberculosis drugs (RIF+INH+EMB+PZA) after eight weeks of therapy had no impact as the bacillary load in lungs and spleens remained unchanged. However, econazole, moxifloxacin alone as well as in combination significantly reduced the bacillary load in lungs as well as in spleens of MDR-TB bacilli infected mice.

Interpretation & conclusions:

Co-administration of the two drugs (econazole and moxifloxacin) to MDR-TB strain JAL-7782 infected mice exhibited additive effect, the efficacy of the drugs in combination being higher as compared with ECZ or MOX alone. These results were substantiated by histopathological studies. This study suggests the utility of econazole for the treatment of MDR tuberculosis and warrants further work in this direction.

Identification of 1-[4-Benzyloxyphenyl)-but-3-enyl]-1H-azoles as New Class of Antitubercular and Antimicrobial Agents

A series of 1-[(4-benzyloxyphenyl)-but-3-enyl]-1H-azoles has been identified as potent antitubercular agents against Mycobacterium tuberculosis. Synthesis of compounds involved acid catalyzed ring-opening of cyclopropyl ring of phenyl cyclopropyl methanols followed by nucleophilic attack of the azoles on the carbocation intermediates. Several of the compounds 26, 34, and 36 exhibited significant antitubercular activities with MIC value as low as 1.56, 1.56, and 0.61 μg/mL, respectively, comparable to many standard drugs. These compounds were also screened against other strains of bacteria and fungi, and few of them showed good antifungal activity against A. fumigatus, responsible for lung infection.

Predicting the in vivo mechanism of action for drug leads using NMR metabolomics

New strategies are needed to circumvent increasing outbreaks of resistant strains of pathogens and to expand the dwindling supply of effective antimicrobials. A common impediment to drug development is the lack of an easy approach to determine the in vivo mechanism of action and efficacy of novel drug leads. Toward this end, we describe an unbiased approach to predict in vivo mechanisms of action from NMR metabolomics data. Mycobacterium smegmatis, a non-pathogenic model organism for Mycobacterium tuberculosis, was treated with 12 known drugs and 3 chemical leads identified from a cell-based assay. NMR analysis of drug-induced changes to the M. smegmatis metabolome resulted in distinct clustering patterns correlating with in vivo drug activity. The clustering of novel chemical leads relative to known drugs provides a mean to identify a protein target or predict in vivo activity.

Glycopeptidolipids: immuno-modulators in greasy mycobacterial cell envelope

Species of opportunistic mycobacteria are the major causative agent for disseminating pulmonary infections in immuno-compromised individuals. These naturally resistant strains recruit a unique type of glycolipid known as glycopeptidolipids (GPLs), noncovalently attached to the outer surface of their thick lipid rich cell envelope. Species specific GPLs constitute the chemical determinants of most nontuberculous mycobacterial serotypes, and their absence from the cell surface confers altered colony morphology, hydrophobicity, and inability to grow as biofilms. The objective of this review is to present a comprehensive account and highlight the renewed interest on this much neglected group of pleiotropic molecules with respect to their structural diversity and biosynthesis. In addition, the role of GPLs in mycobacterial survival, both intracellular and in the environment is also discussed. It also explores the possibility of identifying new targets for intervening Mycobacterium avium complex-related infections. These antigenic molecules have been considered to play a pivotal role in immune suppression and can also induce various cytokine mediated innate immune responses, the molecular mechanism of which remains obscure.

Modifying culture conditions in chemical library screening identifies alternative inhibitors of mycobacteria

In this study, application of a dual absorbance/fluorescence assay to a chemical library screen identified several previously unknown inhibitors of mycobacteria. In addition, growth conditions had a significant effect on the activity profile of the library. Some inhibitors such as Se-methylselenocysteine were detected only when screening was performed under nutrient-limited culture conditions as opposed to nutrient-rich culture conditions. We propose that multiple culture condition library screening is required for complete inhibitory profiling and for maximal antimycobacterial compound detection.

Defects in glycopeptidolipid biosynthesis confer phage I3 resistance in Mycobacterium smegmatis

Mycobacteriophages have played an important role in the development of genetic tools and diagnostics for pathogenic mycobacteria, including Mycobacterium tuberculosis. However, despite the isolation of numerous phages that infect mycobacteria, the mechanisms of mycobacteriophage infection remain poorly understood, and knowledge about phage receptors is minimal. In an effort to identify the receptor for phage I3, we screened a library of Mycobacterium smegmatis transposon mutants for phage-resistant strains. All four phage I3-resistant mutants isolated were found to have transposon insertions in genes located in a cluster involved in the biosynthesis of the cell-wall-associated glycopeptidolipid (GPL), and consequently the mutants did not synthesize GPLs. The loss of GPLs correlated specifically with phage I3 resistance, as all mutants retained sensitivity to two other mycobacteriophages: D29 and Bxz1. In order to define the minimal receptor for phage I3, we then tested the phage sensitivity of previously described GPL-deficient mutants of M. smegmatis that accumulate biosynthesis intermediates of GPLs. The results indicated that, while the removal of most sugar residues from the fatty acyl tetrapeptide (FATP) core of GPL did not affect sensitivity to phage I3, a single methylated rhamnose, transferred by the rhamnosyltransferase Gtf2 to the FATP core, was critical for phage binding.

The Mycobacterium tuberculosis cytochrome P450 system

Tuberculosis remains a leading cause of human mortality. The emergence of strains of Mycobacterium tuberculosis, the causative agent, that are resistant to the major frontline antitubercular drugs increases the urgency for the development of new therapeutic agents. Sequencing of the M. tuberculosis genome revealed the existence of 20 cytochrome P450 enzymes, some of which are potential candidates for drug targeting. The recent burst of studies reporting microarray-based gene essentiality and transcriptome analyses under in vitro, ex vivo and in vivo conditions highlight the importance of selected P450 isoforms for M. tuberculosis viability and pathogenicity. Current knowledge of the structural and biochemical properties of the M. tuberculosis P450 enzymes and their putative redox partners is reviewed, with an emphasis on findings related to their physiological function(s) as well as their potential as drug targets.

Identification and structural basis of the reaction catalyzed by CYP121, an essential cytochrome P450 in Mycobacterium tuberculosis

The gene encoding the cytochrome P450 CYP121 is essential for Mycobacterium tuberculosis. However, the CYP121 catalytic activity remains unknown. Here, we show that the cyclodipeptide cyclo(l-Tyr-l-Tyr) (cYY) binds to CYP121, and is efficiently converted into a single major product in a CYP121 activity assay containing spinach ferredoxin and ferredoxin reductase. NMR spectroscopy analysis of the reaction product shows that CYP121 catalyzes the formation of an intramolecular C-C bond between 2 tyrosyl carbon atoms of cYY resulting in a novel chemical entity. The X-ray structure of cYY-bound CYP121, solved at high resolution (1.4 A), reveals one cYY molecule with full occupancy in the large active site cavity. One cYY tyrosyl approaches the heme and establishes a specific H-bonding network with Ser-237, Gln-385, Arg-386, and 3 water molecules, including the sixth iron ligand. These observations are consistent with low temperature EPR spectra of cYY-bound CYP121 showing a change in the heme environment with the persistence of the sixth heme iron ligand. As the carbon atoms involved in the final C-C coupling are located 5.4 A apart according to the CYP121-cYY complex crystal structure, we propose that C-C coupling is concomitant with substrate tyrosyl movements. This study provides insight into the catalytic activity, mechanism, and biological function of CYP121. Also, it provides clues for rational design of putative CYP121 substrate-based antimycobacterial agents.

Characterization of active site structure in CYP121. A cytochrome P450 essential for viability of Mycobacterium tuberculosis H37Rv

Mycobacterium tuberculosis (Mtb) cytochrome P450 gene CYP121 is shown to be essential for viability of the bacterium in vitro by gene knock-out with complementation. Production of CYP121 protein in Mtb cells is demonstrated. Minimum inhibitory concentration values for azole drugs against Mtb H37Rv were determined, the rank order of which correlated well with Kd values for their binding to CYP121. Solution-state spectroscopic, kinetic, and thermodynamic studies and crystal structure determination for a series of CYP121 active site mutants provide further insights into structure and biophysical features of the enzyme. Pro346 was shown to control heme cofactor conformation, whereas Arg386 is a critical determinant of heme potential, with an unprecedented 280-mV increase in heme iron redox potential in a R386L mutant. A homologous Mtb redox partner system was reconstituted and transported electrons faster to CYP121 R386L than to wild type CYP121. Heme potential was not perturbed in a F338H mutant, suggesting that a proposed P450 superfamily-wide role for the phylogenetically conserved phenylalanine in heme thermodynamic regulation is unlikely. Collectively, data point to an important cellular role for CYP121 and highlight its potential as a novel Mtb drug target.

Synthesis and biological evaluation of NAS-21 and NAS-91 analogues as potential inhibitors of the mycobacterial FAS-II dehydratase enzyme Rv0636

The identification of potential new anti-tubercular chemotherapeutics is paramount due to the recent emergence of extensively drug-resistant strains of Mycobacterium tuberculosis (XDR-TB). Libraries of NAS-21 and NAS-91 analogues were synthesized and evaluated for their whole-cell activity against Mycobacterium bovis BCG. NAS-21 analogues 1 and 2 demonstrated enhanced whole-cell activity in comparison to the parental compound, and an M. bovis BCG strain overexpressing the dehydratase enzyme Rv0636 was resistant to these analogues. NAS-91 analogues with ortho-modifications gave enhanced whole-cell activity. However, extension with biphenyl modifications compromised the whole-cell activities of both NAS-21 and NAS-91 analogues. Interestingly, both libraries demonstrated in vitro activity against fatty acid synthase II (FAS-II) but not FAS-I in cell-free extracts. In in vitro assays of FAS-II inhibition, NAS-21 analogues 4 and 5 had IC₅₀ values of 28 and 19 μg ml⁻¹, respectively, for the control M. bovis strain, and the M. bovis BCG strain overexpressing Rv0636 showed a marked increase in resistance. In contrast, NAS-91 analogues demonstrated moderate in vitro activity, although increased resistance was again observed in FAS-II activity assays with the Rv0636-overexpressing strain. Fatty acid methyl ester (FAME) and mycolic acid methyl ester (MAME) analysis of M. bovis BCG and the Rv0636-overexpressing strain revealed that the effect of the drug was relieved in the overexpressing strain, further implicating and potentially identifying Rv0636 as the target for these known FabZ dehydratase inhibitors. This study has identified candidates for further development as drug therapeutics against the mycobacterial FAS-II dehydratase enzyme.

The mycobacterial glycopeptidolipids: structure, function, and their role in pathogenesis

Glycopeptidolipids (GPLs) are a class of glycolipids produced by several nontuberculosis-causing members of the Mycobacterium genus including pathogenic and nonpathogenic species. GPLs are expressed in different forms with production of highly antigenic, typeable serovar-specific GPLs in members of the Mycobacterium avium complex (MAC). M. avium and M. intracellulare, which comprise this complex, are slow-growing mycobacteria noted for producing disseminated infections in AIDS patients and pulmonary infections in non-AIDS patients. Previous studies have defined the gene cluster responsible for GPL biosynthesis and more recent work has characterized the function of the individual genes. Current research has also focused on the GPL's role in colony morphology, sliding motility, biofilm formation, immune modulation and virulence. These topics, along with new information on the enzymes involved in GPL biosynthesis, are the subject of this review.

Structure, function and drug targeting in Mycobacterium tuberculosis cytochrome P450 systems

The human pathogen Mycobacterium tuberculosis has made a dramatic resurgence in recent years. Drug resistant and multidrug resistant strains are prevalent, and novel antibiotic strategies are desperately needed to counter Mtb's global spread. The M. tuberculosis genome sequence revealed an unexpectedly high number of cytochrome P450 (P450) enzymes (20), and parallel studies indicated that P450-inhibiting azole drugs had potent anti-mycobacterial activity. This article reviews current knowledge of structure/function of P450s and redox partner systems in M. tuberculosis. Recent research has highlighted potential drug target Mtb P450s and provided evidence for roles of selected P450 isoforms in host lipid and sterol/steroid transformations. Structural analysis of key Mtb P450s has provided fundamental information on the nature of the heme binding site, P450 interactions with azole drugs, the biochemical nature of cytochrome P420, and novel mutational adaptations by which azole binding to P450s may be diminished to facilitate azole resistance.

Azole antifungals as novel chemotherapeutic agents against murine tuberculosis

The present study was designed to evaluate the in vivo antimycobacterial potential of econazole alone and in combination with antitubercular drugs against tuberculosis in mice. Econazole was found to reduce bacterial burden by 90% in the lungs and spleen of mice infected with 1 x 10(7) cells of Mycobacterium tuberculosis and was found to be equipotent to rifampicin. Further, our results indicate that econazole can replace rifampicin/isoniazid as well as both rifampicin and isoniazid in chemotherapy of murine tuberculosis. Econazole alone or in combination with antitubercular drugs did not produce any hepatotoxicity in normal or M. tuberculosis-infected mice.

The preponderance of P450s in the Mycobacterium tuberculosis genome

The genome of Mycobacterium tuberculosis (Mtb) encodes 20 different cytochrome P450 enzymes (P450s). P450s are mono-oxygenases, which are historically considered to facilitate prokaryotic usage of unusual carbon sources. However, their preponderance in Mtb strongly indicates crucial physiological functions, as does the fact that polycyclic azoles (known P450 inhibitors) have potent anti-mycobacterial effects. Recent structural and enzyme characterization data reveal novel features for at least two Mtb P450s (CYP121 and CYP51). Genome analysis, knockout studies and structural comparisons signify important roles in cell biology and pathogenesis for various P450s and redox partner enzymes in Mtb. Elucidation of structure, function and metabolic roles will be essential in targeting the P450s as an 'Achilles heel' in this major human pathogen.

Cloning and characterization of CYP51 from Mycobacterium avium

Mycobacterium avium complex (MAC) causes chronic lung disease in immunocompetent people and disseminated infection in patients with AIDS. MAC is intrinsically resistant to many conventional antimycobacterial agents, it develops drug resistance rapidly to macrolide antibiotics, and patients with MAC infection experience frequent relapses or the inability to completely eradicate the infection with current treatment. Treatment regimens are prolonged and complicated by drug toxicity or intolerances. We sought to identify biochemical pathways in MAC that can serve as targets for novel antimycobacterial treatment. The cytochrome P450 enzyme, CYP51, catalyzes an essential early step in sterol metabolism, removing a methyl group from lanosterol in animals and fungi, or from obtusifoliol in plants. Azoles inhibit CYP51 function, leading to an accumulation of methylated sterol precursors. This perturbation of normal sterol metabolism compromises cell membrane integrity, resulting in growth inhibition or cell death. We have cloned and characterized a CYP51 from MAC that functions as a lanosterol 14alpha-demethylase. We show the direct interactions of azoles with purified MAC-CYP51 by absorbance and electron paramagnetic resonance spectroscopy, and determine the minimum inhibitory concentrations (MICs) of econazole, ketoconazole, itraconazole, fluconazole, and voriconazole against MAC. Furthermore, we demonstrate that econazole has a MIC of 4 mug/ml and a minimum bacteriocidal concentration of 4 mug/ml, whereas ketoconazole has a MIC of 8 mug/ml and a minimum bacteriocidal concentration of 16 mug/ml. Itraconazole, voriconazole, and fluconazole did not inhibit MAC growth to any significant extent.