Mycobacterium tuberculosis cell envelope

Mycobacteriophage SWU1 gp39 can potentiate multiple antibiotics against Mycobacterium via altering the cell wall permeability

M. tuberculosis is intrinsically tolerant to many antibiotics largely due to the imperviousness of its unusual mycolic acid-containing cell wall to most antimicrobials. The emergence and increasingly widespread of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) revitalized keen interest in phage-inspired therapy. SWU1gp39 is a novel gene from mycobacteriophage SWU1 with unknown function. SWU1gp39 expressed in M. smegmatis conferred the host cell increased susceptibility to multiple antibiotics, including isoniazid, erythromycin, norfloxacin, ampicillin, ciprofloxacin, ofloxacin, rifampicin and vancomycin, and multiple environment stresses such as H2O2, heat shock, low pH and SDS. By using EtBr/Nile red uptake assays, WT-pAL-gp39 strain showed higher cell wall permeability than control strain WT-pAL. Moreover, the WT-pAL-gp39 strain produced more reactive oxygen species and reduced NAD(+)/NADH ratio. RNA-Seq transcriptomes of the WT-pAL-gp39 and WT-pAL revealed that the transcription of 867 genes was differentially regulated, including genes associated with lipid metabolism. Taken together, our results implicated that SWU1gp39, a novel gene from mycobacteriophage, disrupted the lipid metabolism of host and increased cell wall permeability, ultimately potentiated the efficacy of multiple antibiotics and stresses against mycobacteria.

Spectinamides: a new class of semisynthetic antituberculosis agents that overcome native drug efflux

Although the classical antibiotic spectinomycin is a potent bacterial protein synthesis inhibitor, poor antimycobacterial activity limits its clinical application for treating tuberculosis. Using structure-based design, we generated a new semisynthetic series of spectinomycin analogs with selective ribosomal inhibition and excellent narrow-spectrum antitubercular activity. In multiple murine infection models, these spectinamides were well tolerated, significantly reduced lung mycobacterial burden and increased survival. In vitro studies demonstrated a lack of cross resistance with existing tuberculosis therapeutics, activity against multidrug-resistant (MDR) and extensively drug-resistant tuberculosis and an excellent pharmacological profile. Key to their potent antitubercular properties was their structural modification to evade the Rv1258c efflux pump, which is upregulated in MDR strains and is implicated in macrophage-induced drug tolerance. The antitubercular efficacy of spectinamides demonstrates that synthetic modifications to classical antibiotics can overcome the challenge of intrinsic efflux pump-mediated resistance and expands opportunities for target-based tuberculosis drug discovery.

Test characteristics of urinary lipoarabinomannan and predictors of mortality among hospitalized HIV-infected tuberculosis suspects in Tanzania

Background

Tuberculosis is the most common cause of death among patients with HIV infection living in tuberculosis endemic countries, but many cases are not diagnosed pre-mortem. We assessed the test characteristics of urinary lipoarabinomannan (LAM) and predictors of mortality among HIV-associated tuberculosis suspects in Tanzania.

Methods

We prospectively enrolled hospitalized HIV-infected patients in Dar es Salaam, with ≥2 weeks of cough or fever, or weight loss. Subjects gave 2 mLs of urine to test for LAM using a commercially available ELISA, ≥2 sputum specimens for concentrated AFB smear and solid media culture, and 40 mLs of blood for culture.

Results

Among 212 evaluable subjects, 143 (68%) were female; mean age was 36 years; and the median CD4 count 86 cells/mm³. 69 subjects (33%) had culture confirmation of tuberculosis and 65 (31%) were LAM positive. For 69 cases of sputum or blood culture-confirmed tuberculosis, LAM sensitivity was 65% and specificity 86% compared to 36% and 98% for sputum smear. LAM test characteristics were not different in patients with bacteremia but showed higher sensitivity and lower specificity with decreasing CD4 cell count. Two month mortality was 64 (53%) of 121 with outcomes available. In multivariate analysis there was significant association of mortality with absence of anti-retroviral therapy (p = 0.004) and a trend toward association with a positive urine LAM (p = 0.16). Among culture-negative patients mortality was 9 (75%) of 12 in LAM positive patients and 27 (38%) of 71 in LAM negative patients (p = 0.02).

Conclusions

Urine LAM is more sensitive than sputum smear and has utility for the rapid diagnosis of culture-confirmed tuberculosis in this high-risk population. Mortality data raise the possibility that urine LAM may also be a marker for culture-negative tuberculosis.

Understanding the interaction of Lipoarabinomannan with membrane mimetic architectures

Lipoarabinomannan (LAM) is a critical virulence factor in the pathogenesis of Mycobacterium tuberculosis, the causative agent of tuberculosis. LAM is secreted in urine and serum from infected patients and is being studied as a potential diagnostic indicator for the disease. Herein, we present a novel ultra-sensitive and specific detection strategy for monomeric LAM based on its amphiphilic nature and consequent interaction with supported lipid bilayers. Our strategy involves the capture of LAM on waveguides functionalized with membrane mimetic architectures, followed by detection with a fluorescently labeled polyclonal antibody. This approach offers ultra-sensitive detection of lipoarabinomannan (10 fM, within 15 min) and may be extended to other amphiphilic markers. We also show that chemical deacylation of LAM completely abrogates its association with the supported lipid bilayers. The loss of signal using the waveguide assay for deacylated LAM, as well as atomic force microscopy (AFM) images that show no change in height upon addition of deacylated LAM support this hypothesis. Mass spectrometry of chemically deacylated LAM indicates the presence of LAM-specific carbohydrate chains, which maintain antigenicity in immunoassays. Further, we have developed the first three-dimensional structural model of mannose-capped LAM that provides insights into the orientation of LAM on supported lipid bilayers.

Urine for the diagnosis of tuberculosis: current approaches, clinical applicability, and new developments

PURPOSE OF REVIEW

Urine is increasingly being investigated as a convenient clinical sample for the identification of mycobacterial products for the diagnosis of tuberculosis. The available literature on mycobacterial lipoarabinomannan (LAM) and urine mycobacterial DNA is reviewed.

RECENT FINDINGS

The available data, despite being extracted from heterogeneous clinical populations and different clinical subgroups, indicate that urine LAM has little diagnostic utility in unselected tuberculosis suspects; however, test characteristics improve in HIV-infected patients, particularly those with advanced immunosuppression (CD4 cell count <200 cells/microl). Methodologies for urine PCR for detection of mycobacterial DNA vary across studies and focus is on standardizing assays with respect to specimen collection, assay design, and processing methodology.

SUMMARY

Both the urine LAM and PCR for mycobacterial DNA are being evaluated in different geographical settings. Urine LAM currently offers little utility for the diagnosis of tuberculosis in unselected populations. However, urine LAM appears promising as a diagnostic tool in HIV-infected patients with CD4 cell counts less than 200 cells/microl in different clinical settings. Further developmental studies are required to enhance the performance of the assays, and their usefulness over sputum microscopy in HIV-infected patients with advanced immunosuppression requires definition in large cohort studies.

Inositol monophosphate phosphatase genes of Mycobacterium tuberculosis

BACKGROUND

Mycobacteria use inositol in phosphatidylinositol, for anchoring lipoarabinomannan (LAM), lipomannan (LM) and phosphatidylinosotol mannosides (PIMs) in the cell envelope, and for the production of mycothiol, which maintains the redox balance of the cell. Inositol is synthesized by conversion of glucose-6-phosphate to inositol-1-phosphate, followed by dephosphorylation by inositol monophosphate phosphatases (IMPases) to form myo-inositol. To gain insight into how Mycobacterium tuberculosis synthesises inositol we carried out genetic analysis of the four IMPase homologues that are present in the Mycobacterium tuberculosis genome.

RESULTS

Mutants lacking either impA (Rv1604) or suhB (Rv2701c) were isolated in the absence of exogenous inositol, and no differences in levels of PIMs, LM, LAM or mycothiol were observed. Mutagenesis of cysQ (Rv2131c) was initially unsuccessful, but was possible when a porin-like gene of Mycobacterium smegmatis was expressed, and also by gene switching in the merodiploid strain. In contrast, we could only obtain mutations in impC (Rv3137) when a second functional copy was provided in trans, even when exogenous inositol was provided. Experiments to obtain a mutant in the presence of a second copy of impC containing an active-site mutation, in the presence of porin-like gene of M. smegmatis, or in the absence of inositol 1-phosphate synthase activity, were also unsuccessful. We showed that all four genes are expressed, although at different levels, and levels of inositol phosphatase activity did not fall significantly in any of the mutants obtained.

CONCLUSIONS

We have shown that neither impA, suhB nor cysQ is solely responsible for inositol synthesis. In contrast, we show that impC is essential for mycobacterial growth under the conditions we used, and suggest it may be required in the early stages of mycothiol synthesis.

Synthesis of deoxygenated alpha(1-->5)-linked arabinofuranose disaccharides as substrates and inhibitors of arabinosyltransferases of Mycobacterium tuberculosis

Arabinosyltransferases (AraTs) play a critical role in mycobacterial cell wall biosynthesis and are potential drug targets for the treatment of tuberculosis, especially multi-drug resistant forms of M. tuberculosis (MTB). Herein, we report the synthesis and acceptor/inhibitory activity of Araf alpha(1-->5) Araf disaccharides possessing deoxygenation at the reducing sugar of the disaccharide. Deoxygenation at either the C-2 or C-3 position of Araf was achieved via a free radical procedure using xanthate derivatives of the hydroxyl group. The alpha(1-->5)-linked disaccharides were produced by coupling n-octyl alpha-Araf 2-/3-deoxy, 2-fluoro glycosyl acceptors with an Araf thioglycosyl donor. The target disaccharides were tested in a cell free mycobacterial AraTs assay as well as an in vitro assay against MTB H(37)Ra and M. avium complex strains.

Autophagy, an immunologic magic bullet: Mycobacterium tuberculosis phagosome maturation block and how to bypass it

Mycobacterium tuberculosis is a facultative intracellular pathogen that parasitizes host macrophages where it persists in immature phagosomes by avoiding their maturation into phagolysosomes. The mechanisms of how M. tuberculosis inhibits phagolysosome biogenesis have been researched in detail and the maturation block at least partially depends on the manipulation of host phosphoinositide interconversions, with phosphatidylinositol 3-phosphate (PI3P) being a central target since it has been shown to be required for phagolysosome biogenesis. PI3P earmarks intracellular organelles for binding and assembly of effector molecules that interact with PI3P or its derivatives, including Class E Vps proteins such as Hrs and ESCRT components, early endosome antigen 1, which are required for sequential protein and membrane sorting within the endosomal and, by extension, phagosomal systems. In a search of a cellular mechanism that can bypass the tubercule bacillus-imposed PI3P block, researchers have uncovered a new general bactericidal process, autophagy, which can eliminate intracellular pathogens. This is a new, rapidly growing field replete with possibilities for novel, previously untried immunologic and pharmacologic interventions applicable not only to TB but to other stubborn bacterial, parasitic and viral diseases.

Disclosure of the mycobacterial outer membrane: cryo-electron tomography and vitreous sections reveal the lipid bilayer structure

The cell walls of mycobacteria form an exceptional permeability barrier, and they are essential for virulence. They contain extractable lipids and long-chain mycolic acids that are covalently linked to peptidoglycan via an arabinogalactan network. The lipids were thought to form an asymmetrical bilayer of considerable thickness, but this could never be proven directly by microscopy or other means. Cryo-electron tomography of unperturbed or detergent-treated cells of Mycobacterium smegmatis embedded in vitreous ice now reveals the native organization of the cell envelope and its delineation into several distinct layers. The 3D data and the investigation of ultrathin frozen-hydrated cryosections of M. smegmatis, Myobacterium bovis bacillus Calmette-Guérin, and Corynebacterium glutamicum identified the outermost layer as a morphologically symmetrical lipid bilayer. The structure of the mycobacterial outer membrane necessitates considerable revision of the current view of its architecture. Conceivable models are proposed and discussed. These results are crucial for the investigation and understanding of transport processes across the mycobacterial cell wall, and they are of particular medical relevance in the case of pathogenic mycobacteria.

Mycobacterium tuberculosis Rv2224c modulates innate immune responses

Tuberculosis remains a major global health problem that kills up to 2 million people annually. Central to the success of Mycobacterium tuberculosis (Mtb) as a pathogen is its ability to evade host immunity and to establish a chronic infection. Although its primary intracellular niche is within macrophages, the underlying molecular mechanisms are poorly understood. Here we show that Rv2224c, a cell envelope-associated predicted protease, is critical for Mtb virulence. Disruption of Rv2224c led to prolonged survival of infected mice and highly reduced lung pathology. Absence of Rv2224c enhanced host innate immune responses, compromised the intracellular survival of Mtb in macrophages, and increased its susceptibility to lysozyme. We provide insights into the molecular basis for Rv2224c function by showing that Rv2224c activity promotes processing and extracellular release of the Mtb protein, GroEL2. Inhibition of Rv2224c and its targets offers opportunities for therapeutic interventions and immune-modulatory strategies.

Disaccharide analogs as probes for glycosyltransferases in Mycobacterium tuberculosis

Glycosyltransferases (GTs) play a crucial role in mycobacterial cell wall biosynthesis and are necessary for the survival of mycobacteria. Hence, these enzymes are potential new drug targets for the treatment of tuberculosis (TB), especially multiple drug-resistant TB (MDR-TB). Herein, we report the efficient syntheses of Araf(alpha 1-->5)Araf, Galf(beta 1-->5)Galf, and Galf(beta 1-->6)Galf disaccharides possessing a 5-N,N-dimethylaminonaphthalene-1-sulfonamidoethyl (dansyl) unit that were prepared as fluorescent disaccharide acceptors for arabinosyl- and galactosyl-transferases, respectively. Such analogs may offer advantages relative to radiolabeled acceptors or donors for studying the enzymes and for assay development and compound screening. Additionally, analogs possessing a 5-azidonaphthalene-1-sulfonamidoethyl unit were prepared as photoaffinity probes for their potential utility in studying active site labeling of the GTs (arabinosyl and galactosyl) in Mycobacterium tuberculosis (MTB). Beyond their preparation, initial biological testing and kinetic analysis of these disaccharides as acceptors toward glycosyltransferases are also presented.

Study on the Cell Wall Skeleton Derived from Mycobacterium bovis BCG Tokyo 172 (SMP-105): Establishment of Preparation and Analytical Methods

Mycobacterial cell walls have diverse adjuvant activities, and in particular, cell wall skeleton (CWS) of Mycobacterium bovis BCG has been expected as a drug for tumor immunotherapy. However, its molecular structure-biological activity relationship has not been fully elucidated despite more than 30 years of intensive research. Since it is important to secure purified CWS for such investigation, we established a preparation method of CWS from M. bovis BCG Tokyo 172 (SMP-105) and developed accurate, precise, and reliable analytical methods, based on previous reports. Furthermore, we confirmed that SMP-105 is composed of mycolic acids; arabinogalactan consisting of arabinose, galactose, and rhamnose; and peptidoglycan consisting of alanine, glutamic acid, diaminopimeric acid, muramic acid, glucosamine, and galactosamine. We also determined the levels of potential impurities that might be contaminated in the original bacterium or arise during the manufacturing process, such as glucose, mannose, non-constituted amino acids, as well as nucleic acid, trehaolse di-mycolate, and bacterial endotoxins. These results demonstrated that the prepared SMP-105 was of sufficient quality for research into the chemistry, bioactivity, and structure-activity relationship of CWS.

Structure-based inhibitor design of AccD5, an essential acyl-CoA carboxylase carboxyltransferase domain of Mycobacterium tuberculosis

Mycolic acids and multimethyl-branched fatty acids are found uniquely in the cell envelope of pathogenic mycobacteria. These unusually long fatty acids are essential for the survival, virulence, and antibiotic resistance of Mycobacterium tuberculosis. Acyl-CoA carboxylases (ACCases) commit acyl-CoAs to the biosynthesis of these unique fatty acids. Unlike other organisms such as Escherichia coli or humans that have only one or two ACCases, M. tuberculosis contains six ACCase carboxyltransferase domains, AccD1-6, whose specific roles in the pathogen are not well defined. Previous studies indicate that AccD4, AccD5, and AccD6 are important for cell envelope lipid biosynthesis and that its disruption leads to pathogen death. We have determined the 2.9-Angstroms crystal structure of AccD5, whose sequence, structure, and active site are highly conserved with respect to the carboxyltransferase domain of the Streptomyces coelicolor propionyl-CoA carboxylase. Contrary to the previous proposal that AccD4-5 accept long-chain acyl-CoAs as their substrates, both crystal structure and kinetic assay indicate that AccD5 prefers propionyl-CoA as its substrate and produces methylmalonyl-CoA, the substrate for the biosyntheses of multimethyl-branched fatty acids such as mycocerosic, phthioceranic, hydroxyphthioceranic, mycosanoic, and mycolipenic acids. Extensive in silico screening of National Cancer Institute compounds and the University of California, Irvine, ChemDB database resulted in the identification of one inhibitor with a K(i) of 13.1 microM. Our results pave the way toward understanding the biological roles of key ACCases that commit acyl-CoAs to the biosynthesis of cell envelope fatty acids, in addition to providing a target for structure-based development of antituberculosis therapeutics.

Inhibition of Mycobacterium smegmatis gene expression and growth using antisense peptide nucleic acids

Antisense agents that inhibit genes at the mRNA level are attractive tools for genome-wide studies and drug target validation. The approach may be particularly well suited to studies of bacteria that are difficult to manipulate with standard genetic tools. Antisense peptide nucleic acids (PNA) with attached carrier peptides can inhibit gene expression in Escherichia coli and Staphylococcus aureus. Here we asked whether peptide-PNAs could mediate antisense effects in Mycobacterium smegmatis. We first targeted the gfp reporter gene and observed dose- and sequence-dependent inhibition at low micromolar concentrations. Sequence alterations within both the PNA and target mRNA sequences eliminated inhibition, strongly supporting an antisense mechanism of inhibition. Also, antisense PNAs with various attached peptides showed improved anti-gfp effects. Two peptide-PNAs targeted to the essential gene inhA were growth inhibitory and caused cell morphology changes that resemble that of InhA-depleted cells. Therefore, antisense peptide-PNAs can efficiently and specifically inhibit both reporter and endogenous essential genes in mycobacteria.

Vaccines for tuberculosis: novel concepts and recent progress

Three-quarters of a century after the introduction of Mycobacterium bovis BCG, the first tuberculosis vaccine, new vaccines for tuberculosis are finally entering clinical trials. This breakthrough is based not only on advances in proteomics and genomics which have made the construction of new vaccines possible, but also on a greatly expanded knowledge of the immunology of tuberculosis. Here we review our current understanding of how Mycobacterium tuberculosis subverts or survives the host's immune response to cause disease and why the current vaccination strategy, which relies on BCG, is only partially successful in countering the pathogen. This provides a background for describing the new generation of vaccines designed to supplement or replace the current vaccine and the different approaches they take to stimulate immunity against M. tuberculosis.

Mycobacterium tuberculosis phagosome maturation arrest: mycobacterial phosphatidylinositol analog phosphatidylinositol mannoside stimulates early endosomal fusion

Mycobacterium tuberculosis is a facultative intracellular pathogen that parasitizes macrophages by modulating properties of the Mycobacterium-containing phagosome. Mycobacterial phagosomes do not fuse with late endosomal/lysosomal organelles but retain access to early endosomal contents by an unknown mechanism. We have previously reported that mycobacterial phosphatidylinositol analog lipoarabinomannan (LAM) blocks a trans-Golgi network-to-phagosome phosphatidylinositol 3-kinase-dependent pathway. In this work, we extend our investigations of the effects of mycobacterial phosphoinositides on host membrane trafficking. We present data demonstrating that phosphatidylinositol mannoside (PIM) specifically stimulated homotypic fusion of early endosomes in an ATP-, cytosol-, and N-ethylmaleimide sensitive factor-dependent manner. The fusion showed absolute requirement for small Rab GTPases, and the stimulatory effect of PIM increased upon partial depletion of membrane Rabs with RabGDI. We found that stimulation of early endosomal fusion by PIM was higher when phosphatidylinositol 3-kinase was inhibited by wortmannin. PIM also stimulated in vitro fusion between model phagosomes and early endosomes. Finally, PIM displayed in vivo effects in macrophages by increasing accumulation of plasma membrane-endosomal syntaxin 4 and transferrin receptor on PIM-coated latex bead phagosomes. In addition, inhibition of phagosomal acidification was detected with PIM-coated beads. The effects of PIM, along with the previously reported action of LAM, suggest that M. tuberculosis has evolved a two-prong strategy to modify its intracellular niche: its products block acquisition of late endosomal/lysosomal constituents, while facilitating fusion with early endosomal compartments.

Investigating lipid composition effects on the mechanosensitive channel of large conductance (MscL) using molecular dynamics simulations

Previous experimental work has shown that the functional properties of the mechanosensitive channel of large conductance (MscL) are affected by variations in lipid composition. Here, we utilize molecular dynamics simulations of Mycobacterium tuberculosis MscL to investigate such lipid composition effects on a molecular level. In particular, two sets of simulations were performed. In the first, trajectories using lipids with different headgroups (phosphatidylcholine and phosphatidylethanolamine) were compared. Protein-lipid interactions were clearly altered by the headgroup changes, leading to conformational differences in the C-terminal region of M. tuberculosis MscL. In the second set of simulations, lipid tails were gradually shortened, thinning the membrane over a molecular dynamics trajectory. These simulations showed evidence of hydrophobic matching between MscL and the lipid membrane, as previously proposed. For all simulations, protein-lipid interaction energies in the second transmembrane region were correlated to mutagenic data, emphasizing the importance of lipid interactions for proper MscL function.

Molecular dynamics simulations of wild-type and mutant forms of the Mycobacterium tuberculosis MscL channel

The crystal structure of the Mycobacterium tuberculosis homolog of the bacterial mechanosensitive channel of large conductance (Tb-MscL) provides a unique opportunity to consider mechanosensitive signal transduction at the atomic level. Molecular dynamics simulations of the Tb-MscL channel embedded in an explicit lipid bilayer and of its C-terminal helical bundle alone in aqueous solvent were performed. C-terminal calculations imply that although the helix bundle structure is relatively unstable at physiological pH, it may have been stabilized under low pH conditions such as those used in the crystallization of the channel. Specific mutations to the C-terminal region, which cause a similar conservation of the crystal structure conformation, have also been identified. Full channel simulations were performed for the wild-type channel and two experimentally characterized gain-of-function mutants, V21A and Q51E. The wild-type Tb-MscL trajectory gives insight into regions of relative structural stability and instability in the channel structure. Channel mutations led to observable changes in the trajectories, such as an alteration of intersubunit interactions in the Q51E mutant. In addition, interesting patterns of protein-lipid interactions, such as hydrogen bonding, arose in the simulations. These and other observations from the simulations are relevant to previous and ongoing experimental studies focusing on characterization of the channel.

Biosynthesis of the arabinogalactan-peptidoglycan complex of Mycobacterium tuberculosis

The compositional complexity of the mycobacterial cell envelope differentiates Mycobacterium species from most other prokaryotes. Historically, research in this area has focused on the elucidation of the structure of the mycobacterial cell envelope with the result that the structures of the mycolic acid-arabinogalactan-peptidoglycan complex from M. tuberculosis are fairly well understood. However, the current impetus for studying M. tuberculosis and other pathogenic mycobacteria is the need to identify targets for the development of new drugs. Therefore, emphasis has been shifting to the study of cell envelope biosynthesis and the identification of enzymes that are essential to the viability of M. tuberculosis. The publication of the complete M. tuberculosis genome in 1998 has greatly aided these studies. To date, thirteen enzymes involved in the synthesis of the arabinogalactan-peptidoglycan complex of M. tuberculosis have been identified and at least partially characterized. Eleven of these enzymes were reported subsequent to the publication of the M. tuberculosis genome, a clear indication of the rapid evolution of knowledge stimulated by the sequencing of the genome. In this article we review the current understanding of M. tuberculosis arabinogalactan-peptidoglycan structure and biosynthesis.

Synthetic mannosides act as acceptors for mycobacterial α1-6 mannosyltransferase

A series of synthetic mannosides was screened in a cell-free system for their ability to act as acceptor substrates for mycobacterial mannosyltransferases. Evaluation of these compounds demonstrated the incorporation of [14C]Man from GDP-[14C]Man into a radiolabeled organic-soluble fraction and analysis by thin layer chromatography and autoradiography revealed the formation of two radiolabeled products. Each synthetic acceptor was capable of accepting one or two mannose residues, resulting in a major and a minor mannosylated product. Both products from each acceptor were isolated and their mass was confirmed by fast-atom bombardment-mass spectrometry (FABMS). Characterization of each mannosylated product by exo-glycosidase digestion. acetolysis and linkage analysis by gas chromatography mass spectrometry of partially per-O-methylated alditols, revealed only alpha1-6-linked products. In addition. the antibiotic amphomycin selectively inhibited the formation of mannosylated products suggesting polyprenolmonophosphate-mannose (C15 50-P-Man) was the immediate mannose donor in all mannosylation reactions observed. The ability of synthetic disaccharides to act as acceptor substrates in this system, is most likely due to the action of a mycobacterial polyprenol-P-Man:mannan alpha1-6 mannosyltransferase involved in the biosynthesis of linear alpha1-6-linked lipomannan.

The molecular basis of resistance to isoniazid, rifampin, and pyrazinamide in Mycobacterium tuberculosis

Multidrug-resistant (MDR) strains of Mycobacterium tuberculosis have emerged worldwide. In many countries and regions, these resistant strains constitute a serious threat to the efficacy of tuberculosis control programs. An important element in gaining control of this epidemic is developing an understanding of the molecular basis of resistance to the most important antituberculosis drugs: isoniazid, rifampin, and pyrazinamide. On the basis of this information, more exacting laboratory testing, and ultimately more appropriate and timely treatment regimens, can be developed.

Characterization of the Mycobacterium tuberculosis iniBAC promoter, a promoter that responds to cell wall biosynthesis inhibition

The cell wall provides an attractive target for antibiotics against Mycobacterium tuberculosis. Agents such as isoniazid and ethambutol that work by inhibiting cell wall biosynthesis are among the most highly effective antibiotics against this pathogen. Although considerable progress has been made identifying the targets for cell wall active antibiotics, little is known about the intracellular mechanisms that are activated as a consequence of cell wall injury. These mechanisms are likely to have an important role in growth regulation and in the induction of cell death by antibiotics. We previously discovered three isoniazid-induced genes (iniB, iniA, and iniC) organized in tandem on the M. tuberculosis genome. Here, we investigate the unique features of the putative iniBAC promoter. This promoter was specifically induced by a broad range of inhibitors of cell wall biosynthesis but was not inducible by other conditions that are toxic to mycobacteria via other mechanisms. Induction required inhibitory concentrations of antibiotics and could be detected only in actively growing cells. Analysis of the iniBAC promoter sequence revealed both a regulatory element upstream and a potential repressor binding region downstream of the transcriptional start site. The induction phenotype and structure of the iniBAC promoter suggest that a complex intracellular response occurs when cell wall biosynthesis is inhibited in M. tuberculosis and other mycobacteria.

Three-dimensional structure of M. tuberculosis dihydrofolate reductase reveals opportunities for the design of novel tuberculosis drugs

Dihydrofolate reductase (DHFR) catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate and is essential for the synthesis of thymidylate, purines and several amino acids. Inhibition of the enzyme's activity leads to arrest of DNA synthesis and cell death. The enzyme has been studied extensively as a drug target for bacterial, protozoal and fungal infections, and also for neoplastic and autoimmune diseases. Here, we report the crystal structure of dihydrofolate reductase from Mycobacterium tuberculosis, a human pathogen responsible for the death of millions of human beings per year. Three crystal structures of ternary complexes of M. tuberculosis DHFR with NADP and different inhibitors have been determined, as well as the binary complex with NADP, with resolutions ranging from 1.7 to 2.0 A. The three DHFR inhibitors are the anticancer drug methotrexate, the antimicrobial trimethoprim and Br-WR99210, an analogue of the antimalarial agent WR99210. Structural comparison of these complexes with human dihydrofolate reductase indicates that the overall protein folds are similar, despite only 26 % sequence identity, but that the environments of both NADP and of the inhibitors contain interesting differences between the enzymes from host and pathogen. Specifically, residues Ala101 and Leu102 near the N6 of NADP are distinctly more hydrophobic in the M. tuberculosis than in the human enzyme. Another striking difference occurs in a region near atoms N1 and N8 of methotrexate, which is also near atom N1 of trimethoprim, and near the N1 and two methyl groups of Br-WR99210. A glycerol molecule binds here in a pocket of the M. tuberculosis DHFR:MTX complex, while this pocket is essentially filled with hydrophobic side-chains in the human enzyme. These differences between the enzymes from pathogen and host provide opportunities for designing new selective inhibitors of M. tuberculosis DHFR.

The molecular basis of CD1-mediated presentation of lipid antigens

The CD1 family of proteins mediates a newly described pathway for presentation of lipids and glycolipids for specific recognition by T cells. All four of the known human CD1 proteins (CD1a, CD1b, CD1c and CD1d) as well as murine CD1d have now been shown to mediate T-cell recognition of lipid or glycolipid antigens. These antigens include naturally occurring foreign glycolipids from intracellular pathogens or synthetic glycolipids that are related in structure to mammalian glycolipids. The CD1b and CD1d-presented antigens differ in their fine structures but reveal a general motif in which a rigid hydrophilic cap is bound to two aliphatic hydrocarbon chains. Different T-cell populations recognize individual antigens without cross-reactivity to closely related antigen structures or CD1 isoforms, documenting the complexity and fine specificity of CD1-mediated T-cell responses. Mapping of the molecular determinants of recognition for CD1b and CD1d-presented antigens reveals that T cells discriminate the fine structure of the hydrophilic cap of the antigen, but both the length and structure of the lipid chains may be altered without loss of recognition. This pattern of lipid antigen recognition may be accounted for by a simple molecular mechanism of presentation that parallels the known mechanism for presentation of peptides, but solves the special problems related to the hydrophobic chemical nature of the lipid antigens. We propose that CD1 binds antigen by accommodating the two lipid tails within the hydrophobic groove of its two membrane distal domains, positioning the rigid hydrophilic cap of the antigen on the solvent-exposed surface of the CD1 protein, where it can directly contact the T-cell antigen receptor. This model provides a molecular basis for recognition of a new and diverse set of T-cell antigens contained within the lipid bilayers of cellular membranes.

A Mycobacterium smegmatis mutant with a defective inositol monophosphate phosphatase gene homolog has altered cell envelope permeability

A bacteriophage infection mutant (strain LIMP7) of Mycobacterium smegmatis was isolated following transposon mutagenesis. The mutant showed an unusual phenotype, in that all phages tested produced larger plaques on this strain compared to the parent strain. Other phenotypic characteristics of the mutant were slower growth, increased clumping in liquid culture, increased resistance to chloramphenicol and erythromycin, and increased sensitivity to isoniazid and several beta-lactam antibiotics. Permeability studies showed decreases in the accumulation of lipophilic molecules (norfloxacin and chenodeoxycholate) and a small increase with hydrophilic molecules (cephaloridine); taken together, these characteristics indicate an altered cell envelope. The DNA adjacent to the transposon in LIMP7 was cloned and was shown to be highly similar to genes encoding bacterial and mammalian inositol monophosphate phosphatases. Inositol is important in mycobacteria as a component of the major thiol mycothiol and also in the cell wall, with phosphatidylinositol anchoring lipoarabinomannan (LAM) in the cell envelope. In LIMP7, levels of phosphatidylinositol dimannoside, the precursor of LAM, were less than half of those in the wild-type strain, confirming that the mutation had affected the synthesis of inositol-containing molecules. The impA gene is located within the histidine biosynthesis operon in both M. smegmatis and Mycobacterium tuberculosis, lying between the hisA and hisF genes.

Chemokine secretion by human polymorphonuclear granulocytes after stimulation with Mycobacterium tuberculosis and lipoarabinomannan

Macrophages (MAC) and polymorphonuclear granulocytes (PNG) are professional phagocytes which perform essential functions in antibacterial defense. The intracellular bacterium Mycobacterium tuberculosis persists and replicates in resting macrophages. Although it is generally assumed that activated MAC are central to protection against M. tuberculosis, PNG may also contribute to defense. We wondered whether PNG produce proinflammatory chemokines after stimulation by M. tuberculosis or its major cell wall component, lipoarabinomannan (LAM). In this study, we showed that M. tuberculosis- and LAM-activated human PNG secrete the leukocyte attractant interleukin-8 (IL-8) and the PNG-specific chemokine GRO-alpha in a dose-dependent manner. Treatment of PNG with the leukotriene-B4 inhibitor MK-886 prior to stimulation with M. tuberculosis or LAM partially blocked IL-8 and GRO-alpha induction, suggesting involvement of the 5-lipoxygenase pathway in the secretion of these chemokines. We conclude that PNG contribute to early resistance to M. tuberculosis via chemokine secretion.