Re-emergence of tuberculosis: strategies and treatment

Efficient synthesis of a linear octyl pentaarabinofuranoside, a substrate for mycobacterial EmbA/EmbB proteins

The efficient synthesis of a linear pentasaccharide with the structure 1, β-D-Araf-(1 → 2)-α-D-Araf-(1 → 5)-α-D-Araf-(1 → 5)-α-D-Araf-(1 → 5)-α-D-Araf-(1 → 5), as its octyl glycoside has been achieved through a convergent [3 + 2] coupling strategy. The difficult-to-obtain 1,2-cis-β-arabinofuranosidic bond at the non-reducing end of the target molecule was stereoselectively constructed by the use of a 2-quinolinecarbonyl-directed 1,2-cis glycosylation method.

Influence of silyl protections on the anomeric reactivity of galactofuranosyl thioglycosides and application of the silylated thiogalactofuranosides to one-pot synthesis of diverse β-D-oligogalactofuranosides

We describe in this paper the tuning effect of silyl protecting groups on the donor reactivity of galactofuranosyl phenyl thioglycosides. Silyl ethers on the galactofuranose ring are found to have an arming effect on the glycosylation reactivity, but the cyclic 3,5-acetal protecting group decreases the reactivity. The reactive phenyl 2,6-di-O-Bz-3,5-di-O-TBS-1-thio-β-d-galactofuranoside 3 is proved to be a useful glycosyl building block. By taking advantage of this donor, we achieved the highly efficient one-pot solution-phase assembly of a panel of β-d-galactofuranosyl tri- and tetrasaccharides possessing diverse glycosidic linkages.

Regioselective glycosylation method using partially protected arabino- and galactofuranosyl thioglycosides as key glycosylating substrates and its application to one-pot synthesis of oligofuranoses

We describe in this paper the development of a novel regioselective furanosylation methodology using partially protected furanosyl thioglycosides as central glycosylating building blocks and its application in the efficient one-pot synthesis of a series of linear and branched-type arabino- and galactofuranoside fragments structurally related to the cell wall polysaccharides of Mycobacterium tuberculosis , Streptococcus pneumoniae serostype 35A, and sugar beet.

Ocular Tuberculosis Initially Presenting as Central Retinal Vein Occlusion

A 23-year-old man presented with central retinal vein occlusion. The retinal haemorrhages worsened and signs of retinal vasculitis appeared later as vision dropped from 6/60 to Counting Fingers. No signs of systemic disease were observed. Routine Mantoux test and chest radiograph were negative for tuberculosis. Fundus flourescein angiogram confirmed presence of retinal vasculitis. Both systemic and topical corticosteroid therapy were ineffective. Polymerase chain reaction analysis of vitreous fluid showed presence of Mycobacterium tuberculosis. A full 6-month course of antituberculosis therapy was given and inflammation subsided. Vision improved to 3/60. This is a rare case of ocular tuberculosis without evidence of systemic infection, presenting first as a central retinal vein occlusion.

Silencing Mycobacterium smegmatis by using tetracycline repressors

Many processes that are essential for mycobacterial growth are poorly understood. To facilitate genetic analyses of such processes in mycobacteria, we and others have developed regulated expression systems that are repressed by a tetracycline repressor (TetR) and induced with tetracyclines, permitting the construction of conditional mutants of essential genes. A disadvantage of these systems is that tetracyclines function as transcriptional inducers and have to be removed to initiate gene silencing. Recently, reverse TetR mutants were identified that require tetracyclines as co-repressors. Here, we report that one of these mutants, TetR r1.7, allows efficient repression of lacZ expression in Mycobacterium smegmatis in the presence but not the absence of anhydrotetracycline (atc). TetR and TetR r1.7 also allowed efficient silencing of the essential secA1 gene, as demonstrated by inhibition of the growth of a conditional mutant and dose-dependent depletion of the SecA1 protein after the removal or addition, respectively, of atc. The kinetics of SecA1 depletion were similar with TetR and TetR r1.7. To test whether silencing of secA1 could help identify substrates of the general secretion pathway, we analyzed the main porin of M. smegmatis, MspA. This showed that the amount of cell envelope-associated MspA decreased more than 90-fold after secA1 silencing. We thus demonstrated that TetR r1.7 allows the construction of conditional mycobacterial mutants in which the expression of an essential gene can be efficiently silenced by the addition of atc and that gene silencing permits the identification of candidate substrates of mycobacterial secretion systems.

Structural basis for shikimate-binding specificity of Helicobacter pylori shikimate kinase

Shikimate kinase (EC 2.7.1.71) catalyzes the specific phosphorylation of the 3-hydroxyl group of shikimic acid in the presence of ATP. As the fifth key step in the shikimate pathway for aromatic amino acid biosynthesis in bacteria, fungi, and plants, but not mammals, shikimate kinase represents an attractive target for the development of new antimicrobial agents, herbicides, and antiparasitic agents. Here, we report the 1.8-Angstroms crystal structure of Helicobacter pylori shikimate kinase (HpSK). The crystal structure shows a three-layer alpha/beta fold consisting of a central sheet of five parallel beta-strands flanked by seven alpha-helices. An HpSK-shikimate-PO(4) complex was also determined and refined to 2.3 Angstroms, revealing induced-fit movement from an open to a closed form on substrate binding. Shikimate is located above a short 3(10) helix formed by a strictly conserved motif (GGGXV) after beta(3). Moreover, several highly conserved charged residues including Asp33 (in a conserved DT/SD motif), Arg57, and Arg132 (interacting with shikimate) are identified, guiding the development of novel inhibitors of shikimate kinase.

Modified mannose disaccharides as substrates and inhibitors of a polyprenol monophosphomannose-dependent α-(1→6)-mannosyltransferase involved in mycobacterial lipoarabinomannan biosynthesis

A panel of alpha-(1-->6)-linked mannose disaccharides (5-8) in which the 2'-OH group has been replaced, independently, by deoxy, fluoro, amino, and methoxy functionalities has been synthesized. Evaluation of these compounds as potential substrates or inhibitors of a polyprenol monophosphomannose-dependent alpha-(1-->6)-mannosyltransferase involved in mycobacterial LAM biosynthesis demonstrated that the enzyme is somewhat tolerant substitution at this site. The enzyme recognizes the disaccharides with groups similar or smaller in size than the native hydroxyl (6-8), but not the disaccharide with the more sterically demanding methoxy group (5). The 2'-OH appears not form a critical hydrogen bonding interaction with the protein as the 2'-deoxy analog is a substrate for the enzyme.

Controlling gene expression in mycobacteria with anhydrotetracycline and Tet repressor

Gene expression systems that allow the regulation of bacterial genes during an infection are valuable molecular tools but are lacking for mycobacterial pathogens. We report the development of mycobacterial gene regulation systems that allow controlling gene expression in fast and slow-growing mycobacteria, including Mycobacterium tuberculosis, using anhydrotetracycline (ATc) as inducer. The systems are based on the Escherichia coli Tn10-derived tet regulatory system and consist of a strong tet operator (tetO)-containing mycobacterial promoter, expression cassettes for the repressor TetR and the chemical inducer ATc. These systems allow gene regulation over two orders of magnitude in Mycobacterium smegmatis and M.tuberculosis. TetR-controlled gene expression was inducer concentration-dependent and maximal with ATc concentrations at least 10- and 20-fold below the minimal inhibitory concentration for M.smegmatis and M.tuberculosis, respectively. Using the essential mycobacterial gene ftsZ, we showed that these expression systems can be used to construct conditional knockouts and to analyze the function of essential mycobacterial genes. Finally, we demonstrated that these systems allow gene regulation in M.tuberculosis within the macrophage phagosome.

The MspA porin promotes growth and increases antibiotic susceptibility of both Mycobacterium bovis BCG and Mycobacterium tuberculosis

Porins mediate the diffusion of hydrophilic solutes across the outer membrane of mycobacteria, but the efficiency of this pathway is very low compared to Gram-negative bacteria. To examine the importance of porins in slow-growing mycobacteria, the major porin MspA of Mycobacterium smegmatis was expressed in Mycobacterium tuberculosis and Mycobacterium bovis. Approximately 20 and 35 MspA molecules per microm(2) cell wall were observed in M. tuberculosis and M. bovis BCG, respectively, by electron microscopy and quantitative immunoblot experiments. Surface accessibility of MspA in M. tuberculosis was demonstrated by flow cytometry. Glucose uptake was twofold faster, indicating that the outer membrane permeability of M. bovis BCG to small and hydrophilic solutes was increased by MspA. This significantly accelerated the growth of M. bovis BCG, identifying very slow nutrient uptake as one of the determinants of slow growth in mycobacteria. The susceptibility of both M. bovis BCG and M. tuberculosis to zwitterionic beta-lactam antibiotics was substantially enhanced by MspA, decreasing the minimal inhibitory concentration up to 16-fold. Furthermore, M. tuberculosis became significantly more susceptible to isoniazid, ethambutol and streptomycin. Fluorescence with the nucleic acid binding dye SYTO 9 was 10-fold increased upon expression of mspA. These results indicated that MspA not only enhanced the efficiency of the porin pathway, but also that of pathways mediating access to large and/or hydrophobic agents. This study provides the first experimental evidence that porins are important for drug susceptibility of M. tuberculosis.

The economics of TB control in developing countries

Tuberculosis is one of the world's biggest killers and the burden is skewed towards developing countries, where the relative share of this disease is largest. This has much to do with poor living conditions and less developed public health systems. Nevertheless, even with these factors taken into account, the mortality from this disease is still unjustifiably high given the fact that it is treatable in the vast majority of cases with relatively inexpensive drugs. There are three main reasons for this situation: a low propensity to seek healthcare when sick; poor diagnosis upon seeking care; and high levels of non-compliance with treatment when diagnosed. Taken in combination, this means that of those who seek care, some get appropriate diagnosis at their first contact, while many do not. Unfortunately, not all of those who are diagnosed actually begin treatment and of those who do, a smaller percentage actually complete it. This article will look at each of these issues, and in particular their costs, in turn, and suggest areas in which economics can help to develop solutions to them.

Current status and future development of antitubercular chemotherapy

Tuberculosis (TB), which kills more people than any other infectious disease, was declared a global emergency by the World Health Organization in 1993. The emergence of new Mycobacterium tuberculosis strains that are resistant to some or all current antitubercular drugs seriously hampers the control of the disease. Up to 50 million people may be infected with drug-resistant TB, with resistance being caused by inconsistent or partial treatment when patients do not comply with long-term chemotherapy. Resistance is often a corollary to HIV infection. Besides being more fatal, drug-resistant TB is more difficult and more expensive to treat. In addition to this human cost, TB also represents a significant economic burden for developing countries. Therefore, new approaches to the treatment of TB are needed. During the last few years, important efforts have been made in order to elucidate the molecular mechanism of action of antitubercular drugs and understand the genetic basis of acquired drug resistance in M. tuberculosis. The identification of novel targets requires the characterisation of biochemical pathways specific to mycobacteria. Many unique metabolic processes occur during the biosynthesis of cell wall components, including arabinogalactan and mycolic acids. In this review, the mode of action of first- and second-line agents, as well as the potentiality of some promising drugs that are still at an early stage of development will be described. Finally, some of the attractive targets offered by the mycobacterial cell wall for the rational design of new antitubercular drugs for a future and more effective control of the disease will be examined.