Structure of the wall polysaccharide isolated from a group of corynebacteria

Cell Walls and Membranes of Actinobacteria

Actinobacteria is a group of diverse bacteria. Most species in this class of bacteria are filamentous aerobes found in soil, including the genus Streptomyces perhaps best known for their fascinating capabilities of producing antibiotics. These bacteria typically have a Gram-positive cell envelope, comprised of a plasma membrane and a thick peptidoglycan layer. However, there is a notable exception of the Corynebacteriales order, which has evolved a unique type of outer membrane likely as a consequence of convergent evolution. In this chapter, we will focus on the unique cell envelope of this order. This cell envelope features the peptidoglycan layer that is covalently modified by an additional layer of arabinogalactan . Furthermore, the arabinogalactan layer provides the platform for the covalent attachment of mycolic acids , some of the longest natural fatty acids that can contain ~100 carbon atoms per molecule. Mycolic acids are thought to be the main component of the outer membrane, which is composed of many additional lipids including trehalose dimycolate, also known as the cord factor. Importantly, a subset of bacteria in the Corynebacteriales order are pathogens of human and domestic animals, including Mycobacterium tuberculosis. The surface coat of these pathogens are the first point of contact with the host immune system, and we now know a number of host receptors specific to molecular patterns exposed on the pathogen's surface, highlighting the importance of understanding how the cell envelope of Actinobacteria is structured and constructed. This chapter describes the main structural and biosynthetic features of major components found in the actinobacterial cell envelopes and highlights the key differences between them.

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.

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.

Studies on (beta,1-->5) and (beta,1-->6) linked octyl Gal(f) disaccharides as substrates for mycobacterial galactosyltransferase activity

The emergence of multi-drug resistant (MDR) strains of Mycobacterium tuberculosis (MTB) and the continuing pandemic of tuberculosis emphasizes the urgent need for the development of new anti-tubercular agents with novel drug targets. The recent structural elucidation of the mycobacterial cell wall highlights a large variety of structurally unique components that may be a basis for new drug development. This publication describes the synthesis, characterization, and screening of several octyl Galf(beta,1-->5)Galf and octyl Galf(beta,1-->6)Galf derivatives. A cell-free assay system has been utilized for galactosyltransferase activity using UDP[14C]Galf as the glycosyl donor, and in vitro inhibitory activity has been determined in a colorimetric broth microdilution assay system against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC). Certain derivatives showed moderate activities against MTB and MAC. The biological evaluation of these disaccharides suggests that more hydrophobic analogues with a blocked reducing end showed better activity as compared to totally deprotected disaccharides that more closely resemble the natural substrates in cell wall biosynthesis.

Structure of the cell envelope of corynebacteria: importance of the non-covalently bound lipids in the formation of the cell wall permeability barrier and fracture plane

With the recent success of the heterologous expression of mycobacterial antigens in corynebacteria, in addition to the importance of these bacteria in biotechnology and medicine, a better understanding of the structure of their cell envelopes was needed. A combination of molecular compositional analysis, ultrastructural appearance and freeze-etch electron microscopy study was used to arrive at a chemical model, unique to corynebacteria but consistent with their phylogenetic relatedness to mycobacteria and other members of the distinctive suprageneric actinomycete taxon. Transmission electron microscopy and chemical analyses showed that the cell envelopes of the representative strains of corynebacteria examined consisted of (i) an outer layer composed of polysaccharides (primarily a high-molecular-mass glucan and arabinomannans), proteins, which include the mycoloyltransferase PS1, and lipids; (ii) a cell wall glycan core of peptidoglycan-arabinogalactan which may contain other sugar residues and was usually esterified by corynomycolic acids; and (iii) a typical plasma membrane bilayer. Freeze-etch electron microscopy showed that most corynomycolate-containing strains exhibited a main fracture plane in their cell wall and contained low-molecular-mass porins, while the fracture occurred within the plasma membrane of strains devoid of both corynomycolate and pore-forming proteins. Importantly, in most strains, the amount of cell wall-linked corynomycolates was not sufficient to cover the bacterial surface; interestingly, the occurrence of a cell wall fracture plane correlated with the amount of non-covalently bound lipids of the strains. Furthermore, these lipids were shown to spontaneously form liposomes, indicating that they may participate in a bilayer structure. Altogether, the data suggested that the cell wall permeability barrier in corynebacteria involved both covalently linked corynomycolates and non-covalently bound lipids of their cell envelopes.

Major structural features of the cell wall arabinogalactans of Mycobacterium, Rhodococcus, and Nocardia spp

The cell wall arabinogalactans of strains of Mycobacterium, Rhodococcus, and Nocardia were per-O-methylated, partially hydrolyzed with acid, and the resulting oligosaccharides were reduced and per-O-ethylated to yield per-O-alkylated oligoglycosyl alditol fragments. Analyses of these fragments by gas chromatography-mass spectrometry and of the intact solubilized polysaccharides by 1H and 13C NMR revealed the major structural features of the different arabinogalactans from representatives of the different genera. All of the mycobacterial products contained a homogalactan segment of alternating 5-linked alpha-galactofuranosyl (Galf) and 6-linked beta-Galf residues. The arabinan segment consisted of three major domains, linear 5-linked alpha-arabinofuranosyl (Araf) residues and branched (3-->5)-linked Araf units substituted with either 5-linked Araf or the disaccharide beta-Araf-(1-->2)-alpha-Araf at both branched positions. The recognition of these features in in vivo grown Mycobacterium leprae is an important development. The arabinan from strains of Nocardia contains a nonreducing-end motif composed of the linear trisaccharide, beta-Araf-(1-->2)-alpha-Araf-(1-->5)-Araf, attached to linear 5-linked alpha-Araf units. The galactan segment of the arabinogalactan of Nocardia sp. is composed of linear 5-linked beta-Galf units substituted in part at O-6 with terminal beta-glucosyl units. The two representative strains of Rhodococcus also differed in the composition of the galactan moiety; in addition to the 5-linked Galf, 2- and 3-linked beta-Galf units are present. The reducing end of the galactans, and therefore, apparently, of the entire arabinogalactans from all species from all genera, are apparently composed of the unit, rhamnosyl-(1-->3)-N-acetyl-glucosamine, which, in turn, is apparently attached to peptidoglycan via phosphodiester linkage.

Serodiagnosis survey of tuberculosis by a new ELISA method

In a survey study, the authors used a new Elisa test, designed by Anda Biologicals, Strasbourg, France, to detect specific IgG and IgM antibodies against A60 antigen. Blood samples from 53 subjects were tested with this serological method: 22 with no tubercular diseases and 31 affected by different tubercular lesions. The IgM titers were negative in all control group subjects, in two out of fourteen patients with progressive primary tuberculosis, in thirteen out of sixteen with secondary tuberculosis and in one patient with extrapulmonary tuberculosis. The IgG titers were positive (greater than 1.25 Elisa units) in all cases, except one, of progressive primary tuberculosis, in all cases, except two, of secondary tuberculosis and in the patient affected by extrapulmonary tuberculosis. The performance of this method and the overall results indicate its sensitivity and reliability to detect specific mycobacterial antibodies at different stages of the disease.

Subcellular localisation and sedimentation behaviour of antigen 60 from Mycobacterium bovis BCG

Preparation, composition and immunological properties of A60 of Mycobacterium bovis BCG were previously described (Cocito and Vanlinden 1986). The present study focused on the intracellular distribution of this antigen. Fractionation of mycobacterial homogenates by ultracentrifugation indicated that most of A60 was present within the cytoplasm. Some of the antigen was located within the cell wall, from which it was released by extraction with alkali. Submission of cytoplasm to high speed centrifugation caused A60 to cosediment with ribosomes; however, dissociation of ribosomes in low-Mg buffer did not alter the sedimentation pattern of A60. Labelled A60, after ultracentrifugation in sucrose density gradients without Mg2+, was distributed throughout the entire gradient: treatment of (125I)A60 with urea or detergents produced a peak of radioactivity located in the upper part of the gradient. It is concluded that A60 is represented by a heterogeneous family of molecules of increasing sizes: polymerization being enhanced by Mg2+ and reversibly prevented by urea. Some or all of the biological properties hitherto attributed to ribosomal particles may, in fact, be due to their contamination with cosedimented A60.

DNA methylation in leprosy-associated bacteria: Mycobacterium leprae and Corynebacterium tuberculostearicum

The DNAs of two kinds of microorganisms from human leprosy lesion, Mycobacterium leprae and Corynebacterium tuberculostearicum (also known as "leprosy-derived corynebacterium" or LDC), have been analysed and compared with the genomes of reference bacteria of the CMN group (genera Corynebacterium, Mycobacterium and Nocardia). The guanine-plus-cytosine content (% GC) of DNA was determined by a double-labelling procedure, which is unaffected by the presence of modified and unusual bases (that alter both buoyant density and mid-melting-point determinations). Accordingly, the DNAs of seven LDC strains had GC values of 54-56 mol %, and that of armadillo-grown M. leprae a value of 54.8 +/- 0.9 mol %. Restriction patterns disclosed no methylated cytosine in the DNA sequences CCGG, GGCC, AGCT and GATC of either LDC or M. leprae DNA. N6-methyl adenine was present in the sequence GATC of all LDC strains, but was missing from the genomes of all others CMN organisms analysed, including M. leprae. By HPLC analysis of LDC-DNA hydrolysates, it was found that N6-methyladenine amounted to 1.8% of total DNA adenine, and was present exclusively within GATC sequences, which appeared all to be methylated. It is concluded that LDC represent a group of corynebacteria endowed with high genetic homogeneity and a unique restriction pattern, whereby their genome is easily distinguished from that of M. leprae, which has a similar base composition.

Chemical composition of antigen 60 from Mycobacterium bovis BCG

Antigen 60 (A60), the main thermostable immunogen of tuberculin and PPD, has been purified from Mycobacterium bovis BCG cytoplasm, and identified by crossed immunoelectrophoresis with anti-BCG polyclonal antiserum. Two A60 fractions, free lipids and lipid-conjugated compounds, have been recognized. The free lipids represented about 30% (dry weight), and consisted essentially of C16-C18 fatty acids, and of phosphatidyl-inositol-mannosides. Lipoconjugates, upon DEAE-cellulose chromatography and gel filtration, yielded two main fractions of neutral and polar components. Chromatography of delipidated and deproteinized A60 on Sephadex G-100 yielded: a high molecular weight fraction (Al, 18%, a lipoglucan of congruent to 10(6)), and a low molecular weight fraction (B, 10%, a lipopeptidoglycan of congruent to 10(4)) containing mannose, glucose, and small amounts of arabinose. The polysaccharide moieties of fractions Al and B were submitted to acetylation, methylation, and acid hydrolysis, and the structure of the hydrolysed polymer was deduced by combined gas chromatography/mass spectrometry analysis. The results indicated a branched structure involving 1,4-, 1,6-, and 1,4,6-linked D-gluco- or D-manno-pyranosyl residues. Glucan- and peptidoglycan-bound fatty acids were identified as saturated (C16-C18) and monounsaturated linear acids (C12-C18). Immunodiffusion on agarose gel indicated that delipidation and proteolysis did not suppress the ability of A60 to yield immunoprecipitates with anti-A60 antiserum. The high polymer fractions obtained by chromatography on DEAE cellulose and Sephadex G-100 were also reactive. It is concluded that A60 is made of free lipids and of lipopeptidoglycans of high molecular weights (10(6)-10(7)) endowed with immunogenic properties.

Characterization of 6-deoxy-D-altritol in the cell-wall polysaccharide of Nocardia asteroides R 399

A polyol, found in the cell-wall of Nocardia asteroides R 399 as a component of a neutral polysaccharide mainly composed of D-arabinose and D-galactose, was identified by mass spectrometry, paper chromatography, thin-layer chromatography, and gas chromatography as 6-deoxy-D-altritol.

Description of Corynebacterium tuberculostearicum sp. nov., a leprosy-derived Corynebacterium

Leprosy-derived corynebacteria (LDC) have been extensively studied over the past decade. A composite of their biological properties (cell morphology, staining reactions, cellular inclusions and guanine-plus-cytosine content of their deoxyribonucleic acid; 16 strains studied) and their chemical structures (peptidoglycan type, major cell wall polysaccharide, major glycolipid as well as characteristic mycolic acids) appears to define them as members of the genus Corynebacterium. In relation to other corynebacteria found in humans, including "JK corynebacteria", they seem to be distinct. They are here named Corynebacterium tuberculostearicum sp. nov. because they produce a 10-methyloctadecanoic (tuberculostearic) acid (8 strains studied). This and some of their other attributes are considered in relation to properties of leprosy bacilli and Mycobacterium leprae.