Metabolism in Mycobacterium leprae, M. tuberculosis and other pathogenic mycobacteria

Quantitative lipid composition of cell envelopes of Corynebacterium glutamicum elucidated through reverse micelle extraction

Cells of the Corynebacterium-Nocardia-Mycobacterium group of bacteria are surrounded by an outer membrane (OM) containing mycolic acids that are covalently linked to the underlying arabinogalactan-peptidoglycan complex. This OM presumably acts as a permeability barrier that imparts high levels of intrinsic drug resistance to some members of this group, such as Mycobacterium tuberculosis, and its component lipids have been studied intensively in a qualitative manner over the years. However, the quantitative lipid composition of this membrane has remained obscure, mainly because of difficulties in isolating it without contamination from the inner cytoplasmic membrane. Here we use the extraction, with reverse surfactant micelles, of intact cells of Corynebacterium glutamicum and show that this method extracts the free OM lipids quantitatively with no contamination from lipids of the cytoplasmic membrane, such as phosphatidylglycerol. Although only small amounts of corynomycolate were esterified to arabinogalactan, a large amount of cardiolipin was present in a nonextractable form, tightly associated, possibly covalently, with the peptidoglycan-arabinogalactan complex. Furthermore, we show that the OM contains just enough lipid hydrocarbons to produce a bilayer covering the cell surface, with its inner leaflet composed mainly of the aforementioned nonextractable cardiolipin and its outer leaflet composed of trehalose dimycolates, phosphatidylinositol mannosides, and highly apolar lipids, similar to the Minnikin model of 1982. The reverse micelle extraction method is also useful for extracting proteins associated with the OM, such as porins.

Molecular mechanics of the mycobacterial cell wall: from horizontal layers to vertical scaffolds

Current models depicting the structural organization of the mycobacterial cell wall assume peptidoglycan and galactan strands to run in parallel to the cytoplasmic membrane forming several horizontal layers beneath perpendicularly oriented mycolic acids. Following a thorough re-evaluation of the currently available chemical, biochemical and electron microscopical data, we propose a fundamentally distinct principle of the physical organization and biosynthesis of the mycobacterial cell wall skeleton. According to this new concept, the solid and elastic matrix that makes the mycobacterial cell wall a formidably impermeable barrier is the direct consequence of cross-linked glycan strands which all run in a direction perpendicular to the cytoplasmic membrane.

Trafficking and release of mycobacterial lipids from infected macrophages

Analysis of infected macrophages revealed that lipid-containing moieties of the mycobacterial cell wall are actively trafficked out of the mycobacterial vacuole. To facilitate the analysis of vesicular trafficking from mycobacteria-containing phagosomes, surface-exposed carbohydrates were labeled with hydrazide-tagged markers. The distribution of labeled carbohydrate/lipid moieties and subsequent interaction with cellular compartments were analyzed by immunoelectron microscopy and by fluorescence microscopy of live cells. The released mycobacterial constituents were associated with several intracellular organelles and were enriched strikingly in tubular endocytic compartments. Subcellular fractionation of infected macrophages by density gradient electrophoresis showed temporal movement of labeled bacterial constituents through early and late endosomes. Thin layer chromatography analysis of these subcellular fractions confirmed their lipid nature and revealed five dominant bacteria-derived species. These mycobacterial lipids were also found in extracellular vesicles isolated from the medium and could be observed in un-infected 'bystander' cells. Their transfer to bystander cells could expand the bacteria's sphere of influence beyond the immediate confines of the host cell.

Physical organization of lipids in the cell wall of Mycobacterium chelonae

Mycobacterial cell wall functions as an effective permeability barrier, making these bacteria resistant to most antibacterial agents. It has been assumed that this low permeability was due to the presence of a large amount of unusual lipids in the cell wall, but it was not known how these lipids are able to produce such an exceptional barrier. We report here the first experimental evidence on the physical arrangement of these lipids based on X-ray diffraction studies of purified Mycobacterium chelonae cell wall, a result suggesting that the hydrocarbon chains of the cell-wall lipids are arranged predominantly in a direction perpendicular to the cell wall surface, probably producing an asymmetric bilayer structure.

Porins in the cell wall of mycobacteria

The cell wall of mycobacteria is an efficient permeability barrier that makes mycobacteria naturally resistant to most antibiotics. Liposome swelling assays and planar bilayer experiments were used to investigate the diffusion process of hydrophilic molecules through the cell wall of Mycobacterium chelonae and identify the main hydrophilic pathway. A 59-kilodalton cell wall protein formed a water-filled channel with a diameter of 2.2 nanometers and an average single-channel conductance equal to 2.7 nanosiemens in 1 M potassium chloride. These results suggest that porins can be found in the cell wall of a Gram-positive bacterium. A better knowledge of the hydrophilic pathways should help in the design of more effective antimycobacterial agents.

Stress proteins and the immune response to mycobacteria--antigens as virulence factors?

The immune response to mycobacterial infection includes pathogenic as well as protective activities. It is possible that different types of immune responses are associated with recognition of different antigenic determinants. Amongst the antigens which are prominent in antibody and T cell recognition of mycobacteria, we have identified members of highly conserved stress protein families. Mapping of antigenic determinants on stress proteins shows that both species-specific and conserved regions of these proteins can take part in immune recognition. Induction of an immune response to conserved, "self-like", determinants on stress proteins could play a role in the immunopathology associated with chronic mycobacterial infections.