Fluidity of the lipid domain of cell wall from Mycobacterium chelonae.

Abstract

The mycobacterial cell wall contains large amounts of unusual lipids, including mycolic acids that are covalently linked to the underlying arabinogalactanpeptidoglycan complex. Hydrocarbon chains of much of these lipids have been shown to be packed in a direction perpendicular to the plane of the cell surface. In this study we examined the dynamic properties of the organized lipid domains in the cell wall isolated from Mycobacterium chelonae grown at 30°C. Differential scanning calorimetry showed that much of the lipids underwent major thermal transitions between 30°C and 65°C, that is at temperatures above the growth temperature, a result suggesting that a significant portion of the lipids existed in a structure of extremely low fluidity in the growing cells. Spin-labeled fatty acid probes were successfully inserted into the more fluid part of the cell wall. Our model of the cell wall suggests that this domain corresponds to the outermost leaflet, a conclusion reinforced by the observation that labeling of intact cells produced electron spin resonance spectra similar to those of the isolated cell wall. Use of stearate labeled at different positions showed that the fluidity within the outer leaflet increased only slightly as the nitroxide group was placed farther away from the surface. These results are consistent with the model of mycobacterial cell wall containing an asymmetric lipid bilayer, with an internal, less fluid mycolic acid leaflet and an external, more fluid leaflet composed of lipids containing shorter chain fatty acids. The presence of the low-fluidity layer will lower the permeability of the cell wall to lipophilic antibiotics and chemotherapeutic agents and may contribute to the well-known intrinsic resistance of mycobacteria to such compounds.Mycobacterium tuberculosis causes tuberculosis, a disease that kills 3 million people a year mostly in developing countries. In industrialized countries, the appearance of multiple-drugresistant strains of M tuberculosis has made it once again a major public health problem (1). Furthermore, Mycobacterium leprae causes leprosy in >10 million people worldwide, and several Mycobacterium species, including M. avium complex, M. kansasii, M. fortuitum, and M. chelonae, cause opportunistic infections in immunologically compromised patients, such as AIDS patients. The treatment of mycobacterial infection is often difficult because these bacteria are intrinsically resistant to most common antibiotics and chemotherapeutic agents (2). This general resistance is thought to be related to the unusual structure of mycobacterial cell wall, which is located outside the cytoplasmic membrane yet contains large amounts of lipids, many of them with unusual structure. The major fraction of the cell wall is occupied by unusually long-chain fatty acids containing 70-90 carbons, the mycolic acids (3). The peptidoglycan, which contains N-glycolylmuramic acid instead of the usual N-acetylmuramic acid, is linked to arabinogalactan via a phosphodiester bridge. About 10% of the...

Mycobacterium tuberculosisFunctional Network Analysis by Global Subcellular Protein Profiling

“…The lipids and covalently associated mycolic acids of the cell wall produced by these highly abundant proteins form the primary hydrophobic barrier by packing into a tight, closely packed lipid layer (Liu et al, 1995). This finding is consistent with the fact that the resistance of the pathogen to chemical injury, dehydration, and certain antibiotics is directly related to the low permeability of the unique wall to small hydrophilic molecules (Jarlier and Nikaido, 1994;Trias and Benz, 1994;Liu et al, 1995;Barry, 2001).…”

“…Integral membrane proteins include transmembrane proteins and lipid-anchored proteins. In the Mtb membrane, lipids form a gradient in membrane fluidity so that the region of highest fluidity lies at the outside surface of the pathogen (Liu et al, 1995). Thus the most fluid side of the membrane communicates with the outside world through the temporal association of peripheral membrane macromolecular complexes.…”

“…The lipids and covalently associated mycolic acids of the cell wall produced by these highly abundant proteins form the primary hydrophobic barrier by packing into a tight, closely packed lipid layer (Liu et al, 1995). This finding is consistent with the fact that the resistance of the pathogen to chemical injury, dehydration, and certain antibiotics is directly related to the low permeability of the unique wall to small hydrophilic molecules (Jarlier and Nikaido, 1994;Trias and Benz, 1994;Liu et al, 1995;Barry, 2001). Importantly, large numbers of the enzymes responsible for fatty acid metabolism such as acyl-CoA synthases (fadD) and enoyl-CoA hydratase/isomerase (echA) were also profiled in both the cell wall and the membrane fractions (Supplementary Table and Figure 6).…”

The impact of the absence of glycopeptidolipids on the ultrastructure, cell surface and cell wall properties, and phagocytosis of Mycobacterium smegmatis

“…In most published chemical models of the mycobacterial cell envelope, type-and species-specific lipids such as GPLs are proposed to form a monolayer of lipid that covers the entire mycobacterial surface (Brennan & Nikaido, 1995 ;Liu et al, 1995 ;Minnikin, 1982). One possible consequence of the absence of GPLs from the mutant cells may be the release of surface-exposed components of the mutant into the culture medium, assuming that GPLs play a crucial structural role in the architecture of the outermost layer of the parent strain ; accordingly, the absence of C-type GPLs would cause a disorganization of the layer and the release of some of its components.…”

Broadening Our View About the Role of Mycobacterium tuberculosis Cell Envelope Components During Infection: A Battle for Survival

“…The barrier for the influx of solutes such as nutrients or drugs is associated with the parallel alignment of mycolic acids (Liu et al, 1995;Liu et al, 1996). As a consequence, mycobacteria develop aqueous channels formed by porin molecules in the cell wall structure.…”