Mycolic Acid Structure Determines the Fluidity of the Mycobacterial Cell Wall

Abstract

The low permeability of the mycobacterial cell wall is thought to contribute to the well known resistance of mycobacteria to antibiotics and chemotherapeutic agents. We have used differential scanning calorimetry to demonstrate that the high temperature phase transition observed in purified cell walls, usually in the 60 -70°C range, suggestive of a lipid environment of extremely low fluidity, can also be observed in whole organisms and in cell walls from which much of the free lipids was removed by extraction with Triton X-114. A survey of seven mycobacterial species demonstrated that this high temperature transition was a general property of these organisms. Cell walls isolated from two Corynebacterium species, which contain much shorter corynemycolic acids, displayed a much lower temperature transition, suggesting that the transition temperature was directly correlated to the length of mycolic acid. Methyl esters of mycolic acids were found to have a phase transition temperature that was linearly related to the amount of trans-mycolate. Both Mycobacterium avium and M. smegmatis responded to increasing growth temperature by increasing the proportion of trans-mycolate and displaying a correspondingly higher melting temperature. Whole cells of M. smegmatis grown at higher temperature allowed a less rapid influx of two lipophilic agents, norfloxacin and chenodeoxycholate. These results provide strong evidence that the nature of mycolic acid plays a crucial role in determining the fluidity and permeability of mycobacterial cell wall.Mycobacterial infections are the leading cause of death from infectious diseases (1). There are approximately 1 billion people presently infected with Mycobacterium tuberculosis. Leprosy, caused by M. leprae, affects 10 -12 million people (1). "Atypical mycobacteria," such as M. avium, M. intracellulare, M. xenopi, M. kansasii, M. chelonae, and M. fortuitum, cause opportunistic infections among AIDS patients. A major problem with the infections caused by mycobacteria is their intrinsic resistance to most general purpose antibiotics (1, 2). This problem is compounded by the emergence of multidrug-resistant strains of M. tuberculosis (3, 4).The intrinsic drug resistance of mycobacteria has been attributed, at least in part, to the low permeability of the cell wall (2). The influx of small, hydrophilic agents, which are likely to traverse the cell wall through porin channels (5, 6), is extremely slow (7), presumably because the cell wall contains only a small number of low specific activity porin molecules. On the other hand, mycobacterial cell walls are extremely rich in lipids. Lipophilic and amphiphilic agents may therefore be expected to cross the cell wall through its lipid domain, yet most mycobacteria exhibit high levels of intrinsic resistance even to such agents. We are therefore trying to understand how the lipid domain of the mycobacterial cell wall can serve as an effective permeability barrier by studying the physical organization of the cell wall lipids. The cell wall of myco...

Towards understanding the functional diversity of cell wall mycolic acids of Mycobacterium tuberculosis

“…· Cells of mycobacteria grown at higher temperature contain increased levels of trans-MA and have a higher melting temperature; moreover, they allow a less rapid influx of some drugs [73].…”

Viability states of bacteria—Specific mechanisms of selected probes

“…In the case of Mycobacteria, these teichoic acids are replaced by mycolic acids. These fatty acids have a length up to 90 carbon atoms and are considered to be responsible for the malfunction of the Gram staining reaction and the extremely low fluidity of the mycobacterial cell wall (2). This simple example illustrates how cell wall structure may influence characteristics of dye binding.…”

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

“…5). This probe is a negatively charged hydrophobic molecule that has been previously used to evaluate the fluidity of the mycobacterial cell wall lipids (Liu et al, 1996 ;Yuan et al, 1997). The mutant showed a significantly higher initial rate of uptake and accumulation of chenodeoxycholate than did the parent strain.…”