Cell walls of Mycobacteria

Abstract

The walls of Mycobacteria, like those of most other prokaryotes, contain peptidoglycan but characteristically they also contain glycolipid. Much glycolipid is covalently bound to the structural wall components, but there is also an unbound lipid fraction that is extractable into organic solvents representing in Mycobacterium bovis BCG, for example, about 34–40% of the wall [6, 18]. The walls of Mycobacteria have formed the subject of a valuable review [23] and a more general account of mycobacterial physiology is given by Ratledge [30]. In its amino acid composition the peptidoglycan resembles that of Bacilli, but as mentioned in Chapter 6.1, the most noticeable difference is that the acylation of the amino group of the muramic acid residue is by a glycolyl rather than the more common acetyl group [3, 7] a substitution also found in Nocardia [17]. There is evidence that oxidation of acetyl to glycolyl occurs at the level of UDP-N-acetylmuramic acid, since it has been shown that extracts of Nocardia asteroides will oxidize the latter substrate to UDP-N-glycolylmuramic acid, whence the precursor nucleotide pentapeptide presumably is synthesized in the usual way. Another major difference from most other peptidoglycans is in the type of cross-linking. Some links are made directly from the sub-terminal D-alanine of one pentapeptide side-chain to the D-centre of meso-diaminopimelic acid in another chain, just as in Bacilli or in Gramnegative species, but other links involve a direct link between the mesodiaminopimelic acid residue of one chain and that of another, or even the presence of tripeptides of diaminopimelic acid [34]. The exact linkages between the diaminopimelic acid residues (ie whether the L-centre carboxyl group of one is linked to the D-centre amino group of the next and so on) have not been established, and attempts to demonstrate a specific LD-transpeptidase that would catalyse the formation of that type of linkage have not so far succeeded [28].