Identification of the surface-exposed lipids on the cell envelopes of Mycobacterium tuberculosis and other mycobacterial species

Drug sensitivity and environmental adaptation of mycobacterial cell wall components

The intrinsic resistance of many mycobacterial species to chemotherapy is largely attributable to their impermeable cell wall. The composition of the cell wall of a particular species appears to be influenced by the environmental niche that the species occupies. The complex regulatory and biosynthetic pathways involved in cell wall biosynthesis and construction offer useful chemotherapeutic targets against mycobacteria.

Extracellular and surface-exposed polysaccharides of non-tuberculous mycobacteria

We studied the outermost constituents of the cell envelopes, which are involved in the interaction between the bacilli and the host cells, of five pathogenic and non-pathogenic mycobacterial species for comparison with those we have previously characterized from M. tuberculosis. The extracellular materials (ECMs) were isolated by ethanol precipitation and compared to the surface-exposed materials (SXMs) extracted by mechanical means. The materials from both sources were composed almost exclusively of polysaccharides and proteins. Two groups of mycobacteria were clearly distinguishable. The first group comprised the pathogenic species M. kansasii which produced large amounts of ECM, the glycosyl composition of which was similar to that of the SXM. The second group comprised M. avium and the non-pathogenic strains of M. gastri, M. phlei and M. smegmatis which produced small amounts of ECK This latter group could be subdivided into those which produced carbohydrate-rich ECM (M. avium and M. gastri) and those forming protein-rich ECM (M. phlei and M. smegmatis), a classification that correlated with the difference in the growth rate of the two subgroups. The glycosyl composition of the ECM of a given species was qualitatively similar to that of the SXM, except for M. avium and M. phlei whose SXM were devoid of arabinose. In addition to glucose, mannose and arabinose, xylose was detected in the hydrolysis products of the ECM and SXM of M. smegmatis, the SXM of M. phlei and the ECM of some batches of M. avium. The polysaccharide constituents of the ECM and SXM of the different mycobacteria were purified by anion-exchange and gel-filtration chromatography; all were found to be neutral compounds devoid of acyl substituents. The extracellular polysaccharides consisted of high-molecular-mass glycogen-like glucans, arabinomannans and mannans, structurally similar to the corresponding substances previously characterized from the capsule of M. tuberculosis. The same types of polysaccharides were characterized from the SXM of all the strains, except M. avium and M. phlei which were devoid of arabinomannans. This study questions the unique and universal representation of the mycobacterial cell envelope and the existence of the so-called acidic polysaccharide-rich outer layer.

Structures of the glycolipid antigens of members of the third biovariant complex of Mycobacterium fortuitum

Among the fast-growing mycobacteria, members of the Mycobacterium fortuitum complex are the most-commonly cited opportunistic human pathogens, notably in post-surgical infections. Previous studies showed that this complex was composed of four well-identified species and a group of isolates that did not correspond to recognized species, which has been referred to as the third biovariant complex. The occurrence and chemical structure of the glycolipid antigens of six strains that belong to this latter group were examined in the present study. Based on the TLC profiles, resistance to alkali and seroreactivities of their glycolipids, the examined strains were classified into three groups: one group was devoid of species-specific glycolipid and the two other groups contained alkali-stable or alkali-labile glycoconjugates. The structures of the major glycolipid antigens of the latter two groups were elucidated by fast-atom-bombardment MS, one-dimensional and two-dimensional NMR spectroscopy and conventional chemical analyses. The alkali-stable glycolipids were structurally identical to the C-mycoside-type glycopeptidolipids characterized in the taxonomically related species Mycobacterium peregrinum. The major alkali-labile glycolipid was identified as beta-Glcp-1 --> 6)-alpha-Glcp2Acyl-(1 --> 1)-alpha-GLcp3,4,6Acyl3. The acyl substituents consisted on one acetyl group and three fatty acyl residues composed mainly of tetradecanoyl residues, but significant amounts of 2-methylhexadecanoyl and 2-methyloctadecanoyl substituents were also present. The heterogeneity of the glycolipid content of members of the third biovariant complex of M. fortuitum demonstrated in the present study confirms the heterogeneity of the complex. In addition, the occurrence of a species-specific glycolipid in some strains supports the hypothesis that some strains of this complex of M. fortuitum may belong to a new mycobacterial species.