MORPHOLOGY AND STRUCTURE OF MYCOBACTERIUM

The mycobacteria are slim, Gram-positive bacilli (0.2–0.4 x 2–10 m). They are
nonmotile, obligate aerobes that do not form spores. The cell wall contains peptidoglycan similar to that of other Gram-positive organisms, except that it contains N-glycolylmuramic, rather than N-acetylmuramic, acid. Attached to peptidoglycan are a myriad of branched chain polysaccharides, proteins, and lipids. Of particular importance are long-chain fatty acids called mycolic acids. The mycolic acids, for which the mycobacteria are named, make up more than 60% of the total cell wall mass and are distinctive for each species. Other lipid components include mycosides, sulfolipids, and lipoarabinomannan (LAM), a complex molecule extending from the plasma membrane to the surface. LAM is structurally and functionally analogous to the lipopolysaccharide of Gram-negative bacteria. Porin and other proteins are found throughout the cell wall.
The cell wall lipids make the cell surface hydrophobic, rendering mycobacteria resistant to staining with basic aniline dyes unless they are applied with heat or detergents, or for prolonged periods of time. Once stained, however, mycobacteria resist decolorization with a mixture of 3% hydrochloric acid and 95% ethanol. These properties are described as acid fastness or, more properly, acid–alcohol fastness, and the bacteria possessing them are called acid-fast bacilli. Details are described in Chapter 15. This characteristic allows mycobacteria to be readily distinguished from other genera by microscopic examination of smears stained with carbol fuchsin (Ziehl–Neelsen/Kinyoun techniques), or with the more recently introduced fluorochromes (auramine–rhodamine). Organisms stained with the latter reagents fluoresce brightly when viewed through an appropriate microscope, making the organisms more visually apparent and, thus, decreasing the time required for their detection.