Mycobacteria

Mycobacterium is a genus of Gram-positive bacilli that demonstrate the staining
characteristic of acid-fastness. Its most important species, Mycobacterium tuberculosis, is
the etiologic agent of tuberculosis, the “consumption” referred to above. One of the oldest
and most devastating of human afflictions, tuberculosis remains a leading cause of infectious
disease deaths worldwide today. A second mycobacterium, Mycobacterium leprae,
is the causative agent of leprosy. A large number of less pathogenic species collectively
referred to as “atypical mycobacteria” or “nontuberculous mycobacteria,” are assuming
increasing importance as disease agents in immunocompromised patients, particularly
those with acquired immunodeficiency syndrome (AIDS).

MORPHOLOGY AND STRUCTURE
The mycobacteria are slim, Gram-positive bacilli (0.2 – 0.4
 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 Nglycolylmuramic,
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.