A dread disease in which the struggle between soul and body is so gradual, quiet and solemn, and the result so sure that day by day, and grain by grain, the mortal part wastes and withers away. A disease … which sometimes moves in giant strides and sometimes at a tardy sluggish pace, but, slow or quick, is ever sure and certain. —Charles Dickens: Nicholas Nickleby
Mycobacterium is a genus of Gram-positive bacilli which all demonstrate the staining characteristic of acid-fastness. Its most important species, Mycobacterium tuberculosis, is the etiologic agent of tuberculosis, the dread disease called consumption in Dickens' time. Mostly out of view in wealthy countries, tuberculosis still infects a third of the world population and causes almost 2 million deaths each year. Mycobacterium leprae, is the causative agent of leprosy, an ancient and disfiguring disease. A large number of less pathogenic species are assuming increasing importance as disease agents in immunocompromised patients, particularly those with AIDS.
MYCOBACTERIUM: GENERAL CHARACTERISTICS
The mycobacteria are slim, poorly staining bacilli, which demonstrate the property of acid-fastness. They are nonmotile, obligate aerobes that do not form spores. The cell wall contains peptidoglycan similar to that of other Gram-positive organisms, to which many branched-chain polysaccharides, proteins, and lipids are attached. Porins and other proteins are found throughout the cell wall. Of particular importance is the presence of long-chain fatty acids called mycolic acids (for which the mycobacteria are named) and lipoarabinomannan (LAM), a lipid polysaccharide complex extending from the plasma membrane to the surface (Figure 27–1). LAM is structurally and functionally analogous to the lipopolysaccharide of Gram-negative bacteria. These elements give the mycobacteria a cell wall with unusually high lipid content (>60% of the total cell wall mass), which accounts for many of their biologic characteristics. It can be thought of as a waxy coat that makes them hardy, impenetrable, and hydrophobic. The staining characteristic of acid-fastness is the most frequently observed of these features. The mycobacterial cell wall can be stained only through the use of extreme measures (prolonged time, heat, penetrating agents) but once in, the stain is fast. Even the strongest of decolorizing agents (acid and alcohol) do not wash it out (Figure 27–2).
Mycobacterial cell wall. LAM, lipoarabinomannan. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott's Principles of Microbiology. New York: McGraw-Hill; 2008.)
Mycobacterium tuberculosis in sputum stained by the acid-fast technique. The mycobacteria retain the red carbol fuchsin through the decolorization step. The cells, background, and any other organisms stain with the contrasting methylene blue counterstain. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)