General Microbiology - Schlegel, H. 1987

Prokaryotes: A Systematic Survey
Mycobacteria (Group 17)

All mycobacteria are obligate aerobes. Morphologically, they occupy an intermediate position between corynebacteria and proactinomycetes (nocardiae). They do not form a true mycelium, but instead grow as irregularly shaped, poorly branching Cells. Mycobacteria are Gram-positive and non-motile. They differ from corynebacteria by their "acid-fastness": in 1882, Ehrlich discovered that tubercle bacilli (Mycobacterium tuberculosis), once stained with aniline Dyes, resist decolorization by acids.

Typically, bacterial smears prepared using the Ziehl-Neelsen method are heated in the presence of carbol fuchsin, washed, and then differentiated by Treatment with an acid-alcohol solution (Hydrochloric acid in ethanol). Mycobacteria and nocardiae resist decolorization during this acid treatment, which defines their acid-fastness. While certain saprophytic mycobacteria can be decolorized by acid-alcohol, they remain unaffected by aqueous HCl solutions. This acid-fast property is attributed to the high concentration of mycolic acids in their Cell walls, which imparts a waxy texture and strong Structure/106.html">Hydrophobicity to the mycobacterial surface.

Regarding The Cell wall composition of Corynebacterium, Mycobacterium, and Nocardia, there are both shared features and distinct differences. Their murein Skeleton resembles that of typical Gram-positive bacterial walls; however, it is covalently linked to arabinogalactan—a polysaccharide composed of arabinose and galactose—which, in turn, is esterified with Lipids known as mycolic acids. These are branched 3-hydroxy acids (R¹—СНОН—CHR²—СООН) substituted at the 2 and 3 positions with aliphatic chains of varying lengths. The mycolic acids of corynebacteria contain 32–36 carbon atoms, those of nocardiae contain 48–58, and those of mycobacteria contain 78–95. Notably, only acids with exceptionally long chains confer true acid-fastness to the cells.

Mycobacterium tuberculosis is the CAUSATIVE AGENT OF tuberculosis, a disease that primarily targets physically weakened individuals and became a less formidable threat only following the advent of Antibiotics and isoniazid.

Mycobacteria are also widely distributed in the soil. Many species thrive on media containing paraffins, as well as aromatic and hydroaromatic Hydrocarbons; they can even be cultivated on a simple mineral medium containing only inorganic nutrient salts, exposed to vapors of petroleum, naphthalene, or other volatile hydrocarbons emanating from the soil. Soil mycobacteria do not require complex growth factors and proliferate relatively rapidly, whereas pathogenic species demand enriched media and exhibit slow growth.