What is mycobacterium?

Mycobacterium is a genus of Actinomycetota, given its own family, the Mycobacteriaceae. Over 190 species are recognized in this genus. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae) in humans. The Greek prefix myco- means ‘fungus’, alluding to the way mycobacteria have been observed to grow in a mold-like fashion on the surface of cultures. It is acid-fast and cannot be stained by the Gram stain procedure.
Mycobacteria are Gram-positive, catalase positive, non-motile, non-spore forming rod-shaped bacteria (0.2–0.6 μm wide and 1.0–10 μm long). The colony morphology of mycobacteria varies with some species growing as rough or smooth colonies. Colony colour ranges from white to orange or pink (Iivanainen, 1999). Most mycobacteria are aerobic organisms, although some species are microaerophilc (Falkinham, 1996).

The cell walls of mycobacteria are very thick and consist of four layers. The innermost layer is composed of peptidoglycan and the others of lipids. The presence of lipid provides the bacteria with resistance to acid and alkaline environments and renders the cells relatively impermeable to various basic dyes, which need to be combined with phenol to allow penetration of the cell wall. The cell wall composition renders mycobacteria hydrophobic, and as a result these bacteria tend to grow in aggregates that ‘float’ on the surface of liquid media. Detergents, such as Tween® 80 can be added to culture media to disperse the organisms. Certain mycobacteria, including Mycobacterium paratuberculosis (MAP), can shed their cell walls, forming spheroplasts which are not detected using the acid-fast stain test (Hines and Styer, 2003).