Mycobacterium is a genus of nonmotile, non–spore-forming, aerobic bacilli in the Mycobacteriaceae family. They are slightly curved rods with filamentous and branching forms on occasion. They form mould-like pellicle in liquid medium; thus, the name Mycobacterium is derived from the word “mould,” which means fungus-like bacterium.
The cell wall is thick, complex, and lipid-rich, resulting in a hydrophobic surface. The bacteria’s lipid-rich cell wall also makes them resistant to common disinfectants and laboratory staining reagents. Mycobacteria are difficult to stain due to their lipid-rich waxy cell wall. However, once stained, they are resistant to decolorization with acid solution and alcohol. As a result, they are known as acid-fast bacilli (AFB). They are Gram positive, but they are difficult to stain and do not stain well with Gram stain.
Most mycobacteria grow slowly, with a generation time of 2–24 hours. In order for these bacteria to form visible colonies on solid media, they must be incubated for 3–8 weeks.
Classification of Mycobacterium
There are over 70 species in the genus Mycobacterium, many of which cause disease in humans. Hansen identified Mycobacterium leprae as the first species of the genus Mycobacterium in 1874. Furthermore, in 1882, Robert Koch isolated the mammalian tubercle bacillus and established its role as a causative agent of tuberculosis, as it met all of Koch’s postulates. Mycobacterium tuberculosis was later identified as the causative agent of tuberculosis.
The current term for the four species that can cause tuberculosis in humans and other mammals is Mycobacterium tuberculosis complex. They are as follows:
- M. tuberculosis, human strain
- Mycobacterium bovis, type bovine
- Mycobacterium africanum, a species intermediate between human and bovine tuberculosis that primarily causes human tuberculosis in tropical Africa.
- Mycobacterium microti, a species that is pathogenic to goats and other small animals but does not infect humans.
The third group of mycobacterial species isolated from various sources is saprophytic mycobacteria. This species has been isolated from both cold-blooded and warm-blooded animals, skin ulcers, and environmental sources such as soil and water. Originally known as atypical environmental or opportunistic mycobacteria, these saprophytic mycobacteria are now known as mycobacteria other than typical tubercle mycobacteria (MOTT).
Human infections caused by most common Mycobacterium species
| Bacteria | Diseases |
| Mycobacterium tuberculosis | Tuberculosis |
| Mycobacterium leprae | Leprosy |
| Mycobacterium avium complex | Opportunistic mycobacterial infections (pulmonary disease lymphadenopathy, disseminated disease) |
| Mycobacterium kansasii | Opportunistic mycobacterial infections (pulmonary disease) |
| Mycobacterium fortuitum | Opportunistic mycobacterial infections (post-trauma abscess) |
| Mycobacterium chelonae | Opportunistic mycobacterial infections (post-trauma abscess) |
| Mycobacterium abscessus | Opportunistic mycobacterial infections (skin infection) |
General Properties of Mycobacterium species
Members of the genus Mycobacterium exhibit the following characteristics:
- They are acid fast
- They contain mycolic acid, which is made up of 60–90 carbons that are split to form fatty acid methyl esters.
- They have a high guanine and cytosine content in their DNA (61–71%).
Mycobacterium cell wall: Mycobacterium cell wall is complex. It’s typical of Gram-positive bacteria. It has a distinct cytoplasmic membrane that is surrounded by a thick peptidoglycan layer and a layer of highly antigenic mycolic acid. It has a thick consistency and is high in lipids. The bacteria benefit from a variety of properties due to their complex cell wall. Peptidoglycolipids, glycolipids, and lipids make up mycolic acid. These constitute nearly 60% of the cell wall’s dry weight. The outer layer is made up of peptic chains and serves as an antigen for the bacteria, inducing cellular immunity in infected patients. A purified protein derivative (PPD) of this layer is used as an antigen source in the skin test to demonstrate the host’s hypersensitivity to M. tuberculosis.
