Mycobacteria belong to the order Actinomycetales, family Mycobacteriaceae, and Genus Mycobacterium.
There are more than 80 species of mycobacteria. The two major pathogens are:
- Mycobacterium tuberculosis (Koch, 1882)
- Mycobacterium leprae (Hansen, 1874).
The rest are environmental organisms that are collectively known as Non-tuberculous mycobacteria (NTM). NTM are responsible for opportunistic infections especially in immunosuppressed patients.
- Characteristics of Mycobacteria
- Slim and rod-like obligate anaerobes
- Non-motile
- unencapsulated
- Non-sporing.
- Sensitive to UV light, sunlight, and heat at 60-80 C.
- They have a modified gram positive cell wall that makes them hardy, impenetrable and hydrophobic.
- The mycobacterium cell wall is unique since it is waxy, hydrophobic and has a high lipid content(up to 60% of the dry weight is mycolic acid – a long chain branched fatty acid)
- Mycolic acids and short chain fatty acids form a pseudo outer membrane that has unusual staining characteristics.
- Cord factor (Trehalose dimycolate) is essential for growth and adherence. It is correlated with virulence (virulent strains grow in a characteristic “serpentine” cordlike pattern while avirulent strains lack cord factor)
- Wax-D (glycolipids) have adjuvant properties (in vaccines) and phosphatides play a role in caseation necrosis.
- It also has several proteins (antigens in the purified protein derivative, PPD), which when combined with waxes produce a delayed type hypersensitivity reaction in PPD.
- The cell wall is resistant to alkalis, acids and most disinfectants.
- Acid fast – they resist decolourisation with acid and alcohol once stained with arylmethane dyes.
- Human strains show heavy growth on culture (eugonic) while bovine strains show poor growth on glycerol containing media (dysgonic).
- Intracellular pathogens – the primary host cell infected in humans are macrophages.
- Mycobacteria do not produce endotoxins, or exotoxins and instead elicit a granulomatous response that has a chronic course. The disease process is largely due to reactions to mycobacterial lipids (cord factor and Wax-D)
Components of the mycobacterium cell wall from inside outwards
| Layer | Layer Type | Key component | Function |
|---|---|---|---|
| Capsule | Outermost matrix | Polysaccharides (alpha-D-glucan, arabinomannan), proteins, lipids | Immune evasion, phagocytosis resistance, protection from oxidative stress |
| Mycomembrane (Outer Leaflet) | Extractable free lipids | Trehalose dimycolate (TDM/Cord factor), TMM, PDIM, sulfolipids, GPLs | Virulence factor, macrophage inhibition, masking pathogen-associated patterns |
| Mycomembrane (Inner Leaflet) | Bound lipid layer | Mycolic acids | Forms a highly impermeable hydrophobic barrier; esterified to arabinogalactan |
| Arabinogalactan (AG) | Heteropolysaccharide scaffold | D-arabinofuranose and D-galactofuranose with rhamnose-phosphate linker | Covalently anchors outer mycolic acids to underlying peptidoglycan layer |
| Peptidoglycan (PG) | Basal meshwork | Alternating GlcNAc & MurNGly cross-linked via meso-DAP peptides | Provides structural strength, maintains cell shape, prevents osmotic lysis |
| Periplasmic Space | Aqueous compartment | Solute-binding transport proteins, enzymes, biosynthetic machinery | Nutrient transport, protein sorting, and metabolic recycling |
| Plasma Membrane | Inner lipid bilayer | Phospholipids, transport proteins, anchored lipoglycans (PIM, LM, LAM) | Main metabolic/osmotic barrier; anchors lipoarabinomannan extending upward |
Note: Layers 3, 4, and 5 form the mAGP complex (mycolic acid–arabinogalactan–peptidoglycan), the covalently linked structural core responsible for the characteristic resistance and acid-fast nature of mycobacteria.