Definition

Tuberculosis is an infection caused by micro-organisms of the Mycobacterium tuberculosis complex, and is usually taken to include infections caused by the human and bovine strains of the organism, but not those infections caused by other ‘Non–tuberculous’ mycobacteria such as Mycobacterium avium-intracellulare and others, which are described in full detail in Chapter 11. Such non-tuberculous mycobacteria have also been described as ‘atypical’, ‘anonymous’, ‘opportunistic’ or ‘Mycobacteria other than tuberculosis’ (MOTT), but these terms are generally discouraged.

Characteristics of Mycobacterium tuberculosis

The classic property of the mycobacteria in general is that of being acid fast; that is, when stained with a dye such as carbol fuchsin, as in the Ziehl–Neelsen stain, weak mineral acids do not remove the colour. The stain is also resistant to removal by alcohol, so that they are known as acid and alcohol-fast bacilli, or AAFB. This property is shared by all mycobacteria, including the non-tuberculous mycobacteria.

They are unicellular organisms, containing polysaccharides, antigenic proteins, and lipids. The cell wall has a very high lipid content and is formed in several layers, the innermost being composed of short peptide chains cross-linked with long polysaccharide chains, forming a peptidoglycan called murein. The intermediate layer of the cell wall is formed by arabinogalactan, a polysaccharide attached to long chain fatty acids called mycolic acids, and the outer layer consists of glycolipids named mycosides.