Microbiology and pathogenesis

Mycobacteria are thin rods with a very different cell wall structure compared with other bacteria. The high lipid and mycolic acid content in the cell wall resists conventional Gram staining and special stains such as the acid- and alcohol-based Ziehl–Neelsen stain is required to make them visible under the microscope (see Fig. 3.3.4A). Based on this stain, the bacteria are also known as acid-fast bacilli (AFB). Most mycobacteria are slow growing (up to 6 weeks) and require special culture medium (e.g. Lowenstein–Jensen agar).

Mycobacteria are intracellular organisms that survive and multiply in macrophages. Intact cell-mediated immunity helps to wall off the bacteria in infected tissues by forming granulomas (also called tubercle). Mycobacteria can lay dormant for many years until they are reactivated. This may occur if the patient’s cellular immune system is compromised, such as in old age, corticosteroid use or HIV infection.