Mycobacterial immunity and mycobacterial disease in relation to HIV infection

Mycobacterium tuberculosis is an obligatory parasite with no free-living (saprophytic) forms. It has the capacity to survive and multiply within mononuclear phagocytes (i.e. monocytes and macrophages), as well as survive extracellularly. Unlike pyogenic bacterial pathogens, mycobacteria multiply slowly with a generation time of 12–24h. In consequence, tuberculosis is a chronic disease which is slow to develop and progress (cf. meningococcal septicaemia). The tubercle bacillus is well adapted to be a successful parasite in the human host and can survive for many years within an infected host. Factors which contribute to the prolonged in vivo survival of M. tuberculosis are summarized in Table 7.1.