Tuberculosis (TB), which is caused by bacteria of the Mycobacterium tuberculosis complex, is one of the oldest diseases known to affect humans and the top cause of infectious death worldwide. Population genomic studies suggest that M. tuberculosis may have emerged ~70,000 years ago in Africa and subsequently disseminated along with anatomically modern humans, expanding globally during the Neolithic Age as human density started to increase. Progenitors of M. tuberculosis are likely to have affected prehominids. This disease most often affects the lungs, although other organs are involved in up to one-third of cases. If properly treated, TB caused by drug-susceptible strains is curable in the vast majority of cases. If untreated, the disease may be fatal within 5 years in 50–65% of cases. Transmission usually takes place through the airborne spread of droplet nuclei produced by patients with infectious pulmonary TB.
THE ETIOLOGIC AGENT OF TUBERCULOSIS
Mycobacteria belong to the family Mycobacteriaceae and the order Actinomycetales. Of the pathogenic species belonging to the M. tuberculosis complex, which comprises eight distinct subgroups, the most common and important agent of human disease by far is M. tuberculosis (sensu stricto). A closely related organism isolated from cases in West, Central, and East Africa is M. africanum. The complex includes some zoonotic members, such as M. bovis (the bovine tubercle bacillus—characteristically resistant to pyrazinamide, once an important cause of TB transmitted by unpasteurized milk, and currently responsible for ~150,000 human cases worldwide, half of them in Africa) and M. caprae (related to M. bovis). In addition, other organisms that have been reported rarely as causing TB include M. pinnipedii (a bacillus infecting seals and sea lions in the Southern Hemisphere and recently isolated from humans), M. mungi (isolated from banded mongooses in southern Africa), M. orygis (described in oryxes and other Bovidae in Africa and Asia and a potential cause of infection in humans), and M. microti (the “vole” bacillus, a less virulent organism). Finally, M. canetti is a rare isolate from East African cases that produces unusual smooth colonies on solid media and is considered closely related to a supposed progenitor type. There is no known environmental reservoir for any of these organisms.
M. tuberculosis is a rod-shaped, non-spore-forming, thin aerobic bacterium measuring 0.5 μm by 3 μm. Mycobacteria, including M. tuberculosis, are often neutral on Gram’s staining. However, once stained, the bacilli cannot be decolorized by acid alcohol; this characteristic justifies their classification as acid-fast bacilli (AFB; Fig. 173-1). Acid fastness is due mainly to the organisms’ high content of mycolic acids, long-chain cross-linked fatty acids, and other cell-wall lipids. Microorganisms other than mycobacteria that display some acid fastness include species of Nocardia and Rhodococcus, Legionella micdadei, and the protozoa Isospora and Cryptosporidium. In the mycobacterial cell wall, lipids (e.g., mycolic acids) are linked to underlying arabinogalactan and peptidoglycan. This structure results in very ...