Tuberculosis (TB), which is one of the oldest diseases known to affect humans and is likely to have existed in prehominids, is a major cause of death worldwide. This disease is caused by bacteria of the Mycobacterium tuberculosis complex and usually 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 virtually all 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.
Mycobacteria belong to the family Mycobacteriaceae and the order Actinomycetales. Of the pathogenic species belonging to the M. tuberculosis complex, the most common and important agent of human disease is M. tuberculosis. The complex includes M. bovis (the bovine tubercle bacillus—characteristically resistant to pyrazinamide, once an important cause of TB transmitted by unpasteurized milk, and currently the cause of a small percentage of cases worldwide), M. caprae (related to M. bovis), M. africanum (isolated from cases in West, Central, and East Africa), M. microti (the “vole” bacillus, a less virulent and rarely encountered organism), M. pinnipedii (a bacillus infecting seals and sea lions in the Southern Hemisphere and recently isolated from humans), and M. canetti (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).
M. tuberculosis is a rod-shaped, nonspore-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. 165-1). 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 confers very low permeability of the cell wall, thus reducing the effectiveness of most antibiotics. Another molecule in the mycobacterial cell wall, lipoarabinomannan, is involved in the pathogen-host interaction and facilitates the survival of M. tuberculosis within macrophages. The complete genome sequence of M. tuberculosis comprises 4043 genes encoding 3993 proteins and 50 genes encoding RNAs; its high guanine-plus-cytosine content (65.6%) is indicative of an aerobic “lifestyle.” A large proportion of genes are devoted to the production of enzymes involved in cell wall metabolism.
Acid-fast bacillus smear showing M. tuberculosis bacilli. (Courtesy of the CDC, Atlanta.)
More than 5.8 million new cases of TB (all forms, both pulmonary and extrapulmonary) were reported to the ...