Despite important advances in its treatment over the past 2 decades, tuberculosis (TB) remains a major infectious disease. Approximately one-third of the world’s population harbors Mycobacterium tuberculosis and is at risk for developing disease in the near or distant future. The incidence and prevalence of TB increased in the 1990s and 2000s partly due to the human immunodeficiency virus (HIV) epidemic and the prevalence of drug-resistant TB. The failure to control TB in both developed and developing countries represents one of our greatest public health failures.
M tuberculosis produces a spectrum of clinical entities that have differing diagnostic and management approaches. TB exposure occurs when one individual has been in recent contact with an individual who has a contagious form of TB disease. The exposed individual has negative tests of infection, a normal chest radiograph, and normal physical exam findings. Whereas adults in this stage usually do not get treated, children under 5 years of age are treated because progression to disease may occur rapidly, even before the tests of infection turn positive. TB infection occurs when an individual has inhaled M tuberculosis, has a positive test of infection, has no signs or symptoms of disease, and has a chest radiograph that is normal or only reveals granulomas or calcifications. TB disease occurs when an individual with TB infection develops signs and symptoms and/or radiographic changes characteristic of TB.
PATHOGENESIS AND EPIDEMIOLOGY
Mycobacteria are nonmotile, non–spore-forming, pleomorphic, weakly gram-positive rods that are typically slender and slightly bent. The cell walls contain lipid and wax that make these organisms more resistant than most others to light, alkali, acid, and the bactericidal action of antibodies. Growth is slow, with a generation time of 14 to 24 hours. Acid fastness, the capacity to perform stable mycolate complexes with certain aryl methane dyes, is the hallmark of mycobacteria. Cells appear red when stained with fuchsin (Ziehl-Neelsen or Kinyoun stain; Fig. 264-1), appear purple with crystal violet, or exhibit yellow-green fluorescence under ultraviolet light (auramine and rhodamine, as in Truant stain). Truant stain is the most sensitive method for visualizing mycobacteria in a clinical specimen.
Mycobacterium tuberculosis on a Kinyoun stain.
Identification of mycobacteria species depends on their staining properties and their biochemical and metabolic characteristics. Isolation of these obligate aerobes on solid media often takes 3 to 6 weeks, after which the growth can be replated onto solid media with antituberculosis drugs to allow for drug susceptibility testing. Full drug susceptibility results can take up to 2 to 4 additional weeks for drug susceptibility testing. The automated methods using liquid broth allow isolation and identification from clinical specimens and identification of mycobacteria within 7 to 10 days. Other molecular methods of identification are available, including polymerase chain reaction (PCR) techniques that are discussed in ...