Week 5: Lessons 13, 14, 15 Antimycobacterial drugs

Mycobacteria are slender, rod-shaped bacteria with lipid-rich cell walls that stain poorly with the Gram stain, but once stained, the walls cannot be easily decolorized by treatment with acidified organic solvents. Hence, they are termed “acid-fast.” The most widely encountered  mycobacterial infection is tuberculosis—the leading cause worldwide of death from infection. Members of the genus Mycobacterium also cause leprosy, as well as, several tuberculosis-like human infections. Mycobacterial infections are intracellular and, generally, result in the formation of slow-growing granulomatous lesions that are responsible for major tissue destruction. Diagnostic testing for tuberculosis can be accomplished via the standard tuberculin skin test with purified protein derivative (PPD) or by an interferon-gamma release assay (IGRA) blood test, Quantiferon-TB Gold, approved by the FDA in 2005. The advantages that the blood test offers is that it requires only a single test visit, and it is less susceptible to false- positive results due to BCG vaccination or to infection with mycobacteria other than Mycobacterium tuberculosis. However, the cost of the blood test is more than that of the skin test, yet it reduces the expense of fol- low-up x-rays and lab tests needed with a tuberculin skin test. There are four currently recommended first-line agents utilized for antituberculosis therapy (Figure 34.1). Second-line medications are either less effective, more toxic, or have not been studied as extensively. They are useful in patients who cannot tolerate the first-line drugs or who are infected with myobacteria that are resistant to the first-line agents.

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