518 Chapter 21 Antimicrobial Medications cells, though to a lesser extent, which generally limits their use to topical applications.

Antibacterial Medications that Interfere with Processes Essential to Mycobacterium tuberculosis

Only a limited range of antimicrobials can be used to treat infections caused by Mycobacterium tuberculosis. This is due to several factors, including the chronic nature of the disease caused by these organisms (tuberculosis), their slow growth, and their waxy cell wall, which is impervious to many drugs. A group of five medications, called the first-line drugs, are preferred because they are the most effective as well as the least toxic. These are generally given in combination of two or more to patients who have active tuberculosis. This combination therapy prevents the development of resistant mutants; if some cells in the infecting population spontaneously develop resistance to one drug, the other drug will eliminate them. The second-line medications can be used if the first-line drugs are not an option; however, they are either less effective or more toxic. ■ Mycobacterium tuberculosis, p. 582

Of the first-line medications, some specifically target the unique cell wall that characterizes the mycobacteria. Isoniazid inhibits the synthesis of mycolic acids, a primary component of the cell wall. Ethambutol inhibits enzymes that are required for synthesis of other mycobacterial cell wall components. The mechanism of pyrazinamide is unknown. Other first-line drugs include rifampin and streptomycin, which have already been discussed.

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