Both sulfonamides and trimethoprim (not a sulfonamide) sequentially interfere with folic acid synthesis by bacteria. Folic acid functions as a coenzyme in the transfer of one-carbon units required for the synthesis of thymidine, purines, and some amino acids and consists of three components: a pteridine moiety, PABA, and glutamate (Fig. 44.1). The sulfonamides, as structural analogues, competitively block PABA incorporation; sulfonamides inhibit the enzyme dihydropteroate synthase, which is necessary for PABA to be incorporated into dihydropteroic acid, an intermediate compound in the formation of folinic acid. Since the sulfonamides reversibly block the synthesis of folic acid, they are bacteriostatic drugs. Humans cannot synthesize folic acid and must acquire it in the diet; thus, the sulfon-amides selectively inhibit microbial growth.
Resistance to the sulfonamides can be the result of decreased bacterial permeability to the drug, increased production of PABA, or production of an altered dihy-dropteroate synthetase that exhibits low affinity for sul-fonamides. The latter mechanism of resistance is plas-mid mediated. Active efflux of the sulfonamides has also been reported to play a role in resistance. The inhibitory effect of the sulfonamides also can be reversed by the presence of pus, tissue fluids, and drugs that contain releasable PABA.
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