How do sulfonamides block folic acid production in bacteria?

Sulfonamides act as competitive inhibitors of folic acid synthesis in bacteria. Because most bacteria must synthesize their own folic acid from simpler precursors, this process is an essential step for their growth and the production of nucleic acids.12

The mechanism functions through structural mimicry: sulfonamides are chemically similar to para-aminobenzoic acid (PABA), an intermediate compound that bacteria normally convert into folic acid using specific enzymes. Because of this similarity, the sulfonamide competes with PABA for the active site of the enzyme responsible for this conversion. By occupying the enzyme, the drug prevents the bacteria from synthesizing the folic acid required for multiplication.21

This process is bacteriostatic, meaning it inhibits bacterial growth rather than killing the cells directly. Importantly, because humans and other higher animals obtain folic acid through their diet rather than synthesizing it, sulfonamides can selectively inhibit bacterial growth without harming the host.12