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How do sulfonamide antibiotics block folic acid production in bacteria?

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Sulfonamide antibiotics function as competitive inhibitors of folic acid synthesis in bacteria.12

Bacteria typically synthesize their own folic acid, which is an essential precursor for the production of nucleic acids. The process requires an enzyme to convert an intermediate compound called para-aminobenzoic acid (PABA) into folic acid.12

Because the molecular structure of sulfonamides is similar to that of PABA, the antibiotic competes with PABA for the active site of the necessary enzyme. By occupying this site, the sulfonamide prevents the bacteria from synthesizing the folic acid required for growth and multiplication. This action is bacteriostatic, meaning it inhibits bacterial growth without directly killing the organisms. Because humans and other higher animals must obtain folic acid through their diet rather than synthesizing it, this mechanism allows the drugs to target invading microorganisms while sparing the host.21