Sulfapyridine is a competitive inhibitor of the bacterial enzyme dihydropteroate synthetase. The inhibited reaction is necessary in these organisms for the synthesis of folic acid by means of processing the substrate para-aminobenzoic acid (PABA). Dihydropteroate synthetase activity is vital in the synthesis of folate, and folate is required for cells to make nucleic acids, such as DNA or RNA. So if DNA molecules cannot be built, the cell cannot divide. (en)