Final answer:
Sulfonamide drugs inhibit microbial growth through competitive inhibition of the enzyme dihydropteroate synthase, by mimicking the substrate PABA, thereby preventing bacteria from synthesizing folic acid necessary for nucleic acid synthesis and growth. B. competitive inhibition
Step-by-step explanation:
The general mechanism that sulfonamide drugs use to inhibit microbial growth is known as competitive inhibition. These drugs are structural analogs to para-aminobenzoic acid (PABA), a substance that bacteria use to synthetize their own folic acid, which is essential for the production of nucleic acids and thus for bacterial growth and reproduction. When sulfonamides enter the bacterial cell, they competitively inhibit the enzyme dihydropteroate synthase, hindering the conversion of PABA into dihydrofolic acid, thus blocking the subsequent synthesis of folic acid and ultimately nucleic acids.
Humans are not affected by these drugs in the same way because humans do not synthesize folic acid — we obtain it from our diet. Therefore, sulfonamides exploit a difference between bacterial and human metabolism, achieving a selective toxicity which makes them effective antibacterials. However, some individuals may experience allergic reactions to these drugs.
In addition to competitive inhibition, microbial resistance to sulfonamides can occur through other means such as enzyme overproduction or enzymatic bypass mechanisms.