Background:

Sulfamethoxazole (Ro 4-2130) is a sulfonamide bacteriostatic antibiotic. Sulfamethoxazole is being used for human and veterinary applications[1].

Sulfonamides are structural analogs and competitive antagonists of para-aminobenzoic acid (PABA). They inhibit normal bacterial utilization of PABA for the synthesis of folic acid, an important metabolite in DNA synthesis. The effects seen are usually bacteriostatic in nature. Folic acid is not synthesized in humans, but is instead a dietary requirement. This allows for the selective toxicity to bacterial cells (or any cell dependent on synthesizing folic acid) over human cells. Bacterial resistance to sulfamethoxazole is caused by mutations in the enzymes involved in folic acid synthesis that prevent the drug from binding to it.

参考文献:
[1]. Straub JO, et al. Aquatic environmental risk assessment for human use of the old antibiotic sulfamethoxazole in Europe. Environ Toxicol Chem. 2016 Apr;35(4):767-79.