Sulfamethoxazole is a man-made synthetic antibiotic in the group called Sulfonamides – sulfa drugs. These drugs are broad spectrum bacteriostatic antibiotics – they stop the growth of bacteria but don’t necessarily kill them. Once the drug is removed, the microbe can resume growth.
The mode of action for sulfamethoxazole is inhibition of folic acid synthesis by binding to (and thus inhibiting) a key enzyme in the pathway. Folic acid (vitamin B9) is a critical co-factor used by numerous enzymes for biological reactions, including nucleic acid synthesis and repair. Bacteria that cannot make their own folic acid must have a means for acquiring it from their environment; those that can are resistant to sulfa drugs while those that can’t are killed.