How do sulfonamides treat bacterial infections without harming human cells?

sulfonamides inhibit the synthesis of folic acid in bacterial cells. Humans obtain it from their diet.

Explanation:

Sulfonamides are the sulfa drugs and serve as a competitive inhibitors of p -aminobenzoate (PABA). PABA is one of the substances involved in the synthesis of folic acid. sulfonamides compete with PABA for the enzyme of folic acid synthesis and thereby, block the pathways. In the absence of folic acid, bacterial growth is inhibited. Human cells can not synthesize folic acid but obtain it from the food they eat. Therefore, sulfonamides do not affect humans.  

b - the respond is innate and varies over time.

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