Why are sulfa drugs & trimethoprim often used together?

• A. Sulfa drugs target Gram-positive bacteria, while trimethoprim targets Gram-negative bacteria.
• B. Trimethoprim destroys naturally occurring enzymes that inhibit sulfa drugs.
• C. They exhibit synergism by inhibiting different steps of the same metabolic pathway.
• D. Trimethoprim blocks sulfa drugs from affecting folic acid synthesis in human cells.

Answer: (C)

Verified step by step guidance

1. Understand the mechanism of action of sulfa drugs: Sulfa drugs inhibit the enzyme dihydropteroate synthase, which is involved in the early step of folic acid synthesis in bacteria.
2. Understand the mechanism of action of trimethoprim: Trimethoprim inhibits the enzyme dihydrofolate reductase, which acts later in the folic acid synthesis pathway in bacteria.
3. Recognize that both drugs target different enzymes in the same metabolic pathway (folic acid synthesis), which is essential for bacterial growth and replication.
4. Realize that using both drugs together results in a synergistic effect, meaning the combined effect is greater than the sum of their individual effects, because they block sequential steps in folic acid synthesis.
5. Note that this synergism enhances antibacterial efficacy and reduces the likelihood of resistance development, which is why sulfa drugs and trimethoprim are often used in combination therapy.