Which of the following antibiotics are considered bacteriostatic?

• A. Penicillins
• B. Bactrim
• C. Trimethoprim
• D. FQs

Answer: (C)

Understanding whether an antibiotic is bacteriostatic or bactericidal depends on whether it stops bacterial growth or actually kills the bacteria. Trimethoprim blocks the bacterial enzyme dihydrofolate reductase, which is needed to make tetrahydrofolate and, in turn, synthesize thymidine and other nucleotides essential for DNA replication. With DNA synthesis halted, bacteria stop growing but aren’t immediately killed—their cells remain intact and die off only once the immune system clears them. That stopping of growth while viability is preserved is the signature of a bacteriostatic agent.

Penicillins and fluoroquinolones, in contrast, damage essential structures or processes that lead to bacterial death, so they’re typically bactericidal. Bactrim, the combination of sulfamethoxazole and trimethoprim, acts synergistically to block folate synthesis and is often bactericidal against many pathogens, though the single-agent trimethoprim is commonly described as bacteriostatic.