What is the difference between bactericidal and bacteriostatic antibiotics?

Question
The other day in class, we were discussing why some antibiotics work better for certain infections than others, and it reminded me of this key difference between bactericidal and bacteriostatic antibiotics. One kills bacteria outright, while the other just stops them from growing so your immune system can catch up. Knowing this helps understand treatment choices better in real clinical settings.

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Anttilampinen 2025-06-14T18:53:17+00:00 1 Answer 5 views
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  1. fleming
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    2025-07-13T14:41:36+00:00
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    Basic idea

    Bactericidal antibiotics kill the bug, bacteriostatic ones just stop it from growing so the immune system can finish the job.

    How they work

    Bactericidal

    • Target vital structures like cell wall (beta lactams), cell membrane (daptomycin) or DNA (fluoroquinolones)
    • Damage is irreversible, viable count drops by at least 99.9 % in 24 h in lab tests

    Bacteriostatic

    • Block protein synthesis (tetracyclines, macrolides) or folate pathway (sulfonamides) etc
    • If the drug is removed the cells can start dividing again

    Clinical relevance

    Most of the time either class works fine if the patient has a decent immune response. But in endocarditis, meningitis or severe neutropenia docs prefer bactericidal drugs because there aint much immune help available.

    Lab distinction

    We measure the minimum bactericidal concentration (MBC) and compare it with MIC. If MBC/MIC ≤ 4 the agent is labeled bactericidal.

    Take home

    Bactericidal = kill. Bacteriostatic = hold. Choice depends on bug, site and patient rather than a rigid rule.

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