bactericidal
Summary
Bactericidal refers to antibiotics or antimicrobial agents that directly kill bacteria, as opposed to bacteriostatic agents which merely inhibit bacterial growth and rely on host immune defenses to clear the infection. This distinction is clinically important when choosing antibiotics for immunocompromised patients or severe infections like endocarditis and meningitis.
Detail
Bactericidal agents work through various mechanisms including inhibition of cell wall synthesis (beta-lactams, vancomycin), disruption of cell membrane integrity (daptomycin, polymyxins), inhibition of DNA gyrase/topoisomerase (fluoroquinolones), or irreversible binding to the 30S ribosomal subunit causing misreading of mRNA (aminoglycosides). These mechanisms lead to bacterial cell death rather than just halting replication.
Clinically, bactericidal drugs are preferred in situations where the host immune system cannot adequately clear pathogens even after growth inhibition, such as in neutropenic patients, infective endocarditis, and bacterial meningitis where CSF has limited immune cell penetration. The minimum bactericidal concentration (MBC) is defined as the lowest concentration that kills ≥99.9% of bacteria, and is typically close to the minimum inhibitory concentration (MIC) for bactericidal drugs, whereas bacteriostatic drugs show a much higher MBC:MIC ratio.
Important bactericidal antibiotic classes include: beta-lactams (penicillins, cephalosporins, carbapenems), vancomycin, aminoglycosides, fluoroquinolones, metronidazole, and daptomycin. This contrasts with bacteriostatic agents like macrolides, tetracyclines, clindamycin, sulfonamides, and linezolid. Note that some drugs can be bactericidal against certain organisms but bacteriostatic against others (e.g., linezolid is bactericidal against streptococci but bacteriostatic against staphylococci and enterococci).
A classic exam pearl: combining a bacteriostatic drug with a bactericidal drug that requires active bacterial growth (like beta-lactams targeting cell wall synthesis) can be antagonistic, since the bacteriostatic agent halts the growth phase the bactericidal drug depends on for efficacy.
Sources
- First Aid for the USMLE Step 1
- Katzung's Basic and Clinical Pharmacology
- Sherris Medical Microbiology
- UpToDate: Principles of antimicrobial therapy
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.