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minimum bactericidal concentration

Microbiology / PharmacologyInfectious Disease (multisystem)Cardiovascular (infective endocarditis)Central Nervous System (meningitis)Musculoskeletal (osteomyelitis)

Summary

Minimum bactericidal concentration (MBC) is the lowest concentration of an antimicrobial that kills ≥99.9% (≥3-log10 reduction) of the initial bacterial inoculum after a fixed incubation period (typically 18–24 h). It is always ≥ the MIC (minimum inhibitory concentration); an MBC/MIC ratio ≤4 suggests bactericidal activity, while a ratio >4 suggests bacteriostatic activity or tolerance.

Detail

**Definition & how it is determined** - First determine the MIC by broth (macro/microdilution) testing: the lowest drug concentration that prevents visible growth. - Then subculture aliquots from all clear (no visible growth) tubes/wells onto drug-free agar and incubate. The MBC is the lowest concentration yielding ≥99.9% killing (≤0.1% survivors) of the original inoculum.

**MIC vs MBC** - MIC = inhibits growth (static endpoint). MBC = kills (cidal endpoint). MBC ≥ MIC always. - MBC/MIC ratio ≤4 → bactericidal. Ratio >4 → bacteriostatic or tolerant. - Bactericidal drugs: β-lactams, aminoglycosides, fluoroquinolones, vancomycin, daptomycin, metronidazole, rifampin (variable), isoniazid (against actively dividing mycobacteria), polymyxins. - Bacteriostatic drugs (mnemonic "buy AT 30, CCEL at 50"): Aminoglycosides are cidal; static agents include Tetracyclines, Chloramphenicol, Clindamycin, Erythromycin/macrolides, Linezolid (mostly), Sulfonamides, Trimethoprim. Note that static vs cidal can be organism- and concentration-dependent (e.g., linezolid is cidal against streptococci; chloramphenicol is cidal against H. influenzae, S. pneumoniae, N. meningitidis).

**Pathophysiology/Concepts** - Cidal activity may be concentration-dependent (aminoglycosides, fluoroquinolones: goal is high peak/MIC or AUC/MIC) or time-dependent (β-lactams, vancomycin: goal is time above MIC, fT>MIC). Macrolides/tetracyclines are generally AUC/MIC-dependent but static. - Tolerance: the organism is inhibited at normal MIC but not killed (MBC ≫ MIC, ratio ≥32). Seen with some S. aureus and viridans streptococci exposed to cell wall–active agents (defective autolysin activity). - Persisters: dormant, non-dividing subpopulations that survive cidal drugs without genetic resistance; relevant in biofilms and chronic infections. - Paradoxical (Eagle) effect: reduced killing at very high drug concentrations for some drugs (e.g., penicillin vs group A strep).

**Clinical significance** - Bactericidal therapy is preferred (though supporting evidence for superiority is limited outside these settings) in: infective endocarditis, meningitis, osteomyelitis, severe infections in neutropenic or other immunocompromised patients, where host defenses cannot assist in clearing the organism. - MBC testing is not routinely performed in clinical labs; MIC with susceptibility interpretation (CLSI/EUCAST breakpoints) is standard. MBC is used in special situations (endocarditis, tolerant organisms, treatment failure, research). - Serum bactericidal titer (Schlichter test): highest dilution of patient serum that kills ≥99.9% of the infecting organism; historically used to monitor therapy in endocarditis/osteomyelitis.

**High-yield board points** - MIC → inhibition; MBC → killing (≥99.9%). - MBC ≥ MIC; MBC/MIC ≤4 = cidal. - Disk diffusion (Kirby-Bauer) and E-test give susceptibility/MIC information, not MBC. - Synergy (e.g., β-lactam + aminoglycoside for enterococcal endocarditis) is shown by lower MBC of the combination: the cell-wall agent increases aminoglycoside uptake. - Tolerance ≠ resistance: tolerance is a MBC problem, resistance is a MIC problem.

Sources

  • First Aid for the USMLE Step 1 – Microbiology: Antimicrobial therapy / Pharmacology: Antibiotics
  • Katzung's Basic & Clinical Pharmacology – Antimicrobial pharmacodynamics
  • Murray's Medical Microbiology – Antimicrobial susceptibility testing
  • Goodman & Gilman's The Pharmacological Basis of Therapeutics – Principles of antimicrobial therapy
  • CLSI M26-A: Methods for Determining Bactericidal Activity of Antimicrobial Agents
  • Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases

Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.