For an organism with MIC of 4 mg/L and MBC of 64 mg/L for drug X, the drug is classified as:

• A Bactericidal — MBC/MIC ratio <4
• B Bactericidal — any drug achieving the MBC is bactericidal by definition
• C Bacteriostatic — MBC/MIC ratio >4 (here 16-fold), indicating tolerance ✓
• D Cannot be classified without post-antibiotic effect measurement

Explanation

The MBC/MIC ratio distinguishes bactericidal from bacteriostatic drugs for a given organism. When MBC is ≥4-fold (some references say ≥16-fold) higher than the MIC, the drug is classified as bacteriostatic for that organism, or the organism is said to show 'tolerance'. In this case MBC/MIC = 64/4 = 16, meeting criteria for bacteriostatic behaviour or tolerance. True bactericidal agents (penicillins, aminoglycosides, fluoroquinolones) typically have MBC/MIC ≤4. This distinction is clinically relevant in endocarditis, osteomyelitis, and meningitis where bactericidal drugs are preferred.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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