In the broth microdilution method, a series of antibiotic concentrations is prepared and inoculated with a standardised bacterial suspension (0.5 McFarland). After 18–24 hours incubation, wells 1–4 show turbidity and wells 5–12 show no turbidity. The lowest concentration in a clear well is the MIC. The contents of the clear wells are subcultured to agar plates — wells 5 and 6 show no growth but wells 7–12 show growth. What is the MBC in this experiment?

• A The concentration in well 5 — the lowest concentration yielding ≥99.9% killing on subculture ✓
• B The concentration in well 4 — the last turbid well before the MIC
• C The concentration in well 7 — the first well with regrowth on subculture
• D The MBC equals the MIC because both well 5 and well 6 are sterile

Explanation

The Minimum Bactericidal Concentration (MBC) is the lowest antibiotic concentration that kills ≥99.9% (3-log10 reduction) of the original inoculum. It is determined by subculturing the clear (no-turbidity) MIC wells onto antibiotic-free agar plates and reading for growth after 18–24 hours. The MBC is the lowest concentration among the clear wells from which no growth (or ≤0.1% of inoculum) appears on subculture. In this scenario wells 5 and 6 are both sterile on subculture while well 7 grows — this means MBC = concentration in well 5 (the lowest bactericidal concentration). The MBC always equals or exceeds the MIC; a large MBC/MIC ratio (≥32-fold) defines tolerance and is clinically relevant for endocarditis management.

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

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