In broth dilution susceptibility testing, how is the minimum bactericidal concentration (MBC) determined after overnight incubation?
- A It is read visually as the lowest concentration showing no visible turbidity
- B It is measured spectrophotometrically as the concentration giving 50% reduction in optical density
- C Aliquots from each clear tube are subcultured onto antibiotic-free agar, and the MBC is the lowest concentration yielding no growth or fewer than 0.1% of the original inoculum ✓
- D It equals twice the MIC by definition
Correct answer: C. Aliquots from each clear tube are subcultured onto antibiotic-free agar, and the MBC is the lowest concentration yielding no growth or fewer than 0.1% of the original inoculum
Explanation
MIC is the lowest concentration with no visible growth, read directly from the broth tubes. Determining MBC requires subculturing samples from the clear tubes onto drug-free agar, because residual viable cells invisible to the naked eye must be quantified; the MBC is the lowest concentration killing 99.9% or more of the starting inoculum. Option A describes the MIC, and a fixed twofold relationship between MIC and MBC does not exist; a ratio of 32 or more defines tolerance.
Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.
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