The minimum inhibitory concentration (MIC) is defined as the lowest concentration of an antimicrobial agent that:

• A Kills 99.9% of bacteria in an inoculum
• B Produces a 19 mm zone of inhibition on Kirby-Bauer disc diffusion
• C Reduces bacterial colony count to <1% of inoculum
• D Inhibits visible growth of the test organism after overnight incubation ✓

Explanation

The MIC is the lowest concentration of antibiotic that inhibits visible bacterial growth after overnight incubation in a broth or on agar. It is the fundamental parameter used to classify organisms as susceptible (S), intermediate (I), or resistant (R) according to CLSI/EUCAST breakpoints. The minimum bactericidal concentration (MBC) is the lowest concentration that kills ≥99.9% of bacteria. Zone of inhibition is a surrogate measure from disc diffusion (Kirby-Bauer), not MIC itself.

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

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