The lowest concentration of an antimicrobial that prevents visible growth is the

• A. Bacteriostatic
• B. Bactericidal
• C. Kirby-Bauer Test
• D. Minimum Inhibitory Concentration (MIC)

Answer: (D)

The concept being tested is how we define the potency of an antimicrobial in terms of the amount needed to stop growth. The lowest concentration that prevents visible growth is called the minimum inhibitory concentration, or MIC. In a standard broth dilution test, you prepare a series of decreasing drug concentrations and inoculate them with bacteria. After incubation, you look for the first tube or well that remains clear; that concentration is the MIC—the smallest amount that stops the organisms from multiplying.

This differs from bacteriostatic versus bactericidal effects: bacteriostatic means the drug halts bacterial growth without necessarily killing the cells, while bactericidal means the drug kills the bacteria. The Kirby-Bauer test uses disk diffusion to assess susceptibility by measuring the zone of inhibited growth around an antibiotic disk, not the exact concentration needed to prevent growth. Thus, the lowest concentration that prevents growth is the MIC.