The dilution of an antibiotic at which no bacteria grow is called what?

• A. MIC
• B. MBC
• C. Minimum effective concentration
• D. Therapeutic index

Answer: (A)

In antibiotic susceptibility testing, the key endpoint is the lowest concentration that prevents visible bacterial growth after a standard incubation. This is the minimum inhibitory concentration. It signifies growth is inhibited at that dilution, but it doesn’t necessarily mean the organisms are killed. To determine if the drug actually kills bacteria, you would test for the minimum bactericidal concentration by taking samples from the inhibited wells and seeing whether any organisms survive when transferred to drug-free media; if they do not grow, that concentration is bactericidal and typically lies higher than the MIC.

The other terms don’t fit this specific endpoint: therapeutic index describes safety margins of a drug, not growth inhibition; minimum effective concentration isn’t the standard term used in this context; and minimum bactericidal concentration refers to killing, which requires separate testing beyond the inhibition endpoint.