Which of the following best defines the minimal inhibitory concentration (MIC)?

• A. The highest concentration of an antibiotic that can be given to a patient without causing toxicity and that has been shown in clinical trials to reduce bacterial counts in vivo over time while sometimes permitting regrowth once the drug is withdrawn.
• B. The concentration of antibiotic measured at the peak serum level after a standard dose that correlates with clinical success in patients with severe infections.
• C. The lowest concentration of antibiotic that reduces viable colony counts by 99.9% on subculture onto antibiotic-free agar, demonstrating bactericidal activity.
• D. The lowest antibiotic concentration that prevents visible growth of a bacterium in a standard in vitro assay.

Answer: (D)