The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent required to prevent visible growth of a microorganism after a defined incubation period. It quantifies the potency of antibiotics, antifungals or antivirals against a particular isolate.
Determination and Interpretation
MIC values are determined using standardized laboratory methods. In broth microdilution, a series of two‑fold dilutions of the antimicrobial agent is prepared in a microtiter plate and inoculated with a standardized number of bacterial or fungal cells. After incubation, the lowest drug concentration that shows no visible turbidity is recorded as the MIC. Agar dilution uses agar plates containing graded concentrations of the drug to observe growth inhibition. Gradient diffusion strips (Etest) generate a concentration gradient on an agar plate; the MIC is read where the elliptical zone of inhibition intersects the strip. Interpretation of MIC results relies on clinical breakpoints established by organizations such as the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST), which classify isolates as susceptible, intermediate or resistant based on achievable drug concentrations in patients. MIC determinations can vary with inoculum size, incubation time, medium composition, pH and the presence of serum proteins.
Applications and Clinical Importance
MIC data guide the selection and dosing of antimicrobial therapy. Clinicians use MIC values, together with pharmacokinetic and pharmacodynamic parameters, to choose drugs that achieve concentrations above the MIC at the site of infection. Monitoring MICs across isolates helps detect emerging resistance and informs infection control policies. During drug development, MIC testing screens new compounds for antimicrobial activity and guides structure‑activity optimization. MIC determinations are also used in combination tests to evaluate synergistic effects between drugs. While the MIC measures inhibition of growth, the minimum bactericidal concentration (MBC) defines the lowest concentration that kills the organism; comparing MIC and MBC can reveal bacteriostatic versus bactericidal activity.
The concept of minimum inhibitory concentration provides a quantitative benchmark for antimicrobial efficacy. Determining and interpreting MICs under standardized conditions is essential for effective clinical decision‑making, surveillance of resistance and development of new antimicrobial agents.
Related Terms: Antimicrobial susceptibility testing, Breakpoint, Broth dilution, Etest, Minimum bactericidal concentration