Minimum inhibitory concentration

In microbiology, the minimum inhibitory concentration (MIC) is the lowest concentration of a chemical, usually a drug, which prevents visible in vitro growth of bacteria or fungi.^[1][2] MIC testing is performed in both diagnostic^[1][2] and drug discovery laboratories.^[3][4]

The MIC is determined by preparing a dilution series of the chemical, adding agar or broth, then inoculating with bacteria or fungi, and incubating at a suitable temperature. The value obtained is largely dependent on the susceptibility of the microorganism and the antimicrobial potency of the chemical, but other variables can affect results too.^[5] The MIC is often expressed in micrograms per milliliter (μg/mL) or milligrams per liter (mg/L).

In diagnostic labs, MIC test results are used to grade the susceptibility of microbes. These grades are assigned based on agreed upon values called breakpoints. Breakpoints are published by standards development organizations such as the U.S. Clinical and Laboratory Standards Institute (CLSI), the British Society for Antimicrobial Chemotherapy (BSAC) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST).^[6][7][8] The purpose of measuring MICs and grading microbes is to enable physicians to prescribe the most appropriate antimicrobial treatment.

The first step in drug discovery is often measurement of the MICs of biological extracts, isolated compounds or large chemical libraries against bacteria and fungi of interest.^[9][10] MIC values provide a quantitative measure of an extract or compound's antimicrobial potency. The lower the MIC, the more potent the antimicrobial.^[4] When in vitro toxicity data is available, MICs can also be used to calculate selectivity index values, a measure of off-target to target toxicity.^[4]