minimum inhibitory concentration
The minimum inhibitory concentration, or MIC, is the smallest amount of an antibiotic that stops a particular batch of bacteria from growing visibly in the lab. It is the standard yardstick for measuring how susceptible a bug is to a drug: the lower the MIC, the more readily that drug holds the organism in check.
To measure it, identical samples of the bacterium are exposed to a series of doubling drug concentrations; the lowest tube or well that shows no visible growth after overnight incubation is the MIC. Labs then compare this value against published breakpoints to report the organism as susceptible, intermediate or resistant, which is what guides the clinician's choice of drug.
The MIC is a useful but limited number. It is measured in a test tube, not a patient, so it ignores whether the drug actually reaches the site of infection, how protein binding lowers the free fraction, and how the immune system contributes. It also says nothing about killing speed; a related test, the minimum bactericidal concentration, captures that. MIC is therefore best read alongside the drug's pharmacokinetics rather than on its own.
A lab reports an E. coli isolate with a ciprofloxacin MIC of 0.06 micrograms per millilitre, well below the resistance breakpoint, so the organism is labelled susceptible.
MIC values are interpreted against breakpoints to label an organism susceptible or resistant.
A lower MIC does not automatically mean a better drug for a patient; a drug with a higher MIC may still win clinically if it achieves far higher free concentrations at the infection site.