What Is Minimum Inhibitory Concentration?

Minimum Inhibitory Concentration (MIC) is the smallest amount of a compound that limits visible microbial growth in culture. The lower the number, the more effective the agent is against microorganisms. Testing for MIC can provide important information about the susceptibility of certain organisms, like bacteria and fungi, to drugs used in treating infection. It is typically performed in a lab environment and takes a night to allow the culture time to grow.

In this test, a technician creates a standardized and controlled dilution of the compound being tested and adds it to agar plates or broth with the organism being studied. The mixture is incubated overnight and then evaluated in the morning. If the organisms are growing, it means the concentration is not high enough. If no visible growth is present, the compound works. By testing a range of concentrations, technicians can zero in on the minimum inhibitory concentration.

One use for this testing is in individual susceptibility evaluations. A patient with an infection that fails to respond to initial treatment or is likely to be resistant may be asked to provide a sample for culturing purposes. In the lab, a technician can find out which antimicrobial medications will work, and can make a dosing recommendation. Over time, this data can lead to revisions to prescription guidelines as technicians find the range at which agents are effective with the lowest side effects.

It is also possible to use minimum inhibitory concentration testing in pharmaceutical research. Drug companies want to know which organisms a medication can treat, and how much medication is required to achieve a positive outcome. Such research can also be used to explore natural compounds used historically in the treatment of infection, like tea tree oil, which is known to have antimicrobial properties. The outcome of the minimum inhibitory concentration test can be used to guide dosing for clinical studies.

Testing can be fast, as labs typically only need a single night to process the sample. It is also relatively easy, and provides a wealth of data for care providers and researchers. Labs use a number of controls to reduce error rates with minimum inhibitory concentration testing and other procedures. They can randomly repeat and check tests, for example, to make sure the results are accurate. Suspicious results may also be automatically flagged for retesting to confirm before they are relayed to the scientist or care provider who requested the test.