Minimum Inhibitory Concentration Assay

Principles


The Minimum Inhibitory Concentration Assay is a technique used to determine the lowest concentration of a particular antibiotic needed to kill bacteria. This assay is typically performed on planktonic (free floating) bacterial cells.

Note: MIC values will differ between bacteria.

Procedure

Serial dilutions of the antibiotic (representing different concentrations of the antibiotic) are added to a growth medium in separate test tubes. These tubes are then inoculated with the bacteria you wish to test.

For example, let’s say you wish to determine the MIC of an antibiotic on a bacterium. You decide to test 3 concentrations (10 µg/ml, 1 µg/ml and 0.1 µg/ml). Each of these tubes have growth media inoculated with a standard concentration of bacteria and the respective antibiotic concentration (Figure 1).

The tubes are allowed to incubate overnight. Broth tubes that appear turbid are indicative of bacterial growth while tubes that remain clear indicate no growth. The MIC of the antibiotic is the lowest concentration that does NOT show growth.

After incubating the tubes overnight, you observe the tubes in Figure 2:

From this example, Tube C, (with 0.1 ug/ml of antibiotic) did not inhibit bacterial growth. Tubes A and B, on the other hand, did inhibit growth. Tube B has the minimum inhibitory concentration because B is the lowest concentration of the antibiotic that inhibited cell growth. Therefore, the MIC for this bacterium is 1 µg/ml.

Advantages

  • technically simple set up
  • allows one to quantify antibiotic sensitivity /resistance

Disadvantages

  • not useful as a screen when there are many compounds and/or bacteria to test

Examples