Susceptibility testing: accurate and reproducible minimum inhibitory concentration (MIC) and non-inhibitory concentration (NIC) values

Measuring the minimum inhibitory concentration (MIC) of a substance by curr ent methods is straightforward, whereas obtaining useful comparative inform ation from the tests can be more difficult. A simple technique and a method of data anal! sis are reported which give the experimentalist moro useful information from susceptibility testing. This method makes use of a 100-wel l microtitre plate and the analysis uses all the growth information, obtain ed by turbidometry, from each and every well of the microtitre plate. A mod ified Gompertz function is used to fit the data, from which a more exact va lue can be obtained for the MIC. The technique also showed that at certain concentrations of inhibitor, there was no effect on growth relative to a co ntrol well (zero inhibitor). Above a threshold value, which has been termed the non-inhibitory concentration or NIC, growth becomes limiting until it reaches the MIC, where no growth relative to the control is observed.