Variations in MIC value caused by differences in experimental protocol

The minimal inhibitory concentration (MIC) of an antibiotic for a microorganism quantifies the effectiveness in reducing growth or the bactericidal ability of the compound. Measurements of MIC's carried out using different protocols should be comparable. Several of the factors that influence the outcome of the measurement vary between protocols. Variations in the MIC estimate were examined for E. coli and amoxicillin as well as tetracycline and for Pseudomonas putida and enrofloxacin. Duration of the measurement, density of the starting culture, the use of optical density or cell counts to determine growth and the induction of resistance can cause differences of a factor of up to 8 in the MIC value. While this does not hamper the reporting of trends by researchers adhering to the same protocol, it may affect assessments based on the absolute value of the MIC of a given combination of microorganism and antibiotic.