DETERMINATION OF THE ID₅₀ VALUES OF ANTIBACTERIAL AGENTS IN AGAR

Abstract

Microorganisms were plated on agar plates containing various concentrations of an antimicrobial drug and inhibition of growth was determined at each drug concentration. The ID₅₀ value and the gradient of the line were calculated by the least square method. When 200-800 bacterial cells were inoculated on an agar plate, growth inhibition corresponded linearly with the log concentration of a drug within the range of 5-95% inhibition. The ID₅₀ value and the gradient obtained were reproducible and reliable using microorganisms at stationary phase of growth with all tested bacterial species and all tested antimicrobial agents. It was found that the ID₅₀ values of drugs were more reproducible and may be more reliable than the MIC (minimum inhibitory concentration) values of the drugs.