The Paper-Disc Agar-Plate Method for the Assay of Antibiotic Substances

Abstract

Filter paper discs dipped into antibiotic solns. were found to absorb uniform amts. of liquid. When placed upon the surface of inoculated agar plates, the discs served as reservoirs from which the antibiotic diffused into the medium, causing the development of clear halos of inhibition of growth of Bacillus subtilis which were proportional in size to the conc. of the antibiotic. The range of usefulness of the method was found to lie between 0.009 and 6 Florey units per disc. The method possesses the advantages of being rapid and easy to perform and of having relatively high precision and accuracy. A detailed description of the assay of penicillin is given. By the use of efficient statistical methods it was found that a linear log conc.-response curve could be obtained but that the shape, slope and position of the line were dependent upon the depth of the agar. Variations in the number of organisms per plate affect the slope and the position of the line but not its shape.