[Studies on the disc method for the determination of bacterial sensitivity to sulfobenzylpenicillin (author's...

The MIC''s of sulfobenzylpenicillin (SB-PC) were determined by the two-fold serial agar-dilution method for 108 bacterial strains of 21 species. The diameters of inhibition zones of these bacterial strains by the 30 mug and 200 mug SB-PC discs were also measured. The relation between the MIC and the diameter of the inhibition zone was found to be expressed as a primary regression line in all cases of the conventional method (cultured for about 16 hours), delayed assay method (cultured for about 24 hours) and rapid methods (5 to approximately 6 hours and 3 to approximately 4 hours culture m ethods). Thus, it was confirmed that the single-disc method can be employed for the susceptibility test of SB-PC. Subsequently, variations of MICs obtained by the disc-diffusion method were compared with those obtained by the serial agar-dilution method.