A study on the disc sensitivity test for clavulanic acid/amoxicillin combination

Susceptibilities of 179 strains of 30 bacterial species or subspecies to clavulanic acid/amoxicillin (CVA/AMPC) combination were determined by the 2-fold agar dilution method as well as by diameters of inhibition zones in the single-disc method, under the experimental conditions established by Kanazawa. The experiments demonstrated significant correlation between MICs by the dilution method and diameters of inhibition zones in each of conventional assays of the over-night (about 16 hours) incubation, the delayed assay (about 24 hours incubation), and the rapid assay (after 3-4 or 5-6 hours incubation), thus confirming applicability of the single-disc assay for activities of CVA/AMPC combination. From an analysis of the data obtained using CVA/AMPC (1:2) combination of disc containing 45 micrograms, the primary regression equations were obtained as follows: D (diameter, mm) = 27.4-10.1 log MIC (micrograms/ml) in the conventional assay; D = 33.7-13.4 log MIC (micrograms/ml) in the delayed assay; D = 20.7-6.6 log MIC (micrograms/ml) in the 5-6 hours rapid assay, and D = 14.5-3.6 log MIC (micrograms/ml) in the 3-4 hours rapid assay. The range of variations in MICs of CVA/AMPC combination estimated from diameters of inhibition zones by the disc test was then calculated in comparison with that in MICs determined by the 2-fold agar dilution method to estimate experimental errors involved in assaying MICs of CVA/AMPC combination by the single-disc assay.