A STUDY ON THE DISC SENSITIVITY TEST FOR AMOXICILLIN

Abstract

Susceptibilities of 101 strains of 25 bacterial species or subspecies to amoxicillin (AMPC) were determined by the 2-fold agar dilution method in parallel with the diameter of inhibition zone by the single-disc method, under the experimental conditions established by KANAZAWA.
The experiments demonstrated significant correlation between the MIC by the dilution method and the diameter of inhibition zone in each of conventional assay of the over-night (about 16 hours) incubation, delayed assay (about 24 hours incubation), and rapid assay (after 3-4 or 5-6 hours incubation), thus confirming applicability of the single-disc assay for AMPC.
Analysis of the data obtained by using AMPC disc containing 30μg revealed the primary regression equa-tion to be: D (diameter, mm)=27.1-9.9log MIC (μg/ml) in conventional assay, D=32.2-12.8 log MIC (μg/ml) in delayed assay, D=19.8-6.2 log MIC (μg/ml) in 3-4 hours rapid assay, and D=24.0-7.8 log MIC (μg/ml) in 5-6 hours rapid assay, and particularly for /β-lactamase producing Staphylococci, D=23.7-8.1 log MIC (μg/ml) in conventional assay, D=16.7-9.0 log MIC (μg/ml) in 3-4 hours rapid assay, and D=20.7-9.2 log MIC (μg/ml) in 5-6 hours rapid assay, respectively.
The range of variations in MICs estimated from the diameter of inhibition zone by the disc test was then calculated in comparison with that in MIC determined by the 2-fold agar dilution assays, as reference for the experimental errors which may be involved in the estimation of MIC of AMPC by the single-disc assay.