Abstract
Susceptibility of 203 strains of 34 bacterial species or subspecies to cephapirin (CEPR) was determined by the 2-fold agar dilution method in parallel with the determination of inhibition zone diameter in the singledisc method.
These experiments demonstrated a significant correlation between the MIC by the dilution method and the diameter of inhibition zone determined by the conventional assay using an over-night (about 16 hours) incubation, the delayed assay (about 24 hours incubation), or the rapid assay (after 3-4 or 5-6 hours incubation), hence applicability of the single-disc assay for CEPR was confirmed.
An analysis of the data obtained using CEPR discs (each containing 30 μg) revealed that primary regression equations were as follows: D (diameter, mm)=25.8-9.7 log MIC (μg/ml) in the conventional assay; D=31.2-12.3 log MIC in the delayed assay; D=21.7-7.1 log MIC in the 5-6-hour rapid assay and D=17.9-5.0 log MIC in the 3-4-hour rapid assay, and especially for β-lactamase producing Staphylococci, they were: D=24.9-9.2 log MIC in the conventional assay, D=20.4-7.4 log MIC in the 5-6-hour rapid assay and D=17.5-5.8 log MIC in the 3-4-hour rapid assay.
The range of variations in MICs of CEPR estimated from diameters of inhibition zones in the disc test was then calculated and compared with the range in MICs determined by the 2-fold dilution assay as the reference to estimate the experimental error which may have been resulted from the error in the estimation of MICs by the single-disc assay.
