Influence of disc content on zone sizes obtained in standard disc diffusion antimicrobial susceptibility tests

The influence of disc content on the zone sizes produced was investigated using standard disc diffusion antimicrobial agent susceptibility test protocols. Replicate measurements were made of the zones of inhibition produced by discs containing 2–64 μg of oxytetracycline and oxolinic acid against Escherichia coli, Aeromonas salmonicida and Vibrio parahaemolyticus. Five replicate data sets were obtained for each of the six organism–agent combinations. Analysis of these 30 data sets demonstrated that they all showed a very high correlation (mean r2 = 0.98 ± 0.009) with the predictions of best-fit equations specifying a linear relationship between log10 of the disc content (μg) and the resultant zone sizes (mm). The mean value of the slopes of the best-fit straight-line equations was 9.8 ± 1.8. The overall similarity of these values for the slopes suggested that the quantitative relationship between disc content and zone size was, at least to a first approximation, independent of the organism or the agent involved in the test.