An accurate diffusion method for determining bacterial sensitivity to antibiotics.

A simple method for accurately determining bacterial sensitivity to antibiotics is proposed. Based on the theory of the disc diffusion test, it consists of calculating a continuous variable corresponding to the antibiotic concentration in the agar at the edge of the inhibition zone, the Inhibitory Concentration in Diffusion (ICD). Three aminoglycosides were tested, each against 70-77 strains of Enterobacteriaceae, Staphylococcus aureus and enterococci. For each strain, the ICD was calculated from the size of zones of inhibition with eight to nine differently charged paper discs. Concurrently, the MICs were determined by an agar dilution method. Comparison of the MICs with the ICDs showed that the two measures were identical for the three antibiotics studied. Furthermore, the results of reproducibility assays for the ICDs proved to be better than for the MICs. The ICD could be approximated with two discs, but was more accurately determined with three. Besides its reliability, this method offers the advantage of avoiding the laborious construction of the regression curve for an antibiotic which is usually necessary for the standard diffusion method.