Evaluation of the moving intermediate zone concept for determing susceptibility of pseudomonads to gentamicin by the standardized disk agar-diffusion test

The usefulness of the moving intermediate zone concept for improving the performance of the standardized disk agar-diffusion test in measuring susceptibility of the fluorescent group of pseudomonads to gentamicin was studied. For this purpose, a 3-mm moving intermediate zone lying --3 mm and --6 mm below that measured for the quality control microorganism Pseudomonas aeruginosa, ATCC 27853, and a wider moving intermediate zone lying --2 mm and --7 mm below that for the quality control microorganism were investigated. Data from the authors' previous study of the usefulness of using fixed breakpoints and fixed zones for assessing susceptibility of the fluorescent pseudomonads to gentamicin were used for this analysis. The results indicate that both the 3-mm moving intermediate zone and the wider 5-mm moving intermediate zone produced unacceptably high rates of error for testing the susceptibility of the fluorescent group of pseudomonads to gentamicin by the standardized disk agar-diffusion test.