Gentamicin, tobramycin, and sisomicin disc susceptibility tests. Revised zone standards for interpretation

Studies were undertaken to reevaluate interpretive zone standards for gentamicin and tobramycin disc tests. Disc tests with an investigational aminoglycoside, sisomicin, were also evaluated. The data suggest modification of zone standards for gentamicin disc tests to R (resistant) less than or equal to 12 mm and S (susceptible) greater than or equal to 16 mm. Currently recommended standards for tobramycin disc tests (R less than or equal to 12 mm and S greater than or equal to 15 mm) were found to be satisfactory. For 10-microgram sisomicin discs, zone standards of R less than or equal to 12 mm and S greater than or equal to 15 mm were also appropriate. Although gentamicin is structurally similar to sisomicin, it was less active against Pseudomonas aeruginosa. The activity spectrum of sisomicin against the collection of 470 bacterial isolates studied more nearly resembled that of tobramycin. Data analysis suggested that tobramycin disc tests might be used to predict susceptibility to sisomicin. However, these two drugs differ in their susceptibilities to aminoglycoside-inactivating enzymes produced by some resistant strains. In those institutions where such strains are endemic, the class concept of disc testing may not be applicable.