In vitro susceptibility studies with netilmicin: comparison of a 10-microgram netilmicin disk with a standardized...

Netilmicin is a new, semisynthetic aminoglycoside antibiotic active against some gentamicin-resistant gram-negative bacteria. In this study we compared a 10-mug netilmicin disk with the standardized 10-mug gentamicin disk in terms of their abilities to predict probable clinical susceptibility to netilmicin. The agar dilution procedure of the International Collaborative Study of the World Health Organization and the U.S. Food and Drug Administration standardized disk test procedure were used. The gentamicin disk failed to predict the clinical susceptibility to netilmicin of 26 of 118 isolates previously shown by the agar dilution technique to to be netilmicin susceptible. The netilmicin disk correctly predicted probable susceptibility of all 26 isolates, including 20 shown by the agar dilution procedure to be resistant to gentamicin. These studies demonstrate the need for a separate netilmicin disk for use in agar diffusion disk susceptibility tests.