3-0-demethyl fortimicin A: in vitro activity and interpretive zone standards for disk diffusion susceptibility tests

The in vitro activity of 3-0-demethyl fortimicin A was compared to that of amikacin and tobramycin against 5,230 clinical isolates in four institutions. Amikacin and tobramycin were more active than 3-0-demethyl fortimicin A against Pseudomonas aeruginosa and Acinetobacter spp., but all three drugs had similar activity against the Enterobacteriaceae and Staphylococcus aureus. Additional tests with 335 representative gram-negative bacilli compared five different aminoglycosides, demonstrating differences with some isolates. Standardized disk diffusion tests were also performed with 30 micrograms 3-0-demethyl fortimicin A disks, according to the National Committee for Clinical Laboratory Standards. The following interpretive breakpoints are proposed: less than or equal to 11 mm for resistant (MIC greater than or equal to 32 micrograms/ml) and greater than or equal to 15 mm for susceptible (MIC less than or equal to 16 micrograms/ml).