Fortimicin A, a pseudodisaccharide aminoglycoside, was found to have broadspectrum activity against most clinically important aerobic and facultatively anaerobic bacteria, except Pseudomonas aeruginosa, some other Pseudomonas species, and streptococci. It was comparable to amikacin in its level of activity (miniimum inhibitory concentrations) and spectrum of activity (except for the lack of activity on P. aeruginosa). Fortimicin A was bactericidal and was affected by cations when tested against P. aeruginosa. Minimum inhibitory concentrations were affected by the inoculum used in the susceptibility test. The drug was resistant to most aminoglycoside-inactivating enzymes, but probably is not active against perneability mutants.Fortimicin A is the most active member of the fortimicin complex of aminoglycosidic antibiotics produced by Micromonospora olivoastrospora (6,8). It is a pseudodisaccharide-incorporating fortamine, a novel aminocyclitol (3). It possesses a broad spectrum of activity against gram-positive and gram-negative bacteria, but is less active against Pseudomonas aeruginosa than are some of the other newer aminoglycosides (4-6). It is resistant to the activity of some of the aminoglycoside-inactivating enzymes (4, 9).In this study we compared the antibacterial activity of fortimicin A with those of five other aminoglycosides (amikacin, gentamicin, netilmicin, sisomicin, and tobramycin) by testing them against a group of bacteria selected to represent a wide variety of genera and species with various susceptibility patterns and mechanisms of resistance. We also determined the bactericidal activity and the effect of changes in cation content of the medium and in inocula on the in vitro activity of the drug.MATERLALS AND METHODSProtocol. The primary investigators developed a protocol for the collaborative investigation of the drug in three laboratories (10). The in vitro activity of the drug against the organism was determined in two laboratories, Center for Disease Control, Atlanta, Ga.,and University of California, Davis. Comparable results have been repeatedly obtained in these two laboratories, but to ensure this interlaboratory agreement, some strains of each genus were tested in both laboratories (only one set of data was reported for these strains).The effect of inoculum changes on the minimum inhibitory concentrations (MICs) for some of these strains was determined in a third laboratory (Kaiser Foundation Hospital Laboratories, Portland, Ore.).Antibiotics. Antibiotic powders suitable for antimicrobial susceptibility tests were obtained as follows: fortimicin A from Abbott Laboratories, Chicago, Ill.; gentamicin, sisomicin, and netilmicin from Schering Corp., Bloomfield, N.J.; amikacin from Bristol Laboratories, Syracuse, N.Y.; and tobramycin from Lilly Laboratories, Indianapolis, Ind.Bacteria. This collection of organisms was assembled for the purpose of challenging this aminoglycoside with a wide range of species and of resistance patterns without regard to usual distribution, as described prev...