In Vitro Evaluation of Tobramycin, a New Aminoglycoside Antibiotic

Abstract

One hundred fifty-two strains of Escherichia coli, Klebsiella-Enterobacter, Pseudomonas aeruginosa, Proteus species, and Staphylococcus aureus were inhibited by 3.1 μg of tobramycin/ml in a broth-dilution method and showed zones of inhibition of 16 mm or more around a 10-μg tobramycin disc in the Kirby-Bauer method. Tobramycin was most active against S. aureus , 100% of strains being inhibited by 0.1 μg/ml. All strains of E. coli, K. pneumoniae, P. aeruginosa , and indole-positive Proteus species, and 80% of Enterobacter species were inhibited by 0.8 μg of tobramycin/ml, whereas only 48% of P. mirabilis strains were inhibited by this concentration. Tobramycin was approximately twice as active as gentamicin against S. aureus , four times as active against P. aeruginosa , slightly more active against E. coli and Enterobacter species, equally active against P. mirabilis , and slightly less active against K. pneumoniae . The minimal bactericidal concentrations of tobramycin and gentamicin were the same as or twice the minimal inhibitory concentrations for all strains except those of P. aeruginosa , against which greater concentrations of both gentamicin and tobramycin were required for bactericidal activity. Tobramycin sterilized cultures of S. aureus, E. coli , and P. aeruginosa , but the rate of bactericidal action was faster with a combination of tobramycin and carbenicillin than with either antibiotic alone in the same concentrations. Tobramycin retained potency in the presence of 200 to 600 μg of carbenicillin/ml for at least 6 hr of incubation at 37 C, but lost potency in the presence of 600 μg of carbenicillin/ml by 24 hr of incubation and in the presence of 800 μg/ml by 2 hr of incubation.