Failure of the disk diffusion test to detect tobramycin resistance in kanamycin-resistant Escherichia coli strains.

Approximately 40% of Escherichia coli strains isolated from clinical specimens at the Institute of Medical Microbiology of the University of Zurich were resistant to kanamycin but susceptible to tobramycin in disk diffusion tests. Whereas 50% of these strains required a MIC of 7 micrograms of tobramycin per ml to inhibit 1 x 10(5) to 4 x 10(5) cells, 20% of them required a concentration of 8 micrograms or more of the drug per ml. The disk diffusion test, therefore, failed to detect resistance to tobramycin in kanamycin-resistant E. coli strains. Cell extracts from two representative strains phosphorylated and inactivated kanamycin, amikacin, gentamicin, tobramycin, 3',4'-dideoxykanamycin B (dibekacin), butirosin, lividomycin,and ribostamycin, which together constituted a novel spectrum of substrates for the enzymatic activity.