Evaluation of the resistance mechanisms of gentamicin-resistant Gram-negative bacilli and their susceptibility to tobramycin, netilmicin and amikacin

Two hundred and fifty-three gentamicin-resistant Gram-negative bacilli, isolated from clinical material, were tested for sensitivity to tobramycin, netilmicin and amikacin. Amikacin was effective at 16 mg/l in 90% of cases, whereas there was no difference between netilmicin and tobramycin activity; 13.3% of strains were susceptible to 8 mg/l netilmicin, and 14.8% of strains to 4 mg/l tobramycin. The lack of difference in results between tobramycin and netilmicin is explained by local enzyme patterns. In our population, the AAC(3)II is predominant, and inactivates gentamicin, tobramycin and netilmicin, in contrast to the U.S.A. where ANT (2") seems to be the most common enzyme.