Resistance to gentamicin, tobramycin and amikacin among clinical isolates of bacteria

Susceptibility to the administration of gentamicin, tobramycin and amlkacin was determined for all isolates of aerobic and facultative gram-negative bacilli submitted for testing to the clinical bacteriology laboratory of the Massachusetts General Hospital between July 1, 1974, and June 30, 1976. In this 24-month period more than 46,000 isolates of bacteria were tested by the single-disc diffusion (Bauer-Kirby) method. Resistance to one or more of the aforementioned aminoglycosidic aminocyclitol antibiotics was found among 4, 114 stains. Correlation with quantitative susceptibility test methods revealed that disc-diffusion methods using 10 μg discs accurately predicted resistance to gentamicin and tobramycin, but overestimated the prevalence of resistance to amikacin by 20 to 50 per cent. Most of the gentamicin-reslstant Enterobacteriaceae in this study were also cross-resistant to tobramycin but were susceptible to amikacin. Many gentamicin-resistant strains of Ps. aeruginosa were susceptible to both tobramycin and amikacin. Resistance to amikacin tended to be of relatively low magnltude (most had minimal Inhibitory concentrations (MIC's) between 31 and 125 μg/ml), but organisms which were resistant to the administration of amikacin were usually resistant to the other two aminoglycosidic antibiotics as well.