Susceptibility testing for gentamicin and congeners.

MICs were determined by broth microdilution according to guidelines established by the Clinical and Laboratory Standards Institute (31). Antibiotic stock solutions were prepared fresh prior to each experiment. Inocula were prepared from cells streaked onto Mueller-Hinton agar (MHA) and grown at 35°C overnight. Antibiotic solution was mixed with the bacterial inoculum in cation-adjusted Mueller-Hinton broth in 96-well assay plates, with a final inoculum of approximately 5 × 10⁵ CFU/ml. After incubation of assay plates at 35°C for 18 to 20 h, the lowest concentration of antibiotic that prevented visible growth was recorded as the MIC.