Rapid microbiologic assay of tobramycin.

Different, rapid, plate diffusion assays of tobramycin in serum were compared with each other and with an overnight assay. These assays were done with Klebsiella edwardsii var. atlantae, which was resistant to many antibiotics. One of the rapid assays of tobramycin tested used a 1:50-1:100 dilution of an overnight culture, and results were recorded after incubation for 5 hr at 35 C. Two shorter assays were also tested with a 1:10 dilution of an overnight assay. The 5-hr assay was as reproducible and as accurate as the overnight assay. The 2.5-hr assay at 35 C proved to be an inaccurate method. Incubation of the assay at 40 C instead of 35 C resulted in more rapid growth of K. edwardsii and in the production by tobramycin of larger and sharper zones of inhibition. The 2.5-hr assay at 40 C was as reproducible and as accurate as the 5-hr and the overnight assays.