Stable antibiotic sensitivity disks.

Two methods of preparing sensitivity disks were compared for their effect on disk stability at 25 and 37 C. One method consisted of applying a solution of the antibiotic to blank disks by the conventional procedure; the second method consisted of applying the antibiotic to the disks as a suspension of crystals. Of the four beta-lactam antibiotics that were studied, disks made with suspended crystals were substantially more stable than corresponding disks made by the conventional method. The increased stability is related to the greater chemical stability of the antibiotics in the crystalline versus the amorphous state.