MATERIALS AND METHODS
Diffusion of antibiotic through agar. Experiments
were carried out to determine the time required for
different antibiotics to diffuse from paper and become
uniformly distributed throughout a layer of agar 3 mm
in depth. Squares ofWhatman no. 1 filter paper, 10 by
10 mm, were impregnated with known amounts of
antibiotic by applying 0.02-ml volumes of the appropriate
antibiotic solutions. After drying at 37 C for 30
min, the papers were placed on the upper surface of
blocks of agar 10 by 10 by 3 mm (Blood Agar Base