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Technique for Measurement of Antibiotic Inhibition Zones

National College of Food Technology, St. George's Avenue, Weybridge, Surrey, England

ABSTRACT

Merrill (1) suggested that an overhead projector be used to project on to a screen the image of plates containing bacterial colonies, so as to facilitate group instruction in counting techniques. A similar procedure may be used for class demonstration of zones of exhibition in agar-diffusion assay of vitamins and growth factors, and of zones of inhibition in agar-diffusion assay of antibiotics, Q.A.C.'s, metallic ions, etc. The plates may be projected on to a screen and discussed with classes.

With modifications, this procedure may also be used for measurement of the diameters of zones of inhibition or exhibition. The assay plate (30.5 x 30.5 cm, manufactured locally of plate glass) is placed on an overhead projector (Thermofax Model 36, Minnesota Mining & Manufacturing Co., London) arranged to project the zones of inhibition on to a rigid screen. The projected zones, magnified fivefold, are then measured to the nearest 0.5 mm, this being equivalent to measuring the actual zone to the nearest 0.1 mm.