Formation of Bacterial Inhibitory Zones in Whey Agar by Raw Milk

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Abstract
The disc assay method for detecting antibiotics in milk was modified, to make it suitable for observing the effect of the natural antibacterial property of milk. Substituting lyophilized milk tablets for the paper discs increased sensitivity of detection significantly, through increase in zone size and through appearance of zones hitherto not evident with the fluid raw milks. Two clearly defined zones often were observed from milks possessing the power to form zones against bacteria. A primary zone apparent in twelve hours at 72[degree]F was characterized by a hazy appearance. Appearing after storage at 40[degree]F and reincubation, was another zone, the secondary. Greater in diameter and clear, it was related to the disappearance of bacterial colonies like that associated with lysis. Primary zones were affected seasonly, being almost absent in cow''s milk during the winter months; whereas, secondary zones were less affected by seasons of the year. Breed differences had no effect on size and incidence of zoning.

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