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Formation of Bacterial Inhibitory Zones in Whey Agar by Raw Milk

Department of Dairy Industry, Cornell University, Ithaca, N. Y.

ABSTRACT

This investigation is concerned with the nature of bacterial inhibitory zones of milk, their incidence, and a technique for measuring the antibacterial property of raw milk. The disc assay test for antibiotics in milk was adaptable for the detection of natural inhibitors in raw milk. By substituting lyophilized milk tablets for the saturated paper discs, the sensitivity of the test was increased. Generally, raw milk produced two clearly defined zones, labelled primary and secondary, respectively, against Bacillus subtilis. A primary zone of inhibition first appeared after 12 hr. at 72° F. The secondary zone only appeared after the plates were incubated, refrigerated, and then reincubated. This secondary phenomenon caused disappearance of the bacterial colonies like that associated with lysis. It appears that the presence of the natural inhibitor in milk is independent of the breed of the cow. The initial zone size varied seasonally, and tended to decrease in diameter or disappear during the winter months, whereas secondary zones were less affected seasonally.