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Natural Inhibitory Characteristics of Some Irish Manufacturing Milks¹

Department of Dairy and Food Science, Cornell University, Ithaca, New York

M. O'Leary²

Department of Dairy and Food Bacteriology, University College, Cork, Ireland

ABSTRACT

Penicillin and other pharmaceutical antibiotics in milk show clear zones on disc assay tests, but similar zones may be caused by other unrelated factors of milk. Observance of such factors in manufacturing milk from Cork County, Ireland, was the objective of this study.

During two warm months, May and June, about 56% of Irish milks tested showed thermolabile inhibition, as zoning on disc assay whey and nutrient agar plates, against at least one of five test bacteria of the species Bacillus subtilis, Sarcina lutea, and Staphylococcus aureus, apparently attributable to natural inhibitory sources. Ten, and possibly 11, of the 72 milks were inhibitory to all test organisms. Their average zone size, using 1.3-cm-diameter testing discs, ranged from 1.4 to 1.7 cm.

The pH of a number of the manufacturing milks was significantly lower than normal and this coincided with the ability of many of these milks to inhibit all test organisms on disc assays.

Natural inhibitory zoning by some of the fresh, normal pH milk directed in a random style against one or more test organisms, generally B. subtilis, was observed.

¹ This research was supported in part through Public Health Service Grant EF-3 (C3), Division of Environmental Engineering and Food Protection, while the senior author was residing in Ireland and participating in the first Irish-United States Fellowship Exchange Program.

² Present address—Marsh and Baxter, Ltd., Brierly Hill, Staffordshire, England.

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