A method for detecting antibiotic activity in milk using microscopic examination is presented. The Streptococcus thermophilus culture is exposed to the milk samples for 60–90 min. at 37°C. A methylene blue stain is prepared and examined for changes in morphology or reduction in clump count as compared to a control in antibiotic free milk. Abnormal enlargement or elongation of cells or a 50% reduction in clump count as compared to the control indicates antibiotic activity. Minimum concentrations of various antibiotics detected are: penicillin 0.015 units per ml; bacitracin 0.01 units per ml; terramycin 0.15 micrograms per ml; aureomycin 0.15 micrograms per ml and streptomycin 0.75 micrograms per ml. The test procedure is not affected by normal residues of sanitizers or bacteriophage. The stained milk films can be kept as a permanent record. Leucocycte counts can be made on the stained milk films during microscopic examination.

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Author notes

1Florida Agricultural Experiment Station Journal Series, No. 1005.

2Present address: Department of Dairy Husbandry, Purdue University, Lafayette, Indiana.