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Observations on the Growth Responses of Streptococcus Lactis in Mastitis Milk¹

Washington Agricultural Experiment Station, Division of Dairy Husbandry

ABSTRACT

The growth responses of S. lactis, as judged by changes in the pH values and microscopic examinations have been studied in separate samples of milk drawn from the infected and non-infected udder-quarters of cows suffering with mastitis.

Milk with an initial pH value greater than 6.9 usually failed to support the growth of S. lactis in an active manner, whereas normal milk from the other udder-quarters showed normal acid development. The growth responses of S. lactis varied somewhat in the milk drawn from the three cows under study.

Adjusting the pH value of mastitis milk to that of normal milk resulted in only partially overcoming delayed acid development. Mastitis milk treated in this manner changed little in pH value until after fifteen to twenty hours, after which time acid development was quite rapid.

The addition of as little as 10 per cent of very abnormal mastitis milk to normal milk had a retarding effect on S. lactis development.

Pasteurizing some samples of mastitis milk at 65.5° C.–68.5° C. for 30 minutes tended to partially overcome the retarding influence on the growth of S. lactis; however, with most of the samples studied heat treatment at these temperatures had little or no appreciable effect.

¹ Published as Scientific Paper No. 451, College of Agriculture and Agricultural Experiment Station, State College of Washington.