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The Detection and Significance of Escherichia-Aerobacter in Milk^1,2

III. Correlation of Total Bacterial Count and Presence of the Coli-Aerogenes Group

Department of Bacteriology, University of Maryland, College Park

ABSTRACT

A comparison of the total count on standard agar and presence of colon organisms showed the colon-positive samples of raw milk to have a total count of 2.9 times that of the colon-negative. Less than 10 colon organisms per cc. were found in 69.2 per cent of samples with a total count of less than 10,000 bacteria per cc. No pasteurized samples with counts under 1,000 contained colon organisms, the count of colon-positive samples being 4.3 times higher than colon-negative samples. No colon organisms were found in 0.1 cc. of "certified" milk.

Total bacterial counts made on tryptone glucose milk medium at 32° C. gave excellent correlation with colon titre, since 93.5 per cent of samples with a total count under 10,000 were negative for colon organisms. The average count of colon-positive samples was 12 times higher than that of colon-negative samples, being 6,650 and 80,000 per cc. respectively.

The presence of the coli-aerogenes group in raw milk of high bacterial count, would seem to be of little significance, in milk of low count the number of colon bacteria might be limited to the presence in 1 cc. amounts in 70–80 per cent of samples examined. In order to determine the sanitary condition under which milk is produced, further work to determine a standard for these organisms is desirable.

The presence of coli-aerogenes organisms in 1 cc. of pasteurized milk in 10 to 20 per cent of samples would seem sufficient to indicate contamination, which, as is shown in this investigation, might not be indicated by total bacterial counts. This test in pasteurized milk is particularly desirable.

The present standards for certified milk in respect to colon content are quite lenient.

Lactose taurocholate agar was found unsatisfactory in previous experiments in this series so that the adoption of a more satisfactory method for the detection of these organisms would be desirable and if this is realized a more stringent standard would seem possible. The value of a test to indicate contamination in a milk to be consumed raw can not be too highly stressed.

¹ Approved for publication by the director of the Maryland Agricultural Experiment Station.

² A portion of a thesis submitted to the Graduate School of the University of Maryland by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy.