16. A Note on the Direct Microscopic Count of Bacteria in Milk

Extract

In the course of some experiments on the manufacture of cheese, in which it was necessary to add cultures of various organisms to milk in the cheese vat immediately before the addition of rennet, a curious anomaly in the direct microscopic count (Breed method) was observed. The organisms to be used were grown for 24 hours on a meat-extract peptone agar. The growth was then washed off with sterile saline, and the resulting suspension was added to the milk in the vat. After the milk had been stirred thoroughly, samples were taken and a direct count, according to the method of Breed, was carried out in order to determine approximately the number of organisms which had been added. With several of the types of organisms used (staphylococci, streptococci and Gram-positive bacilli) this procedure gave a reasonable result but, in the case of Gram-negative bacilli of the coliform group, it was rarely possible to detect under the microscope any definite organisms, except those present originally in the milk and observed in the control sample. This observation proved consistent over a long series of experiments.