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A Method of Counting Viable Bacteria in Milk by Means of the Microscope¹

Department of Dairy Industry, Cornell University, Ithaca, New York

ABSTRACT

A direct microscopic method for distinguishing dead from viable bacterial cells in milk is reported. The method is based on the principles of vital staining. One cc. of the milk to be examined is mixed with 0.5 cc. of a dye mixture containing 0.1 per cent methylene blue and 0.1 per cent Nile blue sulfate, and with 0.5 cc. of a gelatin-agar mixture containing 8 per cent gelatin and 2 per cent agar. 0.01 cc. of this mixture is placed on a clean glass slide and covered with a small cover glass (1.28 x 1.28 cm.). The stained cells are then counted and they represent the dead cells already present in the milk. To a similar mixture one drop of 0.7 to 0.8 normal NaOH is added. This causes staining of all the cells present in the milk. The difference between the two counts represents the number of viable cells present.

The object of the agar-gelatin mixture is to solidify the preparation, keeping the bacteria in position. The factor is calculated from a knowledge of the amount of milk present in the preparation and the area of the cover glass.

When desirable, fat can be removed by shaking the milk with heptane followed by moderate centrifuging.

A number of tables are included showing the results obtained when the method is applied to pure cultures and to pasteurized milk. The counts obtained are also compared to petri plate counts made on the same material.

The effect of pasteurization on the stainability of bacteria in milk is studied and briefly discussed.

^1> From a thesis by the junior author in partial fulfillment of the requirements for the degree, Master of Science. The work was aided by a fellowship granted by the Dairy Machinery and Supplies Association.