barely warm. To this melted gelatin a definite quantity of milk is added. The medium is gently shaken, so as to thoroughly mix the milk and gelatine, and the mixture then poured into a sterile, flat, glass dish, and quickly covered, where it is allowed to cool until the gelatin hardens. After the culture plate has been left for twenty-four to thirty-six hours at the proper temperature, tiny spots will begin to appear on the surface, or in the depth of the culture-medium. These spots are called colonies, and are composed of an almost infinite number of individual cells, the result of the continued growth of a single organism that was in the drop of milk and which was firmly held in place when the gelatin solidified. The number of these colonies represents approximately the number of living bacteria that were present in the amount of milk added to the tube of gelatin. If the plate is not too thickly sown with the bacteria, the colonies will continue to grow and increase in size, and as they do, minute differences will begin to appear. These differences may be in the color, the contour, and the texture of the colony, or the manner in which it acts toward gelatin.

Fig. 4.—Plate Culture.
Each of the dots is a colony that has been formed by the growth of an organism embedded in the solid culture-medium. By counting the colonies, the number of living bacteria in the amount of milk added to the culture is determined.

In order to make sure that the number of colonies is not so numerous as to prevent counting and further study of their characteristics, a series of plate cultures is usually made in which varying amounts of milk are added to the tubes of gelatine. This is attained by adding a definite amount of the milk or other substance to be examined to a measured amount of sterile water, e.g., one cubic centimeter of milk to ninety-nine cubic centimeters of water. One cubic centimeter of this mixture may be used for the inoculation of the plate culture. This dilution may be carried on to any desired extent; in the examination of many dairy products, it is necessary to use very minute quantities of material, often only one one-millionth of a cubic centimeter.

To study further the peculiarities of the different bacteria, small portions of the individual colonies are transferred to tubes of sterile culture-media. In order to do this the colony is touched with a piece of platinum wire; the minute amount of growth that adheres to the wire is sufficient to seed the tube of fresh culture-medium. The inoculating needle must always be sterilized before use by passing it through a gas flame.

A culture thus obtained is called a pure culture since it contains but a single kind of an organism, as the colony is the result of the growth of a single cell. These cultures then serve as a basis for continued study, and must be planted and grown upon the different kinds of media that are obtainable. In this way the slightest variations in the growth of different forms are detected, and the peculiar characteristics are determined, so that the student is able to recognize this form when he meets it again.

Fig. 5.—Different Kinds of Bacteria Growing in Gelatin.
A, meager growth, no liquefaction or surface growth; B, profuse surface growth, radiating filaments from the growth below the surface; C, a rapid liquefying form; D, a gas producer that grows equally well in the presence or absence of air; E, form that grows only in the absence of air, an anaerob.

These culture methods are of essential importance in bacteriology, as it is the only way in which it is possible to secure a quantity of germs in a pure state.

The microscope in bacterial investigations. In order to verify the purity of the cultures, the microscope is in constant demand throughout all the different stages of the isolating process. For this purpose it is essential that the instrument used shall be one of high magnifying powers (600 to 800 diameters), combined with sharp definition.

The microscopical examination of any germ is quite as essential as the determination of culture characteristics, in fact, the two must go hand in hand. The examination reveals not only the form and size of the individual germs but the manner in which they are united with each other, as well as any peculiarities of movement that they may possess.

In carrying out the microscopical part of the work, not only is the organism examined in a living condition, but colored preparations are made by using solutions of anilin dyes as staining agents. These are of great service in bringing out almost imperceptible differences. The art of staining has been carried to the highest degree of perfection in bacteriology, especially in the detection of germs that are found in diseased tissues in the animal or human body.

In studying the peculiarities of any special organism, not only is it necessary that these cultural and microscopical characters should be closely observed, but special experiments must be made in different ways, in order to determine any special properties that the germ may possess. Thus, the ability of any form to act as a fermentative organism can be tested by fermentation experiments; the property of causing disease, studied by the inoculation of pure cultures into experimental animals, like rabbits, guinea pigs and white mice.

The methods of the bacteriologist in his laboratory are in their effect not dissimilar to those which the farmer employs in securing his crop of pure-bred grain. The laboratory farmer kills the weed seeds in his culture field by the application of heat. His field, which is embraced in his culture dish, has been fertilized and prepared by the addition of certain favorable ingredients. When he has garnered his crop, he maintains its purity by keeping his selected seed, the pure culture, free from all contamination. The dairyman, even though he may not expect to carry on the detailed operations of the laboratory, will understand the reason for the directions which he is often required to follow much better if he knows how the simple operations of the laboratory are carried out. For a fuller knowledge of these matters, the reader is referred to the special texts on bacteriology.

CHAPTER III.

CONTAMINATION OF MILK.

Spoiling of milk. Materials of animal origin are peculiarly prone to undergo changes, rendering them unfit for use, and of these, milk is exceedingly susceptible to such changes. This is due to the fact that the composition of milk is especially adapted to bacterial growth, and that the opportunity for entrance of such organisms is likewise such as to permit of abundant contamination. The consequence is that milk readily undergoes fermentative changes, due to the development of one or another type of micro-organism.

Milk, a suitable bacterial food. While milk is designed by nature for the nourishment of mammalian life, it is, curiously enough, equally well adapted to the growth of these lowest forms of vegetable life. The nutritive substances required by bacteria are here sufficiently dilute to make possible rapid growth.

Milk also contains all the necessary