This initial power of milk to restrain or destroy bacteria is exceedingly feeble and variable. The germicidal properties of milk have been much misunderstood, especially by dairymen, some of whom argue that advantage may be taken of this property for the preservation of milk without the use of ice. When we stop to consider that bacteria frequently enter the imperfectly closed orifice of the teat and grow well in the milk contained in the milk cisterns, and that they often invade the finer tubules of the gland structure itself where the milk is being freshly secreted, we must be convinced that the "germicidal" power of milk must be exceedingly feeble, if it exist at all. This property varies with the milk of different animals, and also with the milk of the same animal at different times. There is evidence to show that the restraining action of fresh raw milk upon the growth of bacteria varies with the bacterial species, and when we inquire into the causes of the phenomenon we find that this is what we might expect. When micro-organisms are transferred to a strange medium they sometimes hesitate, until they become sufficiently accustomed to the new surroundings, before they begin to grow and multiply. Our experiments show that this is by no means always the case and can not account for the facts now under consideration. We know that the serum of milk may contain "antibodies" in appreciable and variable quantity similar to those found in the blood. For instance, diphtheria, tetanus, and other antitoxins have been demonstrated in the milk of immunized animals. We might also expect slight and variable quantities of the agglutinating, bactericidal, and bacteriolytic substances present in blood serum to pass into the milk serum. Agglutinins in milk would apparently diminish the number of bacteria contained therein when estimated by the number of colonies that develop on agar plates. Such a decrease would be apparent, not real. Microscopic examination of the bacteria in milk made at once after milking, and again in eight hours, demonstrates that such agglutination actually takes place. This is confirmed by our other technique. We know that milk always contains large numbers of leucocytesmany of them of the polymorphonuclear variety. These are known to be active phagocytes, and we might assume that they are active in milk for a short time after it is drawn. In fact we have found that some of the leucocytes actually contain more bacteria after eight hours than when freshly drawn. If phagocytosis plays a part in the diminution in the number of bacteria in milk, we must assume that the milk serum must also contain opsonic power. Our work shows that phagocytosis plays no essential rôle in the apparent reduction in the number of bacteria in fresh milk. It seems that the germicidal property of milk corresponds to a similar property of fresh blood serum. This makes it probable that the causes are numerous and complex. Further, it explains why the action is variable in different milks and in milk from the same cow at different times. It also gives us a clew as to why the action is to a certain extent specific. Although the germicidal property of fresh milk is feeble, it must be of value to the suckling. This self-evident fact emphasizes the importance of using fresh milk for artificial feeding. EXAMPLES OF THE GERMICIDAL ACTION. The following examples show that milk when fresh and raw actually restrains the growth of bacteria as judged by the development of colonies upon agar plates. Whether the bacteria are restrained, destroyed, or clumped is not evident from such technique. This series shows the effect upon the growth of the common bacteria that usually contaminate milk. These results show a restraining, rather than a germicidal action, which varies with the temperature. The reduction is more apparent from a study of our work with pure cultures (vide infra). TABLE NO. 1.- Milk from a healthy cow (No. 1). Bacteria per cubic centimeter at different a In fact Woodhead and Mitchell claim to have demonstrated opsonins in milk. Journ. of Path. and Bactr., vol. 11, 1906-7, p. 408. 2 hours. 4 hours. 6 hours. 8 hours. 10 hours. 24 hours. 48 hours. 72 hours. 96 hours. 2 hours. 3 hours. 4 hours. 5 hours.. 6 hours. 7 hours. 8 hours. TABLE NO. 2.-- Milk from a healthy cow (No. 1). [Immediately after milking contained 500 bacteria per cubic centimeter.] [Immediately after milking contained 8,300 bacteria per cubic centimeter.] Temperature has a decided influence upon this phenomenon. When the milk is kept warm (37° C.) the decrease in the number of colonies is striking, but of short duration. When the milk is kept cool (15° C.) the decrease is less marked, but more prolonged. This is well illustrated by the curves nos. 8 to 13. These curves were plotted from the following tables, which show the germicidal properties of milk for individual species of bacteria. The experiments were conducted as follows: Milk was obtained from a healthy cow, using particular precautions to prevent outside contamination. For this milk we are greatly indebted to Doctor Schroeder and Mr. Cotton, of the Experiment Station, Department of Agriculture, Bethesda, Md. In addition to the usual precautions, a cloth wet with bichloride solution was placed under the cow, permitting only the teats to project through. The foremilk was discarded and about 10 to 15 cubic |