Throughout the investigations upon which a large part of this paper is based the favorable influence of milk feeding on mortality and growth was most apparent. Mortality from all causes was frequently reduced to at least one-half of what obtained among the chicks that received no milk, while the milk-fed chicks in some experiments gained twice as much in weight as those that were without this article of diet. Although the influence of milk feeding was less pronounced on the mortality of chicks that were artificially infected with Bacterium pullorum, quite an appreciable difference in mortality was always noted if the milk was fed at least one or two days before the first administration of the bouillon cultures of the organism in question.
Practically the same results were obtained, whether sweet or sour milk was fed, and no differences could be observed in the relative value of ordinary sour milk and of the so called bulgaricus product. Hence, the unique properties of this food exist in the milk as such, rather than in any milk acids or milk bacteria that may be present.
Milk and lactose diet exert a very important influence on the character of the intestinal bacteria, especially in white rats and in the common domestic fowl. Within a few days after the ingestion of milk or lactose a transformation of the flora takes place in which the usual mixed bacterial flora give way to ones that are more simplified, and in which Bacillus acidophilus and Bacillus bifidus are, as a rule, prominent. It is to be assumed that milk has this influence in virtue of the large amount of lactose which it contains. Other carbohydrates, besides milk sugar, failed to bring about such a transformation.
The ingestion of foreign bacteria, even in large numbers, does not of itself bring about an elimination or displacement of the common intestinal microörganisms. Vastly more important is the influence of diet, especially milk and lactose. The feeding of Bulgara tablets or other preparations which contain as the supposedly active agent the bacillus of Metchnikoff and Mazé, without due regard to the use of milk, can, therefore, be of little, if indeed of any, value. The beneficial effects which it is claimed have been derived from the use of yoghurt, and other oriental sour milk products have in all probability been due to the milk as such, rather than to the bacteria which they contained. This view is strongly supported by the extensive milk feeding experiments on chicks which are recorded in this paper, and also by the results which show the influence of milk and of lactose feeding on the intestinal flora of white rats and of the common domestic fowl.