It is due to Metchnikoff, of the Pasteur Institute, that so much prominence has been given to the use of fermented milks. He gave it as his opinion[40] that senility was caused partly by auto-intoxication or by the poison derived from putrefactive micro-organisms which inhabit the digestive track. These organisms increase with age, and under certain unhealthy conditions multiply enormously, particularly in the large intestine. Having arrived at this knowledge, Metchnikoff set to work to devise some means of combating the influence of these harmful microbes, and set up the hypothesis that the tendency to longevity which is exhibited in Eastern countries is due to the consumption of lactic acid organisms in the shape of soured milk. These organisms are more powerful than those of a putrefactive character and inhibit their growth.
"In the presence of such facts," says Metchnikoff, "it becomes exceedingly important to find some means of combating the intestinal putrefaction which constitutes so incontestable a source of danger. Such putrefaction is not only capable of producing diseases of the digestive tube—enteritis and colitis—but even of becoming a source of intoxication of the organism in its most varied manifestations.
"It is some years since I proposed to combat intestinal putrefaction and its injurious consequences by means of lactic ferments. I thought the acidity produced by such microbes would be much more effective in preventing the germination of putrefying microbes than the small quantity of acids produced by Bacillus coli. On the other hand, I had no illusion as to the difficulty sure to be encountered in any effort to introduce lactic microbes into the intestinal flora which has been preoccupied by a multitude of other microbes. To make surer of the result, I chose the lactic microbe, which is the strongest as an acid producer. It is found in the yahourt (yoghourt), which originates in Bulgaria. The same bacillus has also been isolated from the leben of Egypt; and it is now proved that it is found in the curdled milk of the whole Balkan peninsula, and even in the Don region of Russia."[41]
It is a short step from considerations like these to the adoption of the Bacillus bulgaricus as the most potent of the various lactic organisms which have been examined, and which is likely to play such an important rôle in the destiny of the human race. The Bacillus bulgaricus may claim to be the Bacillus of Long Life.
CHAPTER III
THE CHEMISTRY OF MILK
The Composition of Milk.—Like all other organic substances, or those built up in connection with the life processes of plants and animals, milk is of complex composition. It is also very liable to change—every one is acquainted with its tendency to "go bad." This instability is more or less inherent in all highly organised chemical compounds, and, indeed, it seems to be necessary that the materials used in growth and nutrition should be very plastic in a chemical sense, in order, e.g., that the constituents, say of a plant, may easily be transformed into the substances of the body of the animal which feeds on it.
The perishable nature of milk—the food of young and growing animals—is therefore essential, so that it may be changed easily into the blood, bone, muscle, etc., so abundantly required in the early stages of existence.