Of recent years the consumption of milk fermented by these organisms has been introduced more or less successfully into all European countries. This custom is due, as we have seen,[84] to a very great extent to the announcement of Metchnikoff[85] that the action of such organisms in the alimentary tract conduce to a prolongation of life. Moro found that the dejecta of children contain large numbers of _Bac. lacidophilus_ and _Bac. bifidus_, but, as age advances, the bacterial flora of the intestines tends to change. The number of acid-producing organisms gradually becomes less, and other bacteria capable of producing far-reaching decomposition of albuminoid matter tend to increase. Working on the assumption that senility is partially due to the absorption of by-products formed from albuminoid food by the decomposing or putrefactive bacteria mentioned, Metchnikoff instituted a search for organisms capable of suppressing the growth of the putrefactive bacteria. It has long been known that milk allowed to become sour will keep for a considerably longer period in hot weather than if lactic bacteria had not grown. This preservative action of lactic acid also comes into play in the manufacture of sauerkraut and in the preservation of meat by immersion in sour milk. [Illustration: FIG. 34.—Photo-micrograph of smear of culture of _Bacillus bulgaricus_, recommended by Metchnikoff for use in cases of intestinal auto-intoxication. Unlike the ferments of normally soured milk, which are sometimes indifferent, or even injurious, in their action, this bacillus is capable of growth at blood heat, and, by producing much larger quantities of lactic acid than such organisms as _Streptococcus lacticus_, _Bacillus coli commune_, or _Bacteria lacticus aërogenes_, inhibits the multiplication of bacteria responsible for the putrefaction of albuminoid food in the intestines.]