It will thus be seen that organisms related to those of Oriental and Occidental milk beverages are present in conditions where it is impossible for them to attain to any active growth. The same class of organism has also been found in many cases in butter and cheese throughout the United States.
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.
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 aerogenes, inhibits the multiplication of bacteria responsible for the putrefaction of albuminoid food in the intestines.
Bienstock has shown that the growth of Bac. putrificus is inhibited by the action of Bact. coli commune, which is capable of setting up a slight lactic acid fermentation. Bact. coli commune, however, gives rise to substances of an injurious character, and, although present very abundantly in the intestinal tract, it may by reduced almost entirely by the active growth of lactic acid bacteria. This fact is of great value to the cheese-maker, since by the addition of a lactic acid culture (starter) to milk before renneting, gas-producing bacteria such as Bact. coli may be checked in growth. Since the ordinary lactic acid bacteria such as Streptococcus lacticus, Bac. lactis acidi, and others, are incapable of growth at blood temperature, it appeared necessary to procure cultures of lactic bacteria able to grow at temperatures of 100° F. to 112° F. Such an organism was found in Bulgarian soured milk (yoghourt), and was considered pre-eminently adapted to this purpose. As has been shown in the preceding paragraphs, this organism is merely one of a large group of bacteria found distributed in the intestinal canal of many domestic animals, in manure, and in ordinary market milk. It is then not surprising that the introduction into the intestinal tract of bacteria of the type Bulgaricus in the form of tabloids has not met with any decided success. Although it was considered to be merely necessary to introduce the desired type of organism into the body, and the amount of lactic acid taken into the system by the administration of soured milks was looked upon as of secondary importance, yet, it would seem, in the light of recent investigations, that benefits derived from a soured milk regimen are attributable in part to a chemical as well as a purely bacterial action. This receives support from the fact that soured milk beverages prepared by the use of ordinary lactic bacteria, distinct from those of the Bulgaricus type, often exert a beneficial influence upon human beings even although the organisms responsible for the fermentation are incapable of growth at blood temperature.
It may be mentioned in conclusion that cultures prepared by the use of organisms of the type Streptococcus lacticus combined with Bac. bulgaricus possess a more agreeable flavour and aroma than those prepared from a pure culture of Bulgaricus alone.