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
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 grow