Researches on intestinal flora--Sour milk.
The problem of our intestinal flora is so vast and so difficult that it
demands years of research. Numerous facts had already been accumulated
by Science on this subject, but it was still far from being elucidated.
Certain scientists affirmed that microbes favour digestion by
decomposing food residues in the intestine and are therefore not
merely useful, but necessary to the organism. Others entertained a
diametrically opposed opinion. The first thing, therefore, was to know
which of the two opinions was founded on fact. Metchnikoff studied
the case of the bat, in which the digestive tube is short and the
large bowel not even differentiated. As he had supposed, _a priori_,
in this animal, whose life duration is relatively long, the intestine
contains few or no micro-organisms, which proves that digestion can
be accomplished without their intermediary. Moreover, this was before
long amply confirmed by the researches of MM. Cohendy, Wollman, and
other scientists who succeeded in bringing up chickens and tadpoles in
conditions of absolute sterility.
Having acquired the conviction that microbes are not indispensable to
digestion, Metchnikoff studied the part they play in the organism. It
is universally admitted that the products of putrefaction are toxic,
and he enquired whether the intestine sheltered putrefying microbes.
This question had not yet been solved; certain bacteriologists
thought that little or no putrefaction exists in a normal intestine.
Metchnikoff ascertained through systematic researches that the
intestinal flora includes several kinds of putrefying microbes which
secrete highly toxic products.
With his pupils and collaborators, MM. Berthelot and Wollman, he
carried out a series of experiments which established the fact that
this intoxication is due to poisons of the aromatic group, such
as phenols and indols. With these substances, they succeeded in
artificially provoking arterio-sclerosis in the organs of animals,
and also other modifications similar to those which are observed
in senility. Having proved that putrefying microbes provoke the
intoxication of the tissues, Metchnikoff set to work to find a means of
struggling against those microbes.
It was known that they could only live in an alkaline medium which
is precisely that of the intestinal juices. Metchnikoff thought that
if means were found to render the intestinal contents acid, without
harm being done to the organism, the putrefying microbes might thus be
destroyed. It had been known for a long time that sour milk does not
suffer putrefaction, that being prevented by the acid fermentation. The
lactic microbes of this fermentation must therefore be antagonistic to
the putrefying microbes. He drew a conclusion in favour of the utility
of sour milk, containing acid-producing microbes; once introduced
into the intestine, these should prevent the breeding of the noxious
microbes which require an alkaline medium.
His hypothesis seemed confirmed by the fact that populations who feed
almost exclusively on curded milk live a very long time. In Bulgaria,
for instance, whole villages, thus fed, are known for the longevity
of their inhabitants. Starting from these considerations, he made
experiments upon himself and systematically introduced into his diet
sour milk carefully prepared with pure cultures of certain lactic
bacilli. His health was benefited by it, and his friends followed
his example. Certain doctors recommended sour milk, the use of which
gradually spread as a hygienic food. Metchnikoff considered the result
acquired as a first step towards the artificial transformation of the
wild intestinal flora into a cultivated and useful flora.
Unfortunately, the study of the intestinal flora is extremely
complicated because of the innumerable species of micro-organisms and
the extreme difficulty of disentangling the many influences which
cross each other. He therefore considered collective researches as
indispensable, the life and science of one man being insufficient
to solve so vast a problem. Up to a certain point he succeeded in
realising this scientific collaboration within his own laboratory.