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.