Since Metchnikoff had conceived the idea that a considerable part was played in human life by the intestinal flora, his thoughts had centred around a study which he thought profitable: that of the influence of intestinal microbes on the normal and on the pathological organism.

So, on his return from Russia, he took advantage of the fact that an epidemic of infantile cholera had broken out in order to continue his former investigations of that disease. The numerous cases which he thus studied allowed him finally to establish the specific part of the B. proteus as well as the similarity between infantile cholera and Asiatic cholera. This time he succeeded in contaminating, not only young anthropoid apes, but also new-born rabbits, and that not only through sick children’s excreta, but by pure cultures of the proteus, which eliminated every doubt of the specificity of this microbe.

Metchnikoff explained the contamination of children exclusively breast-fed, either by the presence of a carrier personally refractory, among the entourage, or by the transport of dirt, by means of flies, on the objects which infants so readily put into their mouths. He therefore advised preventive means of absolute hygiene and cleanliness, especially where suckling infants are concerned.

During the year 1912, he studied the intestinal flora and the influence of divers food diets. He experimented upon the rat, an omnivorous animal whose mode of feeding resembles that of man. The rats were divided into three lots, of which one was kept to a meat diet, another to a vegetarian rÉgime, and the third to a mixture of both. The meat diet was least favourable, and the best results obtained by the mixed food.

These observations led Metchnikoff to the study of other problems intimately connected with the same question.

He undertook a series of researches in collaboration with his pupils, MM. Berthelot and Wollman, on the conditions which cause the diminution within the organism of the toxic products of intestinal microbes. They found that the quantity of these products was very small in those animals which feed on vegetable or fruit containing much sugar, such as carrots, beetroot, dates, etc. This is explained by the fact that the products of the decomposition of sugar are acids which prevent the development of putrefying microbes. But the sugar, rapidly absorbed by the walls of the small intestine, only reaches the large intestine in a much reduced quantity, for it is only up to a certain point during its journey that the cellulose of vegetables, rich in sugar, protects that substance. The question, therefore, was to find the means of making it reach the large intestine in greater quantities. In the intestine of a normal dog, an innocuous microbe was found, the Glycobacter peptonicus, which decomposes starch into sugar.

Metchnikoff made some laboratory animals ingest this microbe together with food, and ascertained that it reached the large intestine and decomposed in it the starch of farinaceous food into sugar, of which the acid products prevented the swarming of putrefying microbes. By this process it is possible to reduce to a minimum and even sometimes to eliminate the production of phenol and indol in rats subjected to a mixed diet and made at the same time to ingest cultures of the lactic bacillus and of the glycobacter.

Metchnikoff applied these different diets to himself and to other individuals and obtained concordant results.

However, he ascertained that it is not only the food diet which regulates the quantity of microbian poisons contained in the organism; that quantity sometimes varies very much in spite of an identical diet. He thought that a very important part of influence is due to pre-existing microbes which prevent or favour the development of microbes of putrefaction. All these questions, complicated by the richness and variety of the intestinal flora, still demanded a long series of laborious researches.

At the end of the winter he felt tired, and we went to the seaside during the holidays. But the sharp sea air did not suit him; he had a beginning of cardiac asthma and nearly fainted during a walk. We therefore had to come away from the sea, and went inland, to Eu. At the beginning of our stay, Metchnikoff did not feel well, walking tired him, he suffered from cardiac intermittence; it was only gradually that his condition improved and he was able to write the preface to a Russian edition of his philosophical articles.

This book was entitled Forty Years’ Search for a Rational Conception of Life, and the articles record the evolution of his ideas and his search “not only for a rational understanding of life, but also for the solution of the problem of death, which is so full of contradictions.”

This collection of articles enables us at the same time to follow the gradual transition from the pessimism of his youth to the optimism of his maturity. His first writings[28] relate to the discords of human nature and the lack of a solid basis for morals.

But, already in 1883, he concluded an opening Causerie at the Naturalists’ Congress in Odessa, by the following words: “The theoretical study of natural history problems, in the widest sense of the word, alone can give a sound method for the comprehension of truth and lead to a definite conception of life—or at least to an approach to it.”

Another article, The Curative Forces of the Organism, sums up his phagocyte theory, and states the fact that the organism possesses special powers of struggle against enemy elements.

In 1891, he wrote The Law of Life, in which we find the dawning idea that the lack of harmony in human structure does not make a happy existence and a rational code of morals impossible. Morals must consist “not in rules of conduct adapted to our present defective human nature, but on conduct based upon human nature modified, according to the ideal of human happiness.”

The Flora of the Human Body, published in 1901, is a study in which Metchnikoff’s optimism assumes a definite form, for he speaks of the efficacy of certain means of struggling with our lack of harmony.

The last chapter in the book, “A Conception of Life and of Medical Science,” introducing the word Orthobiosis, strikes the optimistic chord, winged and conclusive, which must result from victory over the disharmonies of human nature. This is Metchnikoff’s ultimate formula, summing up the problems of life and of morals: