One of the research interests of Mechnikov was the relationship between intestinal bacteria and foods. In 1905, a student by the name of Stamen Grigorov, who was teaching a bacteriology course at the University of Geneva’s medical school at the time, discovered several new strains of lactic acid bacteria that were present in Bulgarian yogurt, and elucidated that they were partly responsible for the relative longevity of the Bulgarians. So Mechnikov invited Grigorov to the Pasteur Institute to give a lecture shortly thereafter. This interaction with Grigorov encouraged Mechnikov to start investigating intestinal bacteria from a new angle, which would later allow Mechnikov to prove in an animal experiment that intestinal microbiota contain putrefactive bacteria such as indole and phenol that produce harmful substances, which are responsible not only for autotoxication but also for causing arteriosclerosis and other ailments.

Mechnikov also found that putrefactive bacteria preferred an alkaline environment and could not live in a slightly acidic one. Based on these findings, Mechnikov reasoned that yogurt might effectively prevent the propagation of putrefactive bacteria, as the lactic acid bacteria in it would keep the intestines slightly acidic.
However, he soon discovered that this matter was not as simple as it seemed.

Mechnikov continued his research on this topic by conducting more animal experiments, and learned that a meat-heavy diet increases the production of harmful substances in the body, while the consumption of vegetables and fruits that have a high sugar content results in the intestinal bacteria breaking the sugar down into acid, which in turn suppresses bad bacteria and mitigates the production of harmful substances in the intestines.

He also observed that the ingestion of foods containing lactic acid bacteria yielded similar results. In order to increase the good bacteria in the intestines, such as bifidobacteria and lactic acid bacteria, the body needs sugar and dietary fiber that cannot be digested by human digestive enzymes, which have recently been given the name “bifidus factor” but Mechnikov already knew about this 100 years ago. In addition, Mechnikov was able to observe that intestinal bacteria had an antagonistic function against various pathogens such as typhoid and cholera. Through such experiments, Mechnikov formed his theory that ingestion of the lactic acid bacterium that was identified by Grigorov and named Lactobacillus delbrueckii subsp. bulgaricus induces longevity, and published it in 1907, in a paper titled “The Prolongation of Life: Optimistic Studies (anti-aging and life-extension theory)^*4.” Following Mechnikov’s announcement of the theory, an increasing number of companies started producing and selling yogurts that contained L. bulgaricus.

In 1916, Mechnikov passed away at the age of 71. The following are some words he uttered as he lay in bed nursing his ailment: “I was 53 years old when I noticed the excellent benefit of Bulgarian yogurt and started consuming it regularly, which was unfortunately too late to reap the full benefit. I certainly would have been able to live longer had I started eating yogurt sooner.”
After Mechnikov’s death, the claim that the lactic acid bacteria contained in Bulgarian yogurt would all be killed by stomach acid and would not be able to reach the large intestine, making the ingestion of lactic acid bacteria pointless, gained popularity, and the initial yogurt craze temporarily waned.
However, as we now all know, based on science 100 years after his death, Mechnikov was right all along. He was indeed the first scientist to notice the importance of intestinal microbiota.