In return to the Prof. Massol’s letter, Prof. Mechnikov sent immediately an invitation to Stamen Grigoroff to visit The Institute Pasteur. In the big lecture hall there, Stamen Grigoroff reported the discovery of the lactobacilli. For scientific demonstration he brought with him Bulgarian yoghurt and a microscope. The direction of Pasteur Institute entrusted Prof. Mechnikov with the task to confirm independently the discovery of Stamen Grigoroff and to report the results to the Scientific Council of the institute. Three years later this resulted in a scientific publication: “Some notes regarding the yoghurt” printed in Les Comptes rendus de l'Academie des sciences, 1908. Soon afterwards Coendi, Mikelson, Luerson and Koen Mechnikov’s scientific assistants, named the microorganism discovered by Stamen Grigoroff Bacillus bulgaricus, “Bulgarian milk bacterium”.
There are no exact data for the origin of Bulgarian yoghurt but it is widely accepted that it has been derived from the Thracian tribes 4000 years B.C. The ancient Thracians observed that the fermented milk exhibits better endurance compare to the fresh one. They obtained the product known as “sour milk” by addition of fermented milk in boiled fresh milk. By means of systematical and continuous preparation of “sour milk”, bacterial strains of two species - Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus – have been gradually selected in the Bulgarian yoghurt.
Initially the qualities of Bulgarian yoghurt were scientifically studied by the Russian biologist Iliya Mechnikov. He considered that the activity of putrefactive microorganisms in the human gastrointestinal tract might be reduced and obstructed through colonization of the gut by lactic acid bacteria. These bacteria produce lactic acid which inhibits the growth, multiplication and development of such harmful microorganisms. Mechnikov explained the exaggerated life longevity of Bulgarians with the beneficial health effects of Bulgarian yoghurt.
Nowadays under the designation “Bulgarian yoghurt” one understands fermented milk products, obtained as a result of the activity of symbiotic culture of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. The lactic acid fermentation, caused by those two bacterial species, induces profound changes in the milk content. The fermentation products exhibit positive health effects on the human organism. Scientific investigations demonstrated that yoghurt consumption influences positively the balance of the microbial population in the human intestine. It facilitates the assimilation of lactose, stimulates the immune system and has an anticancer effect. Metabolites produced by Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus during the fermentation process lead to increase of the cell counts of healthy and beneficial intestinal microorganisms.