In the year 1900, the Austrian doctor Ernst Moro noticed something interesting in baby poop. A pointy gram-positive rod only 1.502 micrometers in length was the dominant microbial lifeform in the intestines (and feces) of breast-feeding infants. Moro, who was at the forefront of the emerging school of pediatric medicine, had discovered a new bacteria. Infants are born without bacteria in their intestines and build up their digestive flora from the foods they eat. For some reason, this bacteria was coming from mother’s milk and implanting itself in the newborn’s gut.

Moro had found lactobacillus acidophilus, a lactic acid bacteria that does well in the acidic environment of the human gut. A couple years later in 1908, another prominent European doctor was pioneering the field of geriatrics. Eli Metchnikoff built upon Moro’s discovery. He realized that these bacteria were not only naturally occurring in the intestine, but could be intentionally administered to patients to improve health. Metchnikoff believed that lactic acid bacteria, such as L. acidophilus, could promote longer life by removing toxic organisms that lived in the intestine.

In his book, The prolongation of life; optimistic studies, Metchnikoff wrote, “In addition to such observations on lactic bacilli there is a good deal of knowledge as to the effect of lactic acid taken in bulk. The result of the various observations shows that the acid lessens intestinal putrefaction and lowers the quantity of sulpho-conjugate ethers in the urine. This fact explains why favourable results follow the use of lactic acid in many intestinal diseases such as infantile diarrhea, tuberculous enteritis and even Asiatic cholera.” By the end of the decade, scientists and doctors began to combine the two men’s ideas and researched the health benefits of L. acidophilus bacteria.