Some of the Bifidobacterium animalis bacteria found in a sample of Activia yogurt: The numbered ticks on the scale are 10 micrometres apart.
In 1899, Henri Tissier, a French pediatrician at the Pasteur Institute in Paris, isolated a bacterium characterised by a Y-shaped morphology ("bifid") in the intestinal microbiota of breast-fed infants and named it "bifidus". In 1907, Élie Metchnikoff, deputy director at the Pasteur Institute, propounded the theory that lactic acid bacteria are beneficial to human health. Metchnikoff observed that the longevity of Bulgarians was the result of their consumption of fermented milk products. Metchnikoff also suggested that "oral administration of cultures of fermentative bacteria would implant the beneficial bacteria in the intestinal tract".
