Minoru Shirota

Did You Know?

• Shirota was born to a poor farming family in western Nagano, Japan.
• Inspired by prevalent infectious diseases and malnutrition in his hometown, he chose to study medicine.
• He died in Tokyo, Japan, in 1982.

Scientific contribution

While studying microbes of the digestive system at Kyoto University, Shirota encountered Élie Metchnikoff's theory on aging and longevity. Metchnikoff, a Nobel laureate and founder of immunology, believed that longevity in populations such as Bulgarians resulted from habitual yogurt consumption. He had identified Lactobacillus bulgaricus (later reassigned as Lactobacillus delbrueckii subsp. bulgaricus) as the yogurt ingredient responsible. Shirota, keen to understand this bacterium, conducted experiments and found that the Bulgarian bacterium could not be the cause of health benefits. It could not survive acidic conditions like those in the stomach, and it was difficult to maintain in culture, making isolation and storage impossible. In 1930, Shirota discovered a type of bacteria from human faeces, which he originally identified as Lactobacillus casei. He isolated a strain resistant to gastric acid and digestive enzymes, unlike Metchnikoff's bacterium. This was the discovery of Lactobacillus casei strain Shirota, later reclassified into Lacticaseibacillus paracasei in 2020 (though the name L. paracasei Shirota is not yet recognised under the International Code of Nomenclature of Prokaryotes). The new strain was easily cultured, isolated, and maintained in the laboratory, and could pass through the digestive system to multiply in the intestine. Shirota found it could inhibit pathogenic bacteria in the digestive system and used it to treat diarrhoea during an outbreak. Using his bacterium, he prepared a fermented milk that proved beneficial for the digestive system. In 1935, he produced the first commercial probiotic beverage, named Yakult. The gradual success of his laboratory product led to the establishment of the first probiotic company, Yakult Honsha, in Kyoto in 1955, based on his Shirota Research Institute, which moved to Kunitachi, Tokyo, in 1967 and was renamed Yakult Central Institute for Microbiological Research.