This Y-shaped organism was isolated from infant stool in 1899 at the Pasteur Institute by Tissier (21). There are 24 species in this group. Nine of these are of human origin and the other 15 come from animals (40). These gram-positive organisms are strictly anaerobic. The degree of tolerance to oxygen depends on the species and the growth medium. It appears that the strains of B. bifidus are relatively aerotolerant (40). Optimum growth temperature for species of human origin is 36°-38°C; no growth occurs at 20 °C. They do not tolerate heat; B. bifidus is inactivated at 60 °C. Two moles of glucose are fermented by the fructose-6-phosphate phosphoketolase pathway to 2 moles of L (+) lactate and 3 moles of acetate. Some formic acid and ethanol may also be produced. Bifidobacteria of human origin synthesize several vitamins, thiamine (B1), riboflavin (B2), pyridoxine (B6), folic acid (B9), cyanocabalamin (B12), and nicotinic acid (P) (40).

It has been observed that the number of bifidobacteria falls significantly in the stools of adults and the elderly. The proportions of various species of bifidobacteria vary with age of the humans. It appears that B. infantis and B. breve are favored in breast-fed infants and B. adolescentis predominates in bottle-fed infants and in adults (40). Allergic infants were reported to be colonized mainly by B. adolescentis species with a lower mucus-binding capacity than bifidobacteria from healthy infants (41). Bifidobacteria population may be reduced after the administration of a Western diet. All strains of bifidobacteria do not behave similarly. Morinaga Milk Industry Co, LTD, claims that their B. bifidum (BB536) is hardier and superior to other strains (42).

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