Bifidobacteria: their significance in human intestinal health

Bifidobacterium was first isolated in 1899 from a healthy breast-fed infant by Tissier of the Pasteur Institute in France. It is an anaerobic, Gram-positive, non-sporeforming, pleomorphic rod, and was originally named Bacillus bifidus communis. The name signifies the branching morphology of the bacteria; bifidus in Latin meaning cleft in two parts. Later, the bacterium was once placed within the genus Lactobacillus as L. bifidus. In 1960s, it was accepted as an independent genus and classified as Bifidobacterium. Apart from the properties mentioned above, the main phenotypic characteristics of Bifidobacterium is producing lactic acid and acetic acid as the main products of glucose utilization. Bifidobacterium species and classification There are currently about 30 known Bifidobacterium species. The Bifidobacterium species that inhabit the human intestinal tract are rather distinct from those that inhabit the intestines of animals (Mitsuoka, 1984). The representative species of human origin include B. longum, B. breve, B. infantis, B. bifidum, B. adoltescentis and B. pseudocatenulatum. Representatives of animal-derived species include B. pseudolongum, B. thermophilus and B. animalis. Species of animal origin are never isolated from the human intestinal tract and human origin species are almost never found in animal intestines. The reason for this host-specificity is unknown, but is suspected to be due to differences among species in the ability to colonize the host intestinal tracts. Among the Bifidobacterium species used in various yogurts, B. animalis is often identified (Yaeshima et al, 1996). The reason for using this species is that animal-derived species are more acid-resistant and therefore preserve better in the yogurt. Recently, the B. animalis strains isolated from these yogurts have been found to have some genetic differences compared to the type strain of B. animalis, and these strains have been named B. lactis (Meile et al, 1997). This species is also not an inhabitant of the human intestinal tract. The use of human-origin species as food supplement seems to be the reasonable and correct choice. Ecology in the human intestinal tract Before birth, the human foetus is germ-free and intestinal bacteria do not exist. From the time of birth, bacteria begin to colonize the intestinal tract forming the intestinal microflora. At birth, many bacterial species gain access into the intestinal tract, but bifidobacteria gradually become established as the main bacteria, and predominate in the intestinal microflora during the neonatal period. This tendency is especially marked in breast-fed infants. According to a study,