Bifidobacteria have been known since Tissier (5) first described a species from the feces of breast-fed infants, which was later named as Lactobacillus bifidus by Orla-Jensen (29). Since that time numerous studies have been published concerning the ecology and importance of bifidobacteria in the intestine of humans, especially in infants.
A new taxonomic system was established by creating the genus Bifidobacterium with the description of several new species besides B. bifidum, which was the only existing species at that time (30,31). This was followed by an increasing number of new species isolated from humans and animals (32-35). The new concept of the genus Bifidobacterium taxonomy including 24 taxonomic species was summarized in special chapters of Bergey's Manual (1986). Currently, a total of 26 well-established taxonomic species have been described, among which are nine species which have been found to be exclusive residents of the human intestine. These are B. bifidum, B. longum, B. infantis, B. breve, B. adolescentis, B. angulatum, B. catenulateum, B. pseudocatenulatum, and B. dentium. Bifidobacteria appear between the 2nd and 5th day of life and continue to be one of the most numerous bacteria, amounting to about 1010/g of wet feces. Many studies indicated that each healthy adult has and maintains its own specific composition of Bifidobacterium microbiota during his/her life (33).
Interestingly, the bifidobacterial composition of a human can progressively vary with aging, both qualitatively as well as quantitatively. The predominant species of bifidobacteria in the human GI tract can be differentiated to indicate various stages of life. However, lactobacilli, another of the important genus of endogenous bacteria considered to contribute to host health and well being, are changing only quantitatively, and do not express an apparent species diversity upon aging of the host.
Bifidobacterium was found as one of the predominate bacteria in the intestinal microbiota of both infants and adults (19,32). However, the species and biotype composition of the fecal bifidobacteria progressively varied with increasing age. Species typical for infants were B. bifidium, B. infants, B. breve, and B. parvulorum. Typical for adults were four different biovars of B. adolescentis. B. bifidum and B. longum could often be found in both age groups, but in lower numbers. B. adolescentis biovar b was the most common Bifidobacterium in the microbiota of the elderly. The frequency of the occurrence of B. longum was 71% for infants, 62% and 33% for adults and the elderly, respectively. B. adolescents occurred 100%, 91%, and 79% in the elderly, adults, and infants, respectively. These results have been supported by studies conducted by other research groups (36,37). It was found that B. adolescentis and B. longum dominated the bifidobacteria of healthy adults, which is different from the bifidobacteria composition of infants.
Mitsuoka (20) consistently observed that B. adolescentis biovar. b was significantly higher in the elderly, even when Bifidobacterium counts were similar among children, adults, and the elderly. The number of B. adolescentis and B. longum in healthy adults was significantly higher than those in aged persons. From 1829 fecal bacterial isolates from 15
healthy adults, B. adolescentis, B. longum and B. bifidum were found to be the predominant species of bifidobacteria of these healthy adults (21).
He and coworkers (38) isolated 51 Bifidobacterium strains from the feces of healthy adults (30-40 years old) and seniors (older than 70 years of age). The isolates were identified to species level based on the phenotypic characteristics. The isolates from the adults belonged to B. adolescentis, B. longum, B. infantis, B. breve, while those from the elderly were B. adolescentis and B. longum.
Studies with molecular methods indicate a similar distribution of bifidobacteria species in the various stages of life. In a study using a non-culture-based method using PCR and denaturing gradient gel electrophoresis, B. adolescentis was found to be the most common species in feces of adult subjects as earlier indicated in the studies with traditional culture-based methods (39).
Fecal bacteria from healthy young adults, elderly subjects, and elderly patients with CDAD were identified to species level on the basis of their CFA profiles with MIDI. Species diversity was found to decrease with age. B. angulatum was the most common bifidobacteria! isolate in the healthy young adults. B. bifidum, B. catenulateum, B. pseudocatenulatum and B. infantis were not detected in the feces of the elderly subjects (24).
Human Bifidobacterium species were identificated by Mullie and coworkers (40) with three multiplex PCRs. B. bifidum, B. longum and B. breve species were commonly recovered in infants, while B. adolescentis B. catenulateum/B. pseudocatenulatum and B. longum were predominant in adults.
Matsuki and coworkers (41) applied species- and group-specific PCR directly to fecal samples and found B. catenulatum (B. catenulatum and B. pseudocatenulatum) in 92% of adult fecal samples and B. longum, B. adolescentis and B. bifidum in 65, 60, and 38% of the samples from adults, respectively. Comparison of species-species PCR method with the classical culture method revealed that some species, most frequently B. adolescentis, were detected by the direct PCR method but not by culturing followed by species-specific PCR of the isolates.
The bifidobacteria in the intestinal microbiota of the healthy elderly is characterized by a reduced species diversity as well as quantitative decrease. The bifidobacteria in the elderly are characterized by B. adolescentis as the predominant species as well as a quantitative decrease within the whole intestinal microbiota.
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