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Because of changes in gut physiology, immune system reactivity, and diet, elderly people are more susceptible to gastrointestinal infections than are younger adults. The gut microflora, which provides a natural defense against invading microorganisms, changes in elderly people with the development of potentially damaging bacterial populations, which may lead to alterations in bacterial metabolism and higher levels of infection.A randomized, double-blind, controlled feeding trial was done with 18 healthy elderly volunteers (age, >62 years) using a synbiotic comprising Bifidobacterium bifidum BB-02 and Bifidobacterium lactis BL-01 (probiotics) together with an inulin-based prebiotic (Synergy 1; Orafti). Real-time PCR was employed to quantitate total bifidobacteria, B. bifidum, and B. lactis in fecal DNA before, during, and after synbiotic consumption. Counting all viable anaerobes, bifidobacteria, and lactobacilli and identification of bacterial isolates to species level was also done.Throughout feeding, both bifidobacteria species were detected in fecal samples obtained from all subjects receiving the synbiotic, with significant increases in the number of copies of the 16S rRNA genes of B. bifidum, B. lactis, and total bifidobacteria, compared with the control week and the placebo group. At least 1 of these species remained detectable in fecal samples 3 weeks after feeding in individuals that had no fecal B. bifidum and/or B. lactis in the control week, indicating that the probiotics persisted in the volunteers. Counting of viable organisms showed significantly higher total numbers of fecal bifidobacteria, total numbers of lactobacilli, and numbers of B. bifidum during synbiotic feeding.Synbiotic consumption increased the size and diversity of protective fecal bifidobacterial populations, which are often very much reduced in older people.