Immediately after birth, the infant begins to acquire a commensal microbiota, which is initiated by maternally derived microbes and shaped in large part by the host genotype (1) and diet (2,3). Tissier (4) first reported the predominance of members of the bacterial genus Bifidobacterium in the neonatal gut >100 years ago. More recent nucleic acid–based approaches, including deep sequencing (5) and microarray platforms (6), have illuminated a greater diversity of microbes in the neonatal intestine. They have also confirmed that bifidobacteria constitutes 60% to 91% and ∼50% (28%–75%) of the fecal bacterial community of breast-fed and formula-fed infants, respectively, whereas by adulthood bifidobacteria comprise <5% (2,3). These distinct dietary and developmental differences have led to the hypothesis that components in human milk promote colonization by bifidobacteria, which, in turn, contributes to the beneficial effects of breast-feeding for the infant.
The search for a means to promote the growth of bifidobacteria began nearly 50 years ago with the work of Gauhe et al (7), who discovered that the “bifidus factor” in human milk comprised oligosaccharides containing N-acetyl-glycosamine. We now know that human milk contains remarkably high concentrations of structurally diverse oligosaccharides that contribute to its bifidogenic properties (8) The goal of 2 articles in this month's JPGN was to promote colonization by bifidobacteria through the addition of prebiotics (9) or by directly providing Bifidobacterium longum BL999 as a probiotic along with the modification of infant formula macronutrient composition (10). Veereman-Wauters and colleagues showed that the addition of either oligofructose or a 9:1 combination of galactooligosaccharide:fructooligosaccharide (GOS:FOS) at 0.8 g/L to infant formula increased total bacterial counts and Bifidobacterium in the feces of preterm infants to levels similar to those in breast-fed infants (11). Similar prebiotic additions were shown to be bifidogenetic in preterm infants (11,12). Hascoët and coworkers investigated the bifidogenic effects of a test formula modified to be more like human milk with or without B longum BL999 as compared with standard infant formula or breast milk (10). They found that both test formulae produced similar fecal bifidobacteria counts, which were also similar to those of breast-fed infants. Interestingly, B longum BL999 was only detected in fecal samples collected at the first month, but not thereafter, suggesting lack of colonization of the specific probiotic strain. Taken together, these studies suggest that there are a number of approaches that can increase fecal biofidobacteria.
Questions remain regarding which biofidobacteria species should be targeted and how best to promote their growth. Both of the articles featured this month quantified biofidobacteria abundance using fluorescence in situ hybridization (FISH), which could not discriminate among the species. The genus Bifidobacterium comprises 37 species, with 4 taxa that are further subdivided into subspecies that share more than 93% identity of their 16S rDNA sequences (13). B longum subsp longum, B longum subsp infantis, B bifidum, and, recently, B breve have been reported to be the predominant species in breast-fed infants (6). However, in terms of benefit to the infant, are all bifidobacteria created equal?
Emerging evidence from Miller, Lebrilla, and their colleagues (8,14) at the University of California, Davis, provide clear evidence that B longum subsp infantis is uniquely adapted to survive in the intestine of the breast-fed infant. Parallel glycoprofiling of human milk oligosaccharides (HMO) established that B longum subsp infantis ATCC15697, a strain initially isolated from the stool of a breast-fed infant, efficiently consumes several of the predominant small-mass HMO isomers (lacto-N-tetraose [LNT] and lacto-N-neo-tetraose [LNnT]) present in human milk, whereas the closely related adult type B longum subsp longum does not use HMO, but can use plant oligosaccharides and their constituent sugars (15). Genome sequencing of Bifidobacterium confirmed that B infantis strains share several large clusters containing all of the genes necessary for transport and enzymatic degradation of HMO (16). These gene clusters are absent in other Bifidobacterium strains (16).
Turning to benefit for the infant, several studies suggest that not all bifidobacteria are created equal and that B infantis would be the best one to target for growth. Young and colleagues (17) compared the fecal bifidobacteria populations of ∼1-month-old infants in Ghana (low prevalence of atopy) and New Zealand and the United Kingdom (high-prevalence countries). Samples from Ghana contained almost exclusively Bifidobacterium infantis, whereas fecal samples of the other children contained other species of bifidobacteria. Interestingly, when cord blood dendritic cells (DC) were exposed to the different strains of bifidobacteria, species-specific effects on DC activation were observed. Species that predominated in the feces of the New Zealand and UK infants (B bifidum, B longum, and B pseudocatenulatum) increased the expression of CD83 (a marker of fully mature DC) and IL-10 secretion, whereas B infantis did not, which led the authors to conclude that bifidobacteria strains differentially modulate DC activation, and B infantis could downregulate the immune response and favor tolerance (17).
In summary, several approaches have been successful in increasing bifidobacteria in formula-fed infants (9–12). The prebiotic FOS appears to broadly stimulate the growth of bifidobacteria (9,11,12). Future studies should define the specific subspecies, because all bifidobacteria may not exert the same effects on the developing infant. The prebiotic GOS is used by some bifidobacteria (18); however, the ideal probiotic to promote the growth of B infantis would be the HMO, LNT.
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