Abstract
The developing intestinal microbiota of breast-fed infants is considered to play an important role in the priming of the infants' mucosal and systemic immunity. Generally,BifidobacteriumandLactobacilluspredominate the microbiota of breast-fed infants. In intervention trials it has been shown that lactobacilli can exert beneficial effects on, for example, diarrhea and atopy. However, theLactobacillusspecies distribution in breast-fed or formula-fed infants has not yet been determined in great detail. For accurate enumeration of different lactobacilli, duplex 5′ nuclease assays, targeted on rRNA intergenic spacer regions, were developed forLactobacillus acidophilus,Lactobacillus casei,Lactobacillus delbrueckii,Lactobacillus fermentum,Lactobacillus paracasei,Lactobacillus plantarum,Lactobacillus reuteri, andLactobacillus rhamnosus. The designed and validated assays were used to determine the amounts of differentLactobacillusspecies in fecal samples of infants receiving a standard formula (SF) or a standard formula supplemented with galacto- and fructo-oligosaccharides in a 9:1 ratio (OSF). A breast-fed group (BF) was studied in parallel as a reference. During the 6-week intervention period a significant increase was shown in total percentage of fecal lactobacilli in the BF group (0.8% ± 0.3% versus 4.1% ± 1.5%) and the OSF group (0.8% ± 0.3% versus 4.4% ± 1.4%). TheLactobacillusspecies distribution in the OSF group was comparable to breast-fed infants, with relatively high levels ofL. acidophilus,L. paracasei, andL. casei. The SF-fed infants, on the other hand, contained moreL. delbrueckiiand lessL. paracaseicompared to breast-fed infants and OSF-fed infants. An infant milk formula containing a specific mixture of prebiotics is able to induce a microbiota that closely resembles the microbiota of BF infants.

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