BACKGROUND: Perturbations in the intestinal microbiota may disrupt involved in the development of immunologic tolerance. The present study examine the establishment of the infant microbiota and its association development of atopic dermatitis (AD). METHODS: Within a randomized, placebo-controlled trial on the prevention of AD by oral supplementation bacterial lysate between week 5 and the end of month 7, feces was the ages of 5 weeks (n = 571), 13 weeks (n = 332), and 31 weeks (n = subjected to quantitative PCRs to detect bifidobacteria, bacteroides, lactobacilli, Escherichia coli, Clostridium difficile, and Clostridium RESULTS: Birth mode, breast-feeding but also birth order had a strong the microbiota composition. With increasing number of older siblings the colonization rates at age 5 weeks of lactobacilli (P < .001) and .02) increased, whereas rates of clostridia decreased (P < .001). with clostridia, at the age of 5 and 13 weeks was also associated with increased risk of developing AD in the subsequent 6 months of life (odds ratioadjusted = 2.35; 95% CI, 1.36-3.94 and 2.51; 1.30-4.86, Mediation analyses demonstrated that there was a statistically indirect effect via Clostridium cluster I colonization for both birth birth order in association to AD. CONCLUSION: The results of this study supportive for a role of the microbiota in the development of AD. "beneficial" influence of older siblings on the microbiota composition that this microbiota may be one of the biological mechanisms underlying sibling effect.