Early Development of Intestinal Microbiota: Implications for Future Health José M. Saavedra, MD a,b, *, Anne M. Dattilo, PhD, RD b INTRODUCTION The fetal gut lumen is devoid of any significant amounts of bacteria. But within a few hours after birth, the intestinal tract becomes colonized by microorganisms, collec- tively known as microbiota. This dramatic ecologic change in the gut lumen, from rela- tive bacterial naivete ´ to exposure to billions of organisms of hundreds of species in a matter of days, is a key determinant in the development and maturation of gut barrier function mechanisms and the development and modulation of gut and systemic immune responses, primarily via its effect on gut-associated lymphoid tissue. The time of development, the microbial profile, and the diversity of this ecosystem seem to be critical in eliciting appropriate host responses, particularly adaptive immunity. A few specific bacterial genera and species seem to be important and relevant to a Department of Pediatrics, Johns Hopkins University School of Medicine, 600 North Wolfe Street, Baltimore, MD 21287, USA; b Nestle ´ Nutrition, Medical Scientific and Regulatory Unit, 12 Vreeland Road, Florham Park, NJ 07932, USA * Corresponding author. Nestle Nutrition, Medical Scientific and Regulatory Unit, 12 Vreeland Road, Florham Park, NJ 07932. E-mail address: jose.saavedra@us.nestle.com KEYWORDS  Microbiota  Intestine  Bacteria  Infant  Cesarean section  Prematurity KEY POINTS  Early acquisition and maintenance of a healthy intestinal microbiota is critical to the long- term health of the host; and the mechanisms through which this occurs are beginning to be elucidated.  Factors that lead to alterations in microbial composition during infancy are associated with inadequate and inappropriate host immune responses, which lead to the development of disease.  A greater understanding of these factors and of these mechanisms will unlock potential strategies to modulate the acquisition and maintenance of a human microbiota that main- tains and promotes health.  These strategies will be critical in battling the epidemic of noncommunicable immune- related diseases encountered in the modern world. Gastroenterol Clin N Am 41 (2012) 717–731 http://dx.doi.org/10.1016/j.gtc.2012.08.001 gastro.theclinics.com 0889-8553/12/$ – see front matter Ó 2012 Elsevier Inc. All rights reserved.