Differences were observed in the establishment and composition of the intestinal microbiota between preterm and full-term infants. Preterm infants have an altered intestinal microbiota and a delay in colonization by some anaerobes. Lebanese preterm and full-term infants were highly colonized by Bifidobacterium, a genus known for its beneficial effects. Types of feeding and antibiotic treatments had significant effects on intestinal colonization in preterm infants.