RESUMEN
<p><b>OBJECTIVE</b>To investigate the composition of bacteria in the stools of infants and the colonization of intestinal microbiota during infancy.</p><p><b>METHODS</b>Fresh stools were collected from 15 healthy infants at 0, 2, 4, 7, 10, 14, and 28 days and 3, 6, and 12 months after birth. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) was used to analyze the composition of intestinal microbiota, perform sequencing of dominant bacteria, and to analyze the changes in the composition of intestinal microbiota during infancy.</p><p><b>RESULTS</b>DGGE fingerprint showed that the composition of intestinal microbiota during infancy changed significantly over time after birth. The cloning and sequencing results indicated that Proteobacteria colonized the earliest, mainly the obligate aerobes Enterobacter and Pseudomonas, followed by the obligate anaerobes (Clostridium hathewayi and Veillonella parvula) and the facultative anaerobe Clostridium ramosum in Firmicutes, and Verrucomicrobia. Actinobacteria colonized the latest, mainly Bifidobacterium, and gradually became dominant bacteria.</p><p><b>CONCLUSIONS</b>During infancy, obligate aerobes colonize the intestinal tract the earliest, followed by obligate anaerobes and facultative anaerobes. Proteobacteria colonizes the earliest, followed by Firmicutes and Verrucomicrobia, and Actinobacteria, mainly Bifidobacterium, colonizes the latest.</p>