This study investigates the factors influencing the composition of the intestinal microbiota in early infancy. The researchers analyzed fecal samples from 1032 infants at one month of age, using quantitative real-time polymerase chain reaction assays to measure the levels of various bacteria. They collected data on potential determinants such as mode of delivery, infant feeding, hospitalization, antibiotic use, and family characteristics. Key findings include: 1. **Mode of Delivery**: Infants born through cesarean section had lower levels of bifidobacteria and Bacteroides but higher levels of *Clostridium difficile* compared to vaginally born infants. 2. ** Infant Feeding**: Formula-fed infants were more often colonized with *Escherichia coli*, *C. difficile*, and *Bacteroides* compared to breastfed infants. 3. **Hospitalization and Prematurity**: Hospitalization and prematurity were associated with higher prevalence and counts of *C. difficile*. 4. **Antibiotic Use**: Antibiotic use by the infant was associated with decreased levels of bifidobacteria and *Bacteroides*. 5. **Family Characteristics**: Infants with older siblings had slightly higher levels of bifidobacteria. The study concluded that the most significant determinants of the gut microbiota composition in infants were the mode of delivery, type of infant feeding, gestational age, infant hospitalization, and antibiotic use. Term infants who were born vaginally at home and were breastfed exclusively had the most beneficial gut microbiota, characterized by higher levels of bifidobacteria and lower levels of *C. difficile* and *E. coli*.This study investigates the factors influencing the composition of the intestinal microbiota in early infancy. The researchers analyzed fecal samples from 1032 infants at one month of age, using quantitative real-time polymerase chain reaction assays to measure the levels of various bacteria. They collected data on potential determinants such as mode of delivery, infant feeding, hospitalization, antibiotic use, and family characteristics. Key findings include: 1. **Mode of Delivery**: Infants born through cesarean section had lower levels of bifidobacteria and Bacteroides but higher levels of *Clostridium difficile* compared to vaginally born infants. 2. ** Infant Feeding**: Formula-fed infants were more often colonized with *Escherichia coli*, *C. difficile*, and *Bacteroides* compared to breastfed infants. 3. **Hospitalization and Prematurity**: Hospitalization and prematurity were associated with higher prevalence and counts of *C. difficile*. 4. **Antibiotic Use**: Antibiotic use by the infant was associated with decreased levels of bifidobacteria and *Bacteroides*. 5. **Family Characteristics**: Infants with older siblings had slightly higher levels of bifidobacteria. The study concluded that the most significant determinants of the gut microbiota composition in infants were the mode of delivery, type of infant feeding, gestational age, infant hospitalization, and antibiotic use. Term infants who were born vaginally at home and were breastfed exclusively had the most beneficial gut microbiota, characterized by higher levels of bifidobacteria and lower levels of *C. difficile* and *E. coli*.