Major gut microbiota perturbations in firstborn infants compared to those with older siblings soon after delivery

The gut microbiota may influence immune maturation during infancy. While cesarean delivery is known to delay acquisition of a mature anaerobic microbiota, the influence of being firstborn on early gut colonization is less well studied. Feces were collected regularly from 3 days to 18 months of age in the FARMFLORA cohort (N = 65) and cultured quantitatively for major anaerobic and facultative bacteria. Colonization rates and population counts of different gut bacteria were analyzed in relation to birth order, delivery mode, and antibiotic exposure during delivery. Each exposure was adjusted for the other two. Birth order, delivery mode and antibiotic exposure were each independently associated with gut colonization. Firstborn infants (N = 29) acquired Escherichia coli and bifidobacteria later than infants with older sibings (N = 36) and had signs of microbiota immaturity, including higher levels of facultative bacteria and increased carriage of opportunistic colonizers such as Clostridioides difficile and Staphylococcus aureus. Cesarean-delivered infants (N = 10) showed delayed E. coli acquisition and increased colonization by other enterobacteria, while antibiotic exposure during delivery (N = 10) delayed colonization by Bacteroides. Delayed colonization by typical fecal bacteria in firstborn infants suggests reduced maternal bacterial transfer during delivery. This might contribute to the predilection of firstborn children to develop allergy.