Mode of delivery affects the bacterial community in the newborn gut

The first colonisation of the intestine is one of the most profound immunological exposures faced by the 14 newborn and it is influenced by external and internal factors. The early composition of human microbiota 15 could have long-lasting metabolic effects and the initial composition of human intestinal bacteria is also 16 known to affect postnatal immune system development, as we are already aware that reduced microbial 17 stimulation during infancy would result in slower postnatal maturation of the immune system and 18 development of an optimal balance between TH1 and TH2-like immunity. Mode of delivery has a major role 19 on the composition of intestinal microbiota in early infancy, as it has been shown that infants born by 20 Caesarean section (CS) have lower numbers of Bifidobacteria and Bacteroides compared with vaginally born 21 infants. 22 We designed a study to investigate the influence of mode of delivery (CS vs. vaginal delivery) on intestinal 23 microbial composition on day 3 of life using PCR-denaturing gradient gel electrophoresis (DGGE) and PCR- 24 temperature gradient gel electrophoresis (TGGE). Both DGGE and TGGE analyses have been used before, 25 together with the specific amplifications for 10 Bifidobacterium sp., 3 Ruminococcus sp., and Bacteroides that 26 all have a highly relevant physiological role in the intestinal ecosystem of the newborn. 27 A total of 46 term infants were enrolled in the study, consecutively recruiting all the CS-delivered babies 28 (n=23; 8 males and 15 females) and the immediately following spontaneously delivered babies (n=23; 11 29 males and 12 females). DGGE analysis carried out with Bifidobacterium-specific primers revealed the 30 presence of this genus in 13 of 23 (56.5%) samples derived from vaginally delivered newborns but in none of 31 the samples obtained from newborns delivered by CS. PCR analysis with Bifidobacterium-species-specific 32 primers showed that naturally delivered infants had a large number of bifidobacterial species, whereas in CS- 33 delivered babies only two samples (8.7%) gave positive results, one for B. longum and another for B. gallicum. 34 In all babies enrolled, micro-organisms belonging to Ruminococcus species were absent and Bacteroides was 35 found in 8.7% of spontaneously delivered babies only. 36 Based on our findings, it seems that a newborn's intestinal bacteria during the first 3 days of life are strongly 37 influenced by mode of delivery. The intestinal flora of CS and vaginally delivered infants appears to be very 38 different; the former being altered and characterised by a substantial absence of Bifidobacteria sp., the latter 39 characterised by subject-specific microbial profiles, although predominant groups such as B. longum and 40 B. catenulatum could be identified. 41 In summary, mode of delivery does affect the early stage of intestinal bacterial colonisation, which is altered 42 in CS-delivered infants compared with vaginally delivered infants, with only a minor influence of the type of 43 feeding. In addition, the importance of methodological aspects for determining intestinal microbiota in 44 clinical trials requires emphasis if intestinal microbiota composition is to be considered a measure of 45 postnatal adaptation.