Neonatal Oral Fluid As One of the Transmission Routes for Bifidobacteria to the Infant Gut Immediately after Birth

Background: Bifidobacteria are one of the most abundant bacterial groups in the infant gut microbiota and are closely associated with infant health as well as potentially affecting health in later life. The establishment of bifidobacteria in the infant gut occurs  from birth and is influenced by factors such as birth mode, type of infant feeding, and antibiotic use. However, the details regarding the source of bifidobacteria have yet to be completely elucidated. This study aimed to assess neonatal oral fluid (OF) as a transmission route for bifidobacteria to the infant gut during delivery.   Methods: Neonatal OF and infant fecal (IF) samples were collected immediately and one month after birth from 15 healthy vaginally delivered newborns to analyze the microbiotic composition. Bifidobacterium strains were isolated from OF and IF samples and sequenced using a MiSeq platform to evaluate the similarity of strains between the OF-IF pairs based on the average nucleotide identity (ANI) value of specific genomic coding regions of each strain.   Findings: The 16S rRNA gene sequencing results revealed the presence of Bifidobacteriaceae at >1% relative abundance in all OF samples. Bifidobacterium strains were isolated from OF (9/15) and IF samples (11/15), and those sharing high genomic homology (ANI values >99.5%) between the neonatal OF and IF samples were present in one-third of the OF-IF pairs.  Interpretation: The results of this study indicate that viable bifidobacteria are present in neonatal OF and that bifidobacteria could reach the infant gut though the oral environment.   Funding Statement: Morinaga Milk Industry Co., Ltd. funded this research. Declaration of Interests: K.T., N.K., T.O., E.M., Te.K. and JZ.X are employees of Morinaga Milk Industry Co., Ltd. The remaining authors declare that there are 361 no conflicts of interest. Ethics Approval Statement: This study was approved by the Ethics Committee of the School of Health and Sports Science, Juntendo University, and informed written consent was obtained from all of the mothers.