The establishment and development of the neonatal microbiota play a crucial role in newborn health. In this study, we focused on the early microbiota of the neonate (within 0–3 days of birth, and specifically in the oral cavity, shortly after birth) as the starting point for the establishment of a stable normal flora. This study aimed to examine microbial transmission patterns from the maternal cervix to different body sites in newborns and identify factors influencing the diversity of the newborn oral microbiome. Our findings reveal a clear transmission pattern of microbiota from the maternal cervix to the neonatal gastric, bronchial, and oral sites. We showed that similar genera were detected in the neonatal gastric, bronchial, and oral cavities, and that the neonatal gastric microbiota was the most similar to the maternal cervical microbiota. Various natal factors, such as preterm birth and low birth weight, maternal chorioamnionitis and diabetes, were associated with decreased diversity in the neonatal oral microbiome. Additionally, the composition of neonatal oral microbiome differed according to birth type. These findings underscore the intricate interplay between maternal, neonatal factors in shaping the neonatal oral microbiota and highlight potential implications for newborn health.
A comparison of the bacteria detected at each site at the genus level showed that the diversity of bacteria in the newborn was low compared to that in the maternal cervix, and the neonate gastric, oral, and bronchial cultures showed similar bacterial genera. The composition of the neonate gastric microbiota exhibited the highest similarity with that of maternal cervical microbiota, followed by the oral and bronchial microbiota, respectively. This finding indicates the colonization of maternal-origin microbiota in the oral and intestinal tracts of newborns due to the swallowing of amniotic fluid. Furthermore, the high similarity between the neonatal gastric microbiota and maternal cervical microbiota compared to the neonatal oral microbiota could be because of the insufficient number of samples subjected to oral microbiota culture analysis. Nevertheless, apart from Lactobacillus, Streptococcus, Staphylococcus, and Escherichia were consistently detected among the top genera in all four sites. These findings confirmed the pattern of transmission from the mother to the newborn and underscored the similarity in microbiota patterns among different sites in the newborn.
The formation of the initial microbiota in newborns begins with the microbiota derived from the mother, and various natal factors can influence the colonization of such microbiota. In this study, we categorized natal factors into three groups and identified factors influencing the initial microbiota of newborns. Additionally, elevated microbial diversity is generally considered healthier than constricted microbial diversity.16 Therefore, this study also aimed to identify the natal factors that influence the diversity of the oral microbiome shortly after birth.
Compared with full-term neonates, preterm neonates exhibit decreased gut microbiome diversity, with Proteobacteria and Firmicutes being predominant.17 Additionally, the oral microbiome of preterm neonates is initially disturbed after birth, with specific microbes associated with complications such as bronchopulmonary dysplasia and sepsis.6 Consistent with these previous studies, the present study demonstrated the diversity of oral microbiome was reduced in low birth weight preterm neonates compared to full-term, normal-weight neonates. These findings suggest differences in microbial composition and indicate that birth type and birth weight are important in the formation of early-life oral microbiota.
Several studies have shown the differences in the oral microbiota composition of newborns depending on the mode of delivery, with maternal vaginal microbiota predominating in those born vaginally and maternal skin microbiota predominating in those born by cesarean section.9,18 In this study, we found a significantly higher proportion of neonatal gastric microbiota in vaginal births. Furthermore, we also showed that the neonatal oral microbiome composition differed depending on the mode of delivery. This finding further supported that the newborn microbiota is formed by swallowing the amniotic fluid at birth.
APGAR score evaluates the condition of a newborn immediately after birth by assessing skin color, heart rate, muscle tone, respiration, and reflexes.19 According to this study, neonates with detected bronchial microbiota had significantly reduced 5-min APGAR scores, while no association was observed with oral microbiota. This suggests that the condition of newborns immediately after birth may influence the composition of the early-life microbiota.
Maternal systemic diseases such as chorioamnionitis, diabetes, are associated with the detection of newborn gastric microbiota. In particular, when mothers had chorioamnionitis, the detection of gastric and bronchial microbiota in newborns was found to be associated. Moreover, maternal diabetes and chorioamnionitis significantly reduced the diversity of the neonatal oral microbiome. Chorioamnionitis is an inflammatory infectious disease affecting the placenta and amniotic membranes in pregnant women, known to potentially lead to preterm labor, neonatal sepsis, and respiratory diseases in newborns.20 The presence of such infectious maternal conditions suggests potential impacts on the composition of the early-life microbiota in newborns. Several studies have shown a link between diabetes and changes in the oral microbiota.21 Additionally, the maternal oral microbiota has been speculated to contribute to the seeding of the placental microbiota.22 This suggests that an altered maternal oral microbiota may influence the placental microbiota, with one study showing that the placental microbiota is altered by gestational diabetes.23,24 As the oral microbiota of a newborn is derived from the placenta and amniotic fluid, the diabetes-induced altered placental microbiota could affect the neonate oral microbiota, which supports our findings.
It is well known that the initial microbiota of newborns plays an important role in their health after birth. In this study, we found that the incidence of sepsis was significantly high when the microbiota was detected in neonatal gastric or bronchial areas. Additionally, the probability of NEC was significantly elevated when the microbiota was detected in the neonatal bronchial area. This underscores the previously established association between the early microbiota of newborns and their health outcomes.
Maternal amniotic fluid and placental microbiota are transmitted to newborns, and oral microbiota is transmitted to the gut and respiratory tract. This suggests that the microbiota at each site is interconnected. In this study, the microbiota in the neonate oral, gastric, and bronchial sites were found to be interrelated— when microbiota was detected at these sites, the diversity of the neonate oral microbiome decreased. Investigation of the association between the presence of pathogenic microbes in both mothers and newborns and the microbial composition at each site revealed that Escherichia coli has a significant association with the microbiota composition of neonates. Detection of Escherichia coli in neonatal gastric or oral samples was shown to be associated with reduced diversity of the oral microbiome. Moreover, oral Escherichia coli and cervical A. omnicolens were associated with high bacterial detection rates in gastric samples, whereas cervical K. pneumoniae, gastric Streptococcus agalactiae, Escherichia coli, and K. pneumoniae were associated with high bacterial detection rates in bronchial samples. Most strains of Escherichia coli are harmless, but pathogenic strains are known to cause conditions such as sepsis and NEC.25,26 A. omnicolens act as a pathogen in mothers, leading to preterm premature rupture of membranes.27 K. pneumoniae is associated with preterm birth and neonatal sepsis and is pathogenic in both mothers and neonates.28,29 Streptococcus agalactiae, known as Group B Streptococcus, causes neonatal sepsis.26,30 This study confirms that the presence of specific pathogens in mothers or newborns at certain body sites can influence the composition of the early-life microbiota of neonates, potentially impacting newborn health.
Preterm birth was the most significant factor; therefore, we examined the differences in the composition of the oral microbiome according to birth type. The composition of the oral microbiome between the preterm and full-term groups was significantly different, with Proteobacteria and Firmicutes being enriched in the preterm group, which is consistent with a previous study on the gut microbiome in preterm.14 At the species level, bacteria that can cause neonatal sepsis or NEC, such as Cronobacter sakazakii31 and Staphylococcus captis,32 have been shown to be abundant in preterm neonates. In contrast, Bifidobacterium breve33 with probiotic properties, Veillonellae rogosae,34,35 associated with oral biofilm ecology and has been reported to be abundant in caries-free individuals, were less abundant in preterm neonates. Additionally, Haemophilus parainfluenzae and Porphyromonas pasteri, which have been detected in dental and periodontal health conditions36, are lower in preterm neonates. These findings suggest that beneficial bacteria tend to be less abundant, and pathogens are more prevalent in preterm neonates; however, further research is needed to confirm these speculations. In this study, a comparison with the PHI database revealed that the abundances of Corynebacterium diphtheriae, Lactococcus lactis, Proteus mirabilis, Helicobacter pylori, Enterococcus faecalis, and Streptococcus pneumoniae, were significantly different in preterm neonates, indicating their association with a variety of infectious diseases; nevertheless, this finding warrants further studies to confirm the precise role of these microbes.
This study has several limitations. First, to confirm the mother-to-newborn microbial transmission pattern, retrospective cohort with a larger sample size was necessary. Consequently, the analysis of cervical, gastric, and bronchial microbiota was based solely on the culture test results. Therefore, a direct comparison with the oral microbiome analyzed using 16S rRNA sequencing was difficult, allowing only for the identification of their respective associations. Second, the study focused primarily on 16S rRNA analysis for microbial composition, which limits the ability to identify specific bacteria. Therefore, further studies incorporating additional analyses, such as metagenome analysis, could provide a more comprehensive understanding of the neonatal microbiome. Finally, the study's observational nature limits the ability to establish causal relationships between factors and microbial composition.
Despite these limitations, the study provides valuable insights into the early formation of the neonatal microbiota and its potential implications for newborn health. Firstly, it stands out from existing research by comprehensively investigating the association between bacterial detection across four body sites: the maternal cervix, gastric, bronchial, and oral sites in newborns. This holistic approach provides a more complete understanding of microbial transmission patterns. Secondly, unlike most studies on neonatal oral microbiota, which focus on a limited set of factors, our study analyzes a wide range of factors, categorizing them into four distinct categories. This comprehensive analysis enhances the depth and breadth of our findings. Thirdly, our study includes both prospective and retrospective analyses, with the retrospective component boasting a larger sample size than previous studies in this area. This larger sample size increases the robustness and generalizability of our results. Lastly, by collecting oral microbiota samples immediately after birth, we were able to analyze the early neonatal oral microbiota while minimizing confounding factors, such as feeding type, thereby providing a clearer picture of the initial microbial composition.