Background

The human microbiome comprises a variety of microorganisms that colonize the body in different places and in different compositions. A well-known example of this is the intestinal microbiome [1]. The microbiome differs from person to person. The study presented by Foessleitner et al. [1] focuses on the maternal vaginal and rectal microbiome during pregnancy and the development of the newborn’s microbiome after birth. There is evidence that an altered composition of the vaginal microbiome in pregnant women increases the risk of vaginal infection and can lead to premature birth [2, 3]. The healthy vaginal microbiome is mainly dominated by Lactobacillus species, which can suppress the growth of pathogenic bacteria by producing lactic acid [4, 5]. However, current data show that not all Lactobacillus species have a protective effect on the vaginal flora. For example, the presence of Lactobacillus iners appears to favor bacterial vaginosis, which in turn increases the risk of premature rupture of the membranes and consequently premature birth [2, 6]. During pregnancy, the vaginal microbiome changes early on, in the sense of a decrease in diversity [3, 4]. However, findings from studies regarding the link between the altered vaginal microbiome and an infection, which in turn can lead to premature birth or early premature rupture of the membranes, are inconsistent [4,5,6,7,8].

Since it is generally assumed that the uterus and placenta are sterile, the prevailing doctrine is that the development of the infant microbiome begins at birth through exposure to a variety of germs: during vaginal birth through contact with vaginal and fecal maternal germs and after cesarean section through skin contact and the environment [9]. In recent years, however, studies have been published that claim to have demonstrated a microbiome of the placenta and thus suggest an intrauterine microbial colonization of the fetus [10, 11].

The mode of birth also appears to have an influence on the development of the infant microbiome. In newborns born by cesarean section, there was evidence of a higher exposure of pro-inflammatory cytokines [12]. This is likely to be a major risk factor for the later development of autoimmune diseases such as type 1 diabetes mellitus, allergic bronchial asthma, or various allergies [13]. After vaginal birth, on the other hand, the infant microbiome showed a higher presence of Bifidobacterium species and, in turn, fewer pathogenic opportunistic germs such as Enterococcus and Klebsiella species [14]. Breast milk also plays an important role in the development of the infant’s microbiome, with both infant formula, as an alternative to breast milk, and antibiotics being shown to be disruptive factors in the development of the child’s microbiome [15].

The aim of the presented study was to investigate the maternal vaginal and rectal microbiome in the third trimester of an unremarkable pregnancy, the placental and uterine microbiome at birth by cesarean section, and the subsequent development of the infant microbiome immediately postpartum and in the first days of life. This understanding of the physiological composition of the maternal microbiome during pregnancy and the development of the child’s microbiome should form the basis for recognizing and treating microbiome changes at an early stage in the future and subsequently for preventing premature rupture of the membranes and thus premature birth.

Methods

The study by Foessleitner et al. [1] is a prospective, longitudinal study that was conducted at the Medical University of Vienna at the Department of Obstetrics and Gynecology and the Department of Pediatrics and Adolescent Medicine between August 2020 and September 2021. Patients who presented between 32 + 0 and 37 + 0 weeks of pregnancy to schedule a planned cesarean section around the due date were included in the study. The following swabs were taken as part of the study: a maternal vaginal swab and a rectal swab at inclusion. At the time of the cesarean section, these were repeated, and an intraoperative swab from the fetal side of the placenta and a sterile swab from the uterine cavity were also collected. In the newborn, a buccal, a dermal, and a meconium swab were taken immediately after birth. These neonatal swabs were repeated on the second or third day of life. The sampling protocol is shown in Table 1.

Table 1 Overview of samples taken for microbiome analysis in the longitudinal study settinga
Full size table

The microbiome analysis was carried out in cooperation with the Joint Microbiome Facility (JMF) of the Medical University of Vienna and the University of Vienna and was performed using 16S rRNA amplicon sequencing. To guarantee the validity of the results, only samples with sufficient biomass were analyzed. The samples were analyzed longitudinally in order to detect potential differences in microbial composition between the different sampling times. Values of p < 0.05 were considered statistically significant.

Results

Of a total of 37 pregnant women screened, 30 mothers were included in the analysis. The average age at inclusion was 32.9 years. The birth took place at an average of 38.4 weeks’ gestation, and the newborns had an average birth weight of 3250.2 g. The maternal vaginal and rectal microbiome did not change during the third trimester and thus remained stable in their microbial composition (permutational multivariate analysis of variance [PERMANOVA]; p > 0.05). After exclusion of contamination, both the placental and intrauterine samples were sterile, meaning that no evidence of prepartum bacterial colonization was detected. Of the 18 newborns included in the analysis, 16 buccal, 15 dermal, and 14 meconium swabs showed no evidence of microbiome colonization on the day of birth. Afterwards, the neonatal microbiome developed rapidly in the first days of life. Already on the second or third day of life, the microbial colonization of the newborns was clearly different from the microbiome of the mothers, which was detected in the maternal vaginal and rectal swabs on the day of birth (PERMANOVA; p < 0.01). Furthermore, the infant microbiome was significantly different between buccal and fecal swabs (PERMANOVA; p < 0.01). The buccal microbiome mainly showed colonization with Staphylococcus and Streptococcus species, whereas the microbiome of the stool smears showed a significantly more diverse colonization with different genera of Actinobacteria, Firmicutes, and Proteobacteria. The diversity in the maternal and neonatal microbiome is shown graphically in Fig. 1 (adapted from Foessleitner et al. [1]).

Fig. 1
figure 1

Principal component analysis showing the microbial diversity of seven mother–child pairs. a Maternal microbiome compared with the infant microbiome on the second/third day of life and b infant buccal (blue) and stool smears (orange) on the second/third day of life. (Adapted from Foessleitner et al. [1], used under the Creative Commons CC-BY-NC license)

Full size image

Discussion

The study by Foessleitner et al. [1] examined the composition of the vaginal and rectal microbiome in the third trimester of healthy pregnant women with an unremarkable course of pregnancy whose infants were delivered by elective cesarean section at term. Furthermore, the development of the infant’s microbiome in the first days of life was analyzed longitudinally. No significant change in the maternal vaginal and rectal microbiome was found during the last trimester. The vaginal microbiome of all pregnant women was dominated by Lactobacillus crispatus, with a low incidence of Gardnerella vaginalis. This composition has already been associated with term birth and a low incidence of preterm birth in other studies [5, 16]. Understanding the vaginal microbial colonization of healthy pregnant women is essential as this knowledge can be used in the future to identify disruptive factors that can lead to early premature rupture of the membranes [17]. It would also be desirable to define potential biomarkers in the microbiome which—if present—represent an increased risk of early premature rupture of membranes or premature birth [18]. There is a long-standing controversy regarding the microbial colonization of the placenta and uterine cavity [10, 11, 19,20,21]. Some studies have demonstrated microbial colonization of the placenta or uterine cavity, but the question that often arises is whether this is due to contamination of the samples at birth or during laboratory analyses [10, 19, 22]. In this study, two buccal, three dermal, and four meconium swabs showed microbial colonization on the day of birth, but these were dominated by a Staphylococcus species, which is often found on human skin. Therefore, it is more likely that this colonization originated from skin-to-skin contact after birth than being an intrauterine colonization, especially since no microbiome could be detected in the intrauterine and placental swabs of these mothers. Thus, the work of Foessleitner et al. supports the thesis that newborns do not exhibit microbial colonization before birth. Foessleitner and colleagues demonstrated that the development of the infant microbiome begins only after birth. In addition, they observed that the early neonatal microbiome on the second/third day of life differed significantly from that of their mothers. Therefore, it can be concluded that infant microbiome development is primarily influenced by exposure to the environment.

The strength of this study is the longitudinal study design, which made it possible to investigate the microbiome of the mother in the late phase of pregnancy and the early microbiome of the child after cesarean section of mother–child pairs. A limitation of the study is the lack of comparison of the neonatal microbiome development across different modes of delivery. However, there are already ample data indicating a difference in the infant microbiome after vaginal birth compared to that after cesarean section [9, 15, 23]. The small number of cases in this pilot study should also be mentioned as a limitation, which warrants confirmation of the results in larger studies with a higher number of cases.

Conclusion

In summary, the composition of the maternal microbiome in healthy women at the end of pregnancy was stable and without significant changes. The analysis of maternal and infant swabs on the day of birth showed no evidence of prepartum microbial colonization of the newborns in the context of an unremarkable pregnancy with elective delivery by cesarean section. After elective cesarean delivery, the neonatal microbiome appears to develop through postnatal exposure to the immediate environment. These findings provide a fundamental understanding of the physiological maternal microbiome in the third trimester, as well as the normal development of the infant microbiome after delivery by cesarean section, which will serve as a basis for further studies already underway at the Medical University of Vienna to investigate the microbiome in the context of early preterm rupture of the membranes. These studies will hopefully ultimately contribute to the identification of predictive parameters for early premature rupture of the membranes and subsequent premature birth, allowing this serious pregnancy complication to be detected early and hopefully prevented in the future.