Emmanuel Amabebe
Academic unit of Reproductive and Developmental Medicine, University of Sheffield, UK

https://doi.org/10.36866/pn.109.24

The last two decades have seen a four-fold increase in the incidence of C-section. Over 20% of deliveries in the UK and United States are by C-section. The rate is even higher in Brazil, where up to 80% has been reported. The perception that C-section is life saving and prevents injury to both mother and her baby is rife. This rise is also attributed to increased maternal request due to fear of childbirth, convenience and better control over the timing of delivery. However, a considerable number of C-sections are also medically indicated (Cho & Norman, 2013; Kristensen & Henriksen, 2016).

The influence of the mode of delivery on the infant microbiome and immune system has been topical, with several attempts to elucidate the source(s) of the newborn’s microbial colonisation, immune function and risk of certain diseases later in life. This has led to the commercialisation of a vaginal
‘seeding’ approach to convert C-section infant microbiomes to those of vaginally delivered infants.

Changes in the vaginal microbiome

The human vagina, apart from being a passage for sperm, menstruum and neonates, is a highly versatile organ with a rich diverse microbial landscape. Advances in DNA sequencing techniques have revealed that healthy vaginal microbiota is predominantly characterised by Lactobacillus species, i.e. L. crispatus, L. gasseri, L. iners and L. jensenii. The protective action of these lactobacilli suppresses other anaerobes including Gardnerella, Atopobium, Mobiluncus, Streptococcus, Prevotella, Ureaplasma, etc., which have the potential to cause disease. The distribution of these organisms varies across ethnic groups. For example, it has been reported that Black and Hispanic women harbour more anaerobic species than others (MacIntyre et al., 2015). The reasons behind this are unclear, but a gene-environment interaction has been implicated.

During gestation, there is increased stability and reduced diversity of the vaginal microbiota due to an increase in Lactobacillus dominance. This can be explained by the high level of oestrogen, which promotes maturation, proliferation and accumulation of glycogen in the vaginal epithelial cells. Glycogen is catabolised by human α-amylase to smaller glucose polymers, which are then metabolised to lactic acid by a number of Lactobacillus species (MacIntyre et al., 2015). Both the vaginal epithelium and lactobacilli (which produce antimicrobial peptides and lactic acid and maintain the vaginal pH at <4.5) form the first line of defence against infection by pathogens. The vaginal microbial composition varies with the gestational age and at late gestation parallels that of non-pregnant women. An aberrant (dysbiotic) vaginal microbiome is associated with adverse pregnancy outcome particularly an increased risk of preterm delivery (MacIntyre et al., 2015).

The vaginal microbial composition shifts dramatically shortly after delivery with reduced Lactobacillus dominance and increased α-diversity regardless of the community structure during gestation and independent of ethnicity. It can become more similar to the gut microbiome, regardless of gestational age at and mode of delivery, and is maintained for about 12 months. This can be ascribed to the decline in oestrogen levels after pregnancy as the oestrogen-stimulated maturation, proliferation and deposition of glycogen on vaginal epithelium as well as the resultant Lactobacillus dominance is diminished. This leads to a reduction in vaginal community stability and resilience characteristic of dysbiosis and subsequently increased susceptibility to post-delivery complications including postpartum endometritis. A persistently altered vaginal microbiome can impact the outcome of subsequent pregnancy especially if conception happens too soon after delivery. An inter-pregnancy interval less than 12 months is associated with preterm delivery (MacIntyre et al., 2015).

Development of the fetal microbiome

In humans, bacterial alterations in the placenta and meconium of neonates were observed after administration of probiotics to their mothers during gestation compared with placebo controls. The placenta was previously described as a sterile environment, required for protecting the fetus against infection. Any microbial colonisation of this organ was most likely due to ascending genital infection. However, it has been proven that the placenta in the absence of any histologic evidence of chorioamnionitis possesses its unique microbiome of aerobic and anaerobic bacteria. Due to its similarity to the gut microbiota, it has been suggested that gut bacteria may gain access to the placenta, and this is supported by the increased risk of pregnancy complications in women with periodontal disease (Nuriel-Ohayon et al., 2016).

These imply that from the early stages of gestation there is in vivo transmission of the maternal microbiome to the fetus (Nuriel-Ohayon et al., 2016). Therefore, the fetal microbiome may be established during the gestational period and this amongst other factors may eventually determine the mode of delivery and risk of disease subsequently.

Another possible fetal-microbial exposure could be through swallowing of amniotic fluid contaminated with bacteria. This could prime the immune system of the fetus, which has the capacity to recognise and mount an immune response to pathogens through Toll-like receptor-initiated pathways, production of antimicrobial peptides and lipopolysaccharide-binding protein. This has been termed fetal inflammatory response syndrome and is associated with prematurity.

The effect of mode of delivery on the infant microbiome

Studies have identified distinctions in the microbial composition of various body sites of infants in relation to the mode of their birth. The maternal vaginal microbiome could be an important source of early colonisers of the neonatal gut microbiome, with great impact on the infant’s host metabolism and immunity. The guts of infants born by vaginal delivery possess a microbial community similar to the maternal vagina (dominated by Lactobacillus, Prevotella, Sneathia) and maternal gut bacteria. Whereas infants born by C-section possess gut microbiota similar to those of their mothers’ skin and oral cavity dominated by Streptococcus, Staphylococcus, Propionibacterium and Corynebacterium. Some C-section neonates were even colonised by nosocomial and nonmaternal skin microbiome (Francino, 2014; Nuriel-Ohayon et al., 2016).

There is a vertical mother-to-infant transmission of the gut microbiome that is circumvented by C-section. This is indicated by lesser exchange of Bacteroides and Bifidobacterium in C-section-delivered infants.

Anal samples from vaginally delivered babies and neonates swabbed with a maternal vaginal microbiome after C-section were enriched with Lactobacillus and Bacteroides. In contrast, C-section neonates not exposed lacked these microbes (Dominguez-Bello et al., 2016) and instead harboured increased amounts of Clostridium.

These differences in bacterial colonisation and diversity usually disappear after 6 months of birth but can last up to 7 years in some occasions (Francino, 2014). C-section may also delay the onset of lactation, which is another route of stimulating physiological intestinal microbial colonisation in infants (Nuriel-Ohayon et al., 2016). Together with the absence of vertical transmission of the maternal vaginal microbiome, this can lead to poor immune development with long-lasting sequelae.

Higher proportions of antibiotic resistance genes were also found in the gut microbiome of C-section-delivered infants compared with their vaginally delivered counterparts. Along with abundance of Staphylococcus from maternal skin, this was associated with high rate of methicillin-resistant Staphylococcus aureus skin infections in infants born by C-section (Nuriel-Ohayon et al., 2016).

Furthermore, the oral microbiome of C-section-delivered infants resembles that of maternal skin shortly after birth, while vaginally delivered infants harbour an oral microbiome similar to the maternal vaginal microbiome. However, similar to vaginally delivered babies, the oral microbiome of babies delivered by C-section but exposed to a maternal vaginal microbiome was supplemented with vaginal bacteria diminished in unexposed C-section babies (Dominguez-Bello et al., 2016). The difference between the oral microbiome of C-section and vaginally delivered infants can last up to 3 months with more bacterial species detected in vaginally delivered babies. Also after exposure to Streptococcus mutans, infants delivered vaginally were more resistant to the infection than C-section-delivered infants who were infected nearly a year earlier and harboured a genotype of the bacteria homogenous to that of their mothers (Nuriel-Ohayon et al., 2016). Plausible mechanisms by which C-section alters the newborn’s microbiome and immunity are shown in Fig.1.

The effect of mode of delivery on infant immunity

Despite the widespread perception that C-section averts infant and maternal injury, the infant’s poorly developed or aberrant microbiome and dysfunctional immune system may increase its susceptibility to certain diseases (such as neonatal respiratory morbidity, respiratory syncytial virus infection, bronchiolitis, allergies, asthma, laryngitis, gastroenteritis, inflammatory bowel disease, celiac disease, leukaemia, neuroblastoma, atopic dermatitis, juvenile idiopathic arthritis, obesity and type 1 diabetes) (Cho & Norman, 2013; Kristensen & Henriksen, 2016). The risk of these diseases is reduced in infants ‘seeded’ (swabbing a mother’s vagina and transferring it to her baby’s mouth, eyes and skin after C-section) with their mother’s vaginal microbiome after C-section. The mucosal surfaces have a greater predisposition to the impact of this immune dysfunction, plausibly due to immense immunity-related host–microbial interactions at these sites. For instance, pre- and perinatal intestinal bacterial colonisation stimulates mucosal-associated lymphoid tissue to produce antibodies against pathogens while sparing commensal species, thereby developing immunological tolerance.

Further evidence of a dysfunctional immune function associated with C-section with consequent long-term complications is the observation of reduced concentrations of proinflammatory cytokines (IL-1β, IL-6, IL-8, IFN-γ, TNF-α, etc.) and increased antibody (IgA and IgG) secreting cells compared
with vaginal delivery. Also, C-section-delivered infants had reduced cord blood total leukocyte and neutrophil, monocyte and natural killer cell counts. The cord blood leukocytes of such babies also showed reduced in vitro transmigration capacity and cell surface adhesion molecule expression levels compared with those of vaginally delivered infants. These hypo-inflammatory state and altered immune responses may predispose C-section children to autoimmune diseases, infection and malignancies. Production of cytokines necessary for induction of labour and neonatal immunity is enhanced by vaginal delivery and impaired in C-section. Again, this can be attributed to the Caesarean-associated altered infant microbial colonisation/composition (Cho & Norman, 2013).

In summary, inadequate microbial exposure during delivery adapts the infant’s microbiome and immunity increasing its susceptibility to immune diseases. Neonatal gut microbial colonisation primes the immune system. Children born by C-section may experience delayed postnatal immunological development and priming with potentially deleterious consequences later in life.

Seeding may stimulate microbiome colonisation and immune development similar to vaginally (naturally) born babies and can be a potential ‘health boost’ in the coming years.

References

Cho CE & Norman M (2013). Cesarean section and development of the immune system in the offspring. Am J Obstet Gynecol 208, 249- 254.

Dominguez-Bello MG et al. (2016). Partial restoration of the microbiota of cesarean-born infants via vaginal microbial transfer. Nat Med 22, 250-253.

Francino MP (2014). Early development of the gut microbiota and immune health. Pathogens 3, 769-790.

Kristensen K & Henriksen L (2016). Cesarean section and disease associated with immune function. J Allergy Clin Immunol 137, 587-590.

MacIntyre DA et al. (2015). The vaginal microbiome during pregnancy and the postpartum period in a European population. Sci Rep 5, 8988.

Nuriel-Ohayon M et al. (2016). Microbial changes during pregnancy, birth, and infancy. Front Microbiol 7, 1031.