Researchers have observed that, during a natural vaginal delivery (VD), specific bacteria from the mother's gut are passed on to the baby and stimulate the baby's immune responses. This transmission is impacted in children born by caesarean section. "This may explain why, epidemiologically speaking, caesarean-born children suffer more frequently from chronic, immune system-linked diseases compared to babies born vaginally," says the senior author whose team published the results in the journal Nature Communications.
Humans are born germ-free. Yet, birth is normally the time when vitally important bacteria start to colonise the body including the gut, skin and lungs. Researchers have long suspected that this early colonisation sets the course for one's later health.
It could be, however, that a caesarean section prevents certain bacteria, ordinarily interacting with the baby's immune system, from being passed on from the mother to the new-born. The researchers have now found the first evidence of this in a study of new-borns - half of whom were delivered by caesarean section. The author reports: "We find specific bacterial substances that stimulate the immune system in vaginally born babies. In contrast, the immune stimulation in caesarean children is much lower either because the bacterial triggers are present at much lower levels or other bacterial substances hamper these initial immune reactions to happen."
Several functional pathways are over-represented in VD neonates, including lipopolysaccharide (LPS) bio-synthesis. We link these enriched functions to individual-specific strains, which are transmitted from mothers to neonates in case of VD. The stimulation of primary human immune cells with LPS isolated from early stool samples of VD neonates results in higher levels of tumor necrosis factor (TNF-α) and interleukin 18 (IL-18). Accordingly, the observed levels of TNF-α and IL-18 in neonatal blood plasma are higher after VD.
This bacterial coloniser-immune system link - together with other factors - could explain why caesarean section babies are statistically more prone to develop allergies, chronic inflammatory diseases and metabolic diseases. "It could be that the immune system of these children is set on a different path early on," suggests the author. "We now want to further investigate this link mechanistically and find ways by which we might replace the lacking maternal bacterial strains in caesarean-born babies, e.g. by administering probiotics."
"Of course, it is already clear that we should not intervene too strongly in the birth process. Babies should only be delivered by caesarean section when it is medically necessary", the senior author stresses. "We need to be aware that, in doing so, we are apparently intervening massively in the natural interactions between humans and bacteria."