Article about microbiome inheritance or “vertical transfer”. Part of a series on the maternal microbiome, including its inheritance and influence on foetal health and immunity. Written for the BluMaiden Babypass app.

The human body is made up of tens of trillions of cells — except half of those aren’t exactly our own. Naturally residing in and our bodies are communities of microbes or microorganisms, which include bacteria, fungi, even viruses. These communities are known as “microbiomes”. To these microbes, our bodies are a giant landscape, with various body parts offering different climates and serving as habitats for them.

How vertical transfer of microbiome from mother to infant occurs

Just like genes, microbiomes can also be intergenerationally (or vertically) passed on from mothers to their babies. An infant’s first potential exposure to microbes can occur as early as in the womb, whereby the mother’s microbes travel to her placenta from her gut, or enter her amniotic fluid from her vagina. During delivery, grazing the walls of the birth canal subjects the infant to the mother’s vaginal and faecal microbes, distributing microbes across the skin, mouth, throat and gut. Post-birth, the infant continues to receive essential microbes from the mother through her breastmilk and skin-to-skin contact. Ingesting food from outside and introducing the infant to the external environment can add new microbes to the microbiome.

However, contact itself does not guarantee that a mother’s microbiome will be successfully transferred and implanted onto her infant. Microbes undergo rounds of selection and competition before they can reach and establish their colonies in the infant’s body. For example, during pregnancy, the mother’s gut changes to optimise the growth of several key and beneficial microbial varieties, therefore streamlining composition of the gut microbiome. Breastmilk also favours certain microbial varieties that are able to utilize specific compounds in the milk to sustain themselves, putting those that are unable to at a severe disadvantage.

Additionally, the increasingly acidic and oxygen-free environment of the infant gut and additions of new microbial varieties mean only a certain variety can stay as the majority for a window of time, before being superseded by a better-adapted one. Thus, vertical transfers can procedurally be quite complex and it is difficult to forecast the outcome of the transfer.

Contrary to common belief, scientific evidence suggests that the mother’s gut microbiome is very likely to be the primary source of microbes for the infant, since the breastmilk microbiome share similar members with the gut microbiome. When researchers compared the strains of a particular microbe (Bifididobacteria) in the mother’s breast milk and in the infant, they found that they matched. Other researchers also discovered that the microbial strains in the infant’s faecal samples mirrored those of their mother’s, strongly suggesting that the main source of microbes in infant’s gut is their mother’s own gut.

Vertical transmission is also most frequent during early infancy with breastfeeding, but its prevalence wanes as the infant transitions to formula milk or solid food and the microbiome matures. Thus, the environment or external sources play a progressively bigger role, as evident from the gut gradually becoming more colonised and ultimately dominated by external bacteria group called Clostridia in an adult microbiome.

What affects vertical transfer

Nevertheless, vertical transfer of important microbes is critical for the infant because the microbes that colonise earlier will dictate future microbes that take up residence after, such as through shaping the properties of their habitat. However, there are other major factors that influence vertical transfer, including the mode of delivery, type of milk-feeding and antibiotic consumption in either the mother or infant.