While the human microbiota comprises only 1–3% of an individual’s total body mass, this small percentage represents over 100 trillion microbial cells, outnumbering human cells 10 to 1 and adding over 8 million genes to our set of 22,000. This complexity establishes a network of interactions between the host genome and microbiome spanning gut development, digestion, immune cell development, dental health, and resistance to pathogens. Recent studies have also provided a greater understanding of how the composition of an individual’s microbiota changes throughout development, especially during the first year of life. While the general dogma is that the placental barrier keeps infants sterile throughout pregnancy, increasing evidence suggests that an infant’s initial inoculum can be provided by its mother before birth and is supplemented by maternal microbes through birth and breastfeeding.
While maternal transmission of microbes in humans has attracted considerable attention in the last few years, nearly a century’s worth of research is available for vertical transmission of symbionts in invertebrates. Since these indispensable symbionts cannot live outside of host cells, they cannot be acquired from the environment and are faithfully transferred from mother to offspring – from sponges all the way up to humans.
The sterile womb paradigm is an enduring premise in biology that human infants are born sterile. Recent studies suggest that infants incorporate an initial microbiome before birth and receive copious supplementation of maternal microbes through birth and breastfeeding. Moreover, evidence for microbial maternal transmission is increasingly widespread across animals. This knowledge requires a paradigm shift – one in which maternal transmission of microbes advances from a taxonomically specialized phenomenon to a universal one in animals. It also requires fresh views on the assembly of the microbiome, its role in animal evolution, and applications to human health and disease.