Colonization of a newborn infant’s microbiome depends on a number of factors, but barring odd circumstances, oftentimes the initial seeding of the microbiome comes directly from the immediate family, particularly the mother herself.
Firstly, initial colonization is impacted by the method of birth. Babies born vaginally have an initial microbiome aligned with the maternal vaginal microbiome, while babies born via C section have initial microbiomes closer to the maternal skin microbiome.
Past this first moment, much of the infants microbiome is determined by what they put in their mouth. Traditionally speaking, this could be determined by formula feeding vs breastfeeding, with the latter seeding the microbiome with a wider variety of healthy maternal microbes.
In a less elegant manner, newborns, infants, toddlers, etc, like to put things into their mouths. This serves to introduce environmental microbes to their systems which may or may not have an impact on their microbiomes or their immune systems. Therefor, any impact that a decision has on the environmental microbial community will have an impact on the infant. Anything from owning a pet to opening a window can serve to expose the infant to a broader microbial community by way of them putting things into their mouth.
