microbes on our skin, around our ears, in our hair, in our
mouths, on our genitals, and in our guts. Even parts of
ourselves once thought to be sterile, like the lungs and the
mammary glands of the breasts, are now known to be posh
country clubs for microbes.^1 Each location has its own
population that lives in symbiosis with its unique
environment. The gut microbe population, for example,
which consists primarily of bacteria that live without
oxygen, would die instantly if placed next to the microbes
on your face, which delight in their exposure to fresh air.
The general term that we use for the cumulative genetic
content of all of these simple, single-celled organisms is the
microbiome. Your house has its own microbiome, which
represents the genetic material carried by the microbes that
live in it. Your home microbiome may differ dramatically
from your neighbor’s depending on whether you have a dog
or a small child, and whether you live in a city or in a
suburb. Even entire cities have their own microbial
signatures.^2 The microbiome of Los Angeles, for example,
is different from that of New York. Do West Coast bacteria
prefer on-camera work to the stage, where the East Coast
microbes really shine? These questions remain to be
answered by science.
Though you have microbes all over the exterior of your
body, the vast majority of microbial cells that you carry with
you reside in your gut. This is your gut microbiome.
Though we once believed they outnumbered our own
human cells ten to one, we’ve since arrived at a more
accurate estimate—about thirty trillion—placing their
population roughly at same number of cells that contain