As I look at my son, it’s hard to believe that only forty-three percent of his body contains human cells. How he feels, digests food, and fights off illness is not just an individual essay, but an intricate tale woven by ‘him’ and his microbial comrades.

Each of us consists of trillions of microorganisms numbering around one-to-one with our human cells. Not only that, but our 23,000 human genes pale in comparison to the 2,000,000 microbial genes inhabiting our body. Our notion of what it means to be human has shifted.

Since the dawn of animals, microbes have been setting up shop, colonising our bodies not randomly, but in specific niches. Some of these microbes are conserved (meaning we all share them), while others are host-specific (the microbes an individual has that not everyone shares).

Over time we have co-evolved with our microbes, an ancient symbiotic partnership where bacteria and animals have learnt to converse; they provide signals to us, and we in turn signal back. It’s a vigorous and resilient setup allowing bacteria to interact with us by modulating our metabolism and immune system. But how exactly did these little creatures come to be our personal roomies?

Mammals develop in the womb, a mostly sterile environment. When we’re born however, we are introduced to microbes the moment our mother’s water breaks and bacteria in her vagina ascends.

Descending through the birth canal, microbes coat our skin and fill our mouths as we swallow – it’s our first inoculum. During first cuddles more bacteria from mum’s skin is acquired. Mouths, filled with new bacteria, inoculate her breast as we feed for the first time. The bacteria now in the milk are consumed on subsequent feeds. This is one of the ways bacteria establish in our infant gut. It’s this intergenerational transfer of ancestral microbes which can profoundly influence our health not only as infants but as adults too.

Out of the hospital and into the real world, it’s during the first three years of life when our microbiota is the most dynamic. Modes of transmission of bacteria now include sloppy kisses from family, premasticated food, frolicking in the garden, and playtime with pets.

Interestingly, this is also the time when children are rapidly developing in terms of immunity, metabolism and cognition. Bacteria acquired at birth and during the first three years of life are crucial to this development. Conversely, the loss of ancestral bacteria at this age has been shown to have a deleterious effect on our future health.

Life isn’t what it used to be. Today new mums receive many courses of antibiotics, including during labour. They give birth for the most part in sterile environments. And it’s not just mothers whose conditions have changed. More and more babies are being born by cesarean section, bathed extensively after birth, fed formula and administered antibiotics (all potentially vital interventions).

These ecological changes have profoundly altered the transmission and maintenance of our microbiota. As a result, human physiology has been affected, with a knock-on effect on disease risk.

So what exactly happens when modern methods perturb our ancient relationships with microbes? What happens when these microbes are lost? Well that’s for another post.

Brigid xx