What is the microbiome?

Meet the Ecosystem Living Inside Your Gut

This is well-established science, built on decades of genetic sequencing across large population studies.

The microbiome is the full community of microbes, mostly bacteria, that live in and on the human body, together with the genes they carry.

It isn't a single organ or a single species. It's an entire ecosystem, and most of it lives in one place: the gut.

A second genome, carried by trillions of tenants

The human genome contains around 20,000 to 23,000 genes. The bacteria living in a single person's gut collectively carry an estimated 3.3 million unique genes, roughly 150 times more than the person hosting them.

This matters because genes are instructions for making proteins and enzymes. Many of the chemical reactions gut bacteria can perform, breaking down plant fibre the human body can't digest alone, producing certain vitamins, are things human genes simply don't code for. The bacterial community fills in genetic gaps the body doesn't have.

Sequencing studies estimate around 1,000 different bacterial species live in a typical human gut, though most people carry a smaller core set of 50 to 100 species that make up the bulk of the community, with the rest varying more from person to person.

Source: Human gut microbiome: the second genome of human body

Where all this actually lives, and how much of it there is

Microbes live all over the body, on the skin, in the mouth, in the nose, but the vast majority live in the large intestine, where low oxygen levels favour the kinds of bacteria that dominate the gut.

A widely repeated claim held that bacteria outnumber human cells by ten to one. A careful 2016 recalculation found the true ratio is closer to 1.3 to 1, roughly 38 trillion bacteria against about 30 trillion human cells, still bacteria-dominant, but by a much smaller margin than the old figure suggested.

These bacteria have real physical mass too, estimated at about 200 grams in total. It's a small enough number to be easy to overlook, and a large enough, active enough community to genuinely matter.

Source: Revised Estimates for the Number of Human and Bacteria Cells in the Body

This community starts building itself from the moment of birth

Nobody is born with an established microbiome. It develops rapidly in the first months and years of life, shaped by how someone is born, how they're fed, and what they're exposed to.

Babies born vaginally pick up bacteria from the birth canal, while babies delivered by caesarean section start with a different initial bacterial mix, typically showing lower diversity in the first weeks. Breastfeeding then plays a major role in growing beneficial bacteria such as Bifidobacterium, partly restoring some of that early gap over time.

Diversity climbs steadily through infancy and childhood, and the community doesn't settle into a more stable, adult-like pattern until somewhere around age 3 to 5. Everything that happens in those early years, birth, feeding, illness, antibiotics, leaves a lasting mark on the community that forms.

Source: Early-life gut and oral microbiota development: a multi-niche study including mother-partner-infant triads

What this means for you

Understanding what the microbiome is doesn't require memorising these numbers, just the basic shape of the picture: a vast, genetically rich bacterial community living mostly in the gut, doing metabolic work human genes can't do alone. A varied, plant-rich diet gives that community the range of fuel it needs to keep performing that work well. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.