The Microbial System

You have probably heard that bacterial cells outnumber human cells ten to one. It is one of the most quoted figures in health writing, and it is not correct. It traces back to a rough estimate made in 1972 that was never intended as a precise measurement, and it was then repeated for four decades without anyone recalculating it.

In 2016 a group of researchers finally did the arithmetic properly, organ by organ. Their estimate: about 38 trillion bacterial cells against about 30 trillion human cells in a 70 kg adult. That is a ratio near 1.3 to 1 -- close to parity, not an order of magnitude. They also found that most of your own cell count is red blood cells, which is its own surprise. Worth adding: by mass the bacteria come to only a couple of hundred grams, because a bacterium is tiny compared with a human cell.

We use the corrected figure here deliberately. Roughly as many bacterial cells as human cells is astonishing on its own and it has the advantage of being true. If a page tells you ten to one, it has not checked its sources since 2016 -- which tells you something useful about the rest of the page.

This is one of the most exciting areas in biology and also one of the most oversold, so it is worth separating carefully. Solid: the anatomy and the signalling. There is a dense nerve connection, microbes produce and modulate neuroactive compounds, and the immune and endocrine systems carry information both ways. Solid: in animals, altering the microbiome changes behaviour, sometimes dramatically.

The honest position: the gut-brain axis is a real communication system that is being actively mapped, and it is not yet a lever anyone knows how to pull reliably. Emotional coping is shaped by it. It is not explained by it.

Microbiome science has produced very few universal rules, because a healthy community looks different in different people. The nearest thing to a consistent finding is that a more diverse community tends to be a more resilient one, and that the things which reduce diversity -- a narrow diet, low fibre, repeated broad-spectrum antibiotics, an environment scrubbed of contact with soil and other living things -- tend to be the things associated with worse outcomes.

That last one has been tested rather than assumed. When researchers added biodiverse forest soil and vegetation to urban daycare yards, the children's skin and gut communities became more diverse within a month and their regulatory immune markers shifted, with the effect persisting over a two-year follow-up. Children playing in dirt is a real intervention with a measurable immune signature.

Different microbial groups in soil respond differently to how land is used, which is why soil health is not one number. It also closes a loop: the diversity of the soil around you is part of the diversity of the community inside you. The two are not separate systems that happen to be similar. They exchange members.

The Fungal System is the other half of this story, since bacteria and fungi are competitors and partners in the same two places -- your gut and the soil. The Ecosystem page follows the loop outward to the biodiversity that stocks both.

The vagus nerve as a gut-brain pathway, microbial synthesis of vitamin K and B vitamins, and soil respiration as a major carbon flux are referenced above as established science rather than through single studies. Where human evidence for mood effects is associational or from small trials, the text says so.