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Roughly one bacterium per human cell

About Half of You Is Not You

You are carrying about as many bacterial cells as you have cells of your own. Most of them are helping.

Your own cellsBacterialcells livingon and in youRoughly one toone, by count
Not a tidy exact ratio — but far closer to even than most people expect.

  1. Step 1 of 5

    Almost all of them live in one place.

    The large intestine:almost all of themSkin: a thin, dry,much smallerpopulationMouth and nose: theirown distinctcommunitiesStomach: acid keepsnumbers low

    The overwhelming majority are in the large intestine. Your skin, mouth and nose have their own populations, but they are small by comparison. The gut is where the crowd is, and it is dense — packed at levels you would struggle to reach in a laboratory.

  2. Step 2 of 5

    They are cells, but they are tiny cells.

    By cell count Roughly halfBy weight About 0.2 kilograms —a small fraction ofone per cent

    A bacterium is far smaller than most of your own cells, so equal numbers does not mean equal mass. The whole population weighs somewhere around 200 grams — about the weight of an orange. Half of you by count is a fraction of a per cent of you by weight.

    Both numbers are correct. They just answer different questions, and the striking one is the one people quote.

  3. Step 3 of 5

    They earn their keep.

    Digesting fibreWhat your enzymescannotMaking vitaminsVitamin K andsome BCrowding out invadersOccupied space isdefended spaceTraining the immune systemTeaching it whatto ignoreProducing signalling chemicalsSome of whichreach the brain

    Gut bacteria break down fibre your own enzymes cannot touch, releasing energy you would otherwise lose. They make vitamin K and several B vitamins. And by occupying the space, they make it much harder for something harmful to move in.

  4. Step 4 of 5

    The famous ten-to-one figure was an estimate that stuck.

    1 1972: aroughestimateTen to one,labelledapproximate2 It getsquoted, thenquoted againThe caveatsfall off3 2016:someonerecountsCell bycell, tissueby tissue4=Close to oneto oneWhich isstillastonishing

    For decades the number quoted was ten bacteria for every human cell. It traced back to a rough calculation from the 1970s that was never presented as precise. A careful 2016 recount put it at close to one to one, and that is now the accepted figure.

    A good example of how a number can become famous by repetition rather than by evidence. The corrected figure is less extreme and much better supported.

  5. Step 5 of 5

    Your own count is not fixed either.

    The ratio, day to dayFibre, variety and time build it upAntibiotics and illness knock it down
    It is a population, and populations move.

    One large bowel movement can shift the ratio measurably. Diet changes which species dominate within days. Antibiotics can flatten the population and it takes weeks to months to recover. There is no single number that is permanently true of you.

    Which is one reason 'about half' is the right way to say it, rather than a precise figure.

The short version

You carry roughly as many bacterial cells as human ones — mostly gut bacteria weighing around 200 grams in total — and the old ten-to-one figure was a rough estimate corrected in 2016.

Try it yourself

Eat noticeably more fibre for a week — beans, oats, vegetables. Digestion changes within days. That is a population of bacteria responding to a change of menu.

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