The Tenants in My Body — No Germ Is Good or Bad by Nature

As many as my own cells

If you tapped the body in the piece 1 tool, you already know the gut microbiota and the neighbours elsewhere: the gut is the capital, the mouth the second city, the skin a desert and the armpit an oasis. All together they match the number of cells in your body. By weight, about one brain.

And the mix differs from person to person, like a fingerprint. Families share a little; households with a dog share a little with the dog. Whom you met between birth and age three largely sets the mix for life.

What they do

I called them tenants, but they pay rent. Four jobs.

1. They eat what we cannot digest. Tough vegetable fibre passes the stomach and small intestine undigested and reaches the colon. There the neighbours eat it, and what they leave behind feeds the colon wall itself.
2. They make vitamins. The vitamin that clots blood and several B vitamins are made not by us but by them.
3. They train the immune system. Meeting neighbours in childhood teaches immunity who is an enemy and who is not. Without that practice, immunity gets angry at dust and peanuts.
4. They occupy the rooms. This matters most. When every room is taken, a bad guest has nowhere to stay. When rooms are empty, anyone moves in.

Does yogurt make you live longer?

The first to say these neighbours affect the whole body was Metchnikoff at the Pasteur Institute, a century ago. He heard that Bulgarian villages were full of old people and ate yogurt every day. He never proved it. But the question stayed, and the probiotic drink industry began here.

'Good germs' and 'bad germs'

Adverts sort germs into two teams. But germs have no team. They have a place and a number.

E. coli is a vitamin-making neighbour in the gut and an infection in the bladder. Staphylococcus is a tenant in the nose and a troublemaker in a wound. Even lactic bacteria cause disease on a heart valve. Same germ. Different address.

Helicobacter in the stomach is stranger still. It really does cause ulcers. Yet studies find more asthma and allergy in children who never carried it. After thousands of years together, our immunity may have learned to practise on it. Culprit and neighbour at once.

The table decides

So what tips the balance? Which crowd thrives and which goes quiet is decided by whom you feed. And that is decided at your table.

Fibre is not digested in the stomach or small intestine, so it reaches the colon: vegetables, brown rice, beans, fruit, onion, garlic. What we cannot eat is the neighbours' food. Sugar and white bread are absorbed higher up. Nothing reaches the colon. The neighbours starve, and only the cavity germs in the mouth get a feast.

Kimchi, doenjang, yogurt and cheese bring living germs with them. But most are passing guests; they stay a few days and leave. The real reason kimchi is good is not the lactic bacteria but the cabbage fibre that travels with them — a packed lunch for the neighbours.

Makgeolli deserves a word. A bottle of fresh makgeolli holds as many lactic bacteria as yogurt, plus living yeast, so you hear it called a 'probiotic drink'. Half true. But as piece 2 said, fermented does not mean healthy. Makgeolli is alcohol, and alcohol thins the gut neighbours. Nobody drinks it for the bacteria. You drink it because you like it, and that is fine as it is.

What matters most is variety. The more kinds of neighbour, the healthier the gut. Many vegetables in small amounts beat one vegetable in large amounts. People who eat thirty different plants a week have busier guts than those who eat ten. See who cheers for what you put on the plate in panel 3 of the tool.

Tool The Body Microbe Map — Where Your Tenants Live Tap a spot on the body and see who lives there, how many, what they do and what they like. The gut is the capital, the skin a desert, the armpit an oasis. One common myth corrected at every spot, plus a gallery of microbe faces, a plate game and a myth quiz. (Korean only)

What comes after antibiotics

One course of antibiotics wipes out about half the gut neighbours. For a bacterial disease, that is necessary. But recovery takes weeks to months, some never return, and rooms stand empty.

Into the empty rooms moves a troublemaker that had lived quietly in a corner, and now it thrives. At worst, diarrhoea that will not stop. One cure for that is astonishing: transplanting a healthy person's gut neighbours. Literally, a stool transplant. Nine in ten patients that antibiotics could not cure recover this way. Filling the rooms was the medicine.

A little further in

The first neighbours come from mother

A baby receives its first tenants at birth, passing through and inheriting the mother's neighbours. Breast milk contains sugars the baby cannot digest — they are not for the baby but for the neighbours in the baby's gut. A mother feeds the baby and its tenants together.

That is why a childhood that is too clean can be a problem. A child who handles soil, hugs the dog and tumbles with siblings gives its immunity plenty of practice. Piece 4 said hand-washing saves lives, and it does. Disinfecting your whole body all day is a different matter: it evicts the neighbours.

Antibiotics wipe out half the neighbours, I said. But we did not invent antibiotics. They are weapons moulds made to fight bacteria, and we borrowed them. A story that began with a contaminated dish — and the story of bacteria learning faster than we do. Next piece.

The question that remainsIf strangers as numerous as my own cells live inside me — whom is my dinner really for?