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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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The zoo inside you

Microbes and worms that are necessary for the immune system to work

My latest Mind and Matter column in the Wall Street Journal:

One of the delights of science is its capacity for showing us that the world is not as it seems. A good example is the startling statistic that there are at least 10 times as many bacterial cells (belonging to up to 1,000 species) in your gut as there are human cells in your entire body: that "you" are actually an entire microbial zoo as well as a person. You are 90% microbes by cell count, though not by volume-a handy reminder of just how small bacteria are.

This fact also provides a glimpse of the symbiotic nature of our relationship with these bugs. A recent study by Howard Ochman at Yale University and colleagues found that each of five great apes has a distinct set of microbes in its gut, wherever it lives. So chimpanzees can be distinguished from human beings by their gut bacteria, which have been co-evolving with their hosts for millions of years.

A new experiment by Hachung Chung in Prof. Dennis Kasper's laboratory at Harvard Medical School shows that this microbial specificity has consequences for health. Researchers bred mice with no gut "flora" at all, then filled their guts with either normal mouse bacteria or normal human bacteria. In both cases the microbes flourished, producing an equal quantity of both individual cells and species.

But the immune system of the mice with human gut flora was markedly less active. In some way, the mouse immune system did not recognize the human gut flora and did not properly develop. When the researchers filled a mouse gut with rat bacteria, the same thing happened: that is, not even the rat bacteria are similar enough to stimulate the mouse immune system. And when the mice with human flora were dosed with salmonella, they contracted a worse infection, their immune system proving less able to respond.

It has been clear for a long time that the microbes in your gut are not just passengers but colleagues that help with the digestion of food: releasing vitamins, breaking down toxins and metabolizing nutrients into more useful forms. What's becoming clear from such experiments is that they are also vital to the immune system's capacity to fight infection. It's as if they train the body's defense forces.

For instance, breast-fed babies, whose gut microbes are dominated by creatures called bifidobacteria, are less likely than formula-fed babies to suffer not only from diarrhea but also from allergies later in life.

The evidence also suggests that the addition of "probiotic" supplements to formula may help the normal development of the immune system.

A recent study by Jeffrey Weiser and colleagues at the University of Pennsylvania found that immune-system cells called neutrophils were less responsive to pathogens in mice that had grown up germ-free or on antibiotics. This may be why people taking long courses of broad-spectrum antibiotics can often get secondary infections.

Our sometimes excessive hygiene may not only affect the body's ability to fight infection when it does come, but may even cause it to turn on itself. There has long been suspicion among medical researchers that the rise of autoimmune disorders such as asthma could be abetted by the sterility of our homes compared with when we lived "wild."

The results of early-phase clinical trials seem to suggest, for example, that the symptoms of multiple sclerosis can be ameliorated by deliberate infection with intestinal worms, such as hookworms or pig whipworms. People with hookworm rarely suffer from allergies or autoimmune problems.

Prof. David Pritchard, of Nottingham University in the U.K., thinks this is because hookworms have an innate ability to moderate the immune system to allow them to survive in the body. This moderating influence may also diminish the self-harming immune response that leads to the symptoms of MS. Worms are parasites, but ones whose presence over the evolutionary eons we may have come to rely on for normal immune function.