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My latest Mind and Matter column in the Wall
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
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
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
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
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.