Bug Me: Our Bodies Need Microbes and Worms
By MATT RIDLEY June 29, 2012, 7:58 p.m. ET
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.
Researchers are finding that the creatures in our
guts are vital to the body's capacity to fight
infection.
This fact also provides a glimpse of the symbiotic
nature of our relationship with these bacteria. A
recent study by Howard Ochman at Yale
University and colleagues found that each of five
Chris Silas Neal
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 broadspectrum 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 helped 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.
A version of this article appeared June 30, 2012, on page C4 in the U.S. edition of The Wall Street Journal, with the headline:
Bug Me: Our Bodies Need Microbes and Worms.
Bug Me: Our Bodies Need Microbes and Worms
Questions
1. How many bacterial cells are in your body compared to “body” cells?
What does it mean that you are a “microbial zoo”?
2. What is a symbiotic relationship? How do bacteria benefit from living
inside of us? How do we benefit from having bacteria live inside us?
3. Draw a diagram that illustrates the experiment performed Hachung
Chung in Prof. Dennis Kasper's laboratory at Harvard Medical School.
What were the results of the experiment?
4. What is the problem with excessive hygiene?
5. What kind of treatments trials are being conducted for individuals with
multiple sclerosis?
6. How might hookworm or pinworm infections prevent autoimmune
disorders or allergies?
7. How is our bodies need for microbes or worms related to our
evolutionary history?