[Norman Swan]
Have you ever wondered what it means to be
human? Well, it turns out that only a tiny
percentage of what you and I are made of is
in fact human. And we need these non-human
bits to survive. But we're doing dreadful
things to this hidden majority and it's damaging
our health in ways that we are only just discovering.
[Professor David Relman]
It is said that we are ten parts microbe and
one part human. And that's counting microbial
cells as the unit. If we count unique genes
then the difference is a factor of a hundred.
We are only one percent human in terms of unique genes.
[Norman Swan]
So you and I are mostly bacteria of some kind.
We need each other to stay healthy, and there's
now a name for this part of ourselves.
[Professor David Relman]
The human microbiome is a set of microbial
communities that have co-evolved with us,
and we with them, and have become part of
the human landscape. It is an extension of
self. They provide to us untold functions
that we don't have the means of executing.
We're born deficient in our ability to digest
certain kinds of foodstuffs, for example the
complex polysaccharides of plants. We can't
manage those very well. Our microbes provide
us the means of taking advantage They also
help us manage lots of other encounters with
the environment.
[Norman Swan]
One of the things our microbiome does is give
us an internal shield of friendly microbes
which protect us against invasion by some
of the nasties out there. The good guys try
to crowd out the bad ones
[Professor David Relman]
It's competitive exclusion. Another term for
it is colonisation resistance. So -- you know
-- they man the beachhead and make it hard
for others to arrive and find space.
[Norman Swan]
So where are these microbes? Well, while most
are in our bowel, they can in fact be anywhere
where we're exposed to the outside world.
[Professor David Relman]
So we have them in our mouth. In the mouth
it's a measly ten million per gram, but it's
only measly because in the colon we're talking
about a hundred thousand times as many per
gram.
[Norman Swan]
So the microbes in our bowel, for example,
help us to recirculate valuable nutrients,
and in fact assist us to keep our cholesterol
down. So we need them, and they need us. But
we do some terrible things to our microbiome.
Like assault it with antibiotics.
[Professor David Relman]
Yeah, unfortunately for us -- and for them
-- everything I'm going to tell you about
the untoward effects of antibiotics has to
be balanced against the obvious important
benefits. But I think for too long we have
felt as though you can't do much wrong with
an antibiotic. So if in doubt, pull it out.
And that clearly is not a good position to be taking.
 
We've been asking volunteers to take an antibiotic
even though they didn't really need it. We
do it for short periods of time. We watch
them before, we watch them after, and we're
now doing second exposures a half year later,
and the bottom line is, maybe not surprisingly,
far more members of the microbial communities
of the human body experience a severe impact.
They're decimated. Many more than we thought.
[Norman Swan]
Now, in healthy people, who might only have
an occasional course of antibiotics, the microbiome
seems to recover quite well. That's not so
true in people who are sick and weakened.
As a side effect of antibiotics, they can
become infected with a germ called Clostridium difficile,
which causes hundreds of thousands
of deaths worldwide.
[Professor David Relman]
So, you're right. Colstridium difficile is
the cause of large numbers of deaths, but
even larger numbers of illnesses, where there
is a recovery, but after some cost.
[Norman Swan]
One solution that some people suggest for
this devastation to our microbiome is probiotics;
substances which put back the friendly bacteria.
[Professor David Relman]
The evidence on probiotics is still early
to interpret. But I would say it's suggestive;
that there is some potential benefit there.
[Norman Swan]
As I said before, these friendly bugs -- our
microbiome -- does a lot more than protect
us against nasty infections. They keep our
cholesterol down and detoxify poisons. So
the question is, if we damage our microbiome
with antibiotic overuse, are we causing heart
disease and cancer?
[Professor David Relman]
The short answer is, we don't know. Because
no-one has looked for this particular link
in the story. We simply don't know.
[Norman Swan]
So while in the past we've seen bacteria as
the enemy, and got excited about how we can
kill off disease-causing germs with antibiotics,
it's time for a new way of thinking. Remember,
we're talking about ninety-nine percent of
what we are.
