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- Hello, my name's Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this video,

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I want to show you how
cells can change the tools,

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machinery, and spare
parts that they produce

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so that they can respond to
changes in their environments.

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This is a bag of tricks
that's been perfected

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by the bacteria,

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those organisms that we most love to hate,

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and they form the focus

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of chapter five of my book
"Biology: The Whole Story."

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(birds chirping)

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(frog croaking)

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Humans love to eat food,

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but mostly we prefer not
to think about what happens

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to that food once we've eaten it.

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But inside our guts, food
has to be broken down.

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And that means breaking
down the long molecules

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into their smaller components.

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For example, proteins
need to be broken down

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into their amino acids,

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and that's a job that's
carried out by enzymes.

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Now, humans produce
different kinds of enzymes

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in order to break down
different molecules,

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but one molecule that
they can't really digest

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is called cellulose.

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Humans don't produce the
enzymes to digest cellulose.

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And cellulose is a major part of our diet

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because plants produce loads of it

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to put in their cell walls.

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Now, the good news for humans
is that there are organisms

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that can digest cellulose

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and those are bacteria.

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And so in our large intestines,

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we have huge numbers of bacteria living,

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and they are helping
us to digest cellulose

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and get nutrition from our food.

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In fact, there's so many
bacteria living in our bodies

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that for every human cell in our bodies,

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it's estimated that there's
at least one bacterial cell.

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But this doesn't mean that we're ill.

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This is entirely normal.

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In fact, that community of bacterial cells

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that lives in and on us
is called the microbiome.

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And it's causing a stir in medical circles

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as people realise just how
important it is for human health.

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So a very common bacteria that
lives in the guts of humans

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is called E. Coli.

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And that's been very well studied

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so we understand E. coli very well.

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Now, there are one or
two strains of E. coli

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that can be very damaging for humans

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and cause public health problems,

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but the vast majority of strains

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of E. coli are entirely harmless.

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Now, living in your gut, E.
coli is going to be confronted

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with many different foodstuffs

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and it's going to need different tools

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and machinery to deal with them all.

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But it might not want to
produce all of those tools

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and machinery at the same time.

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It might want to fine-tune what it's doing.

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So how does it do that?

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Well, to understand that,
let's look inside it.

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Inside an E. coli cell,

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we're going to find a genome of course.

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And in bacteria,

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that's just one enormous
long strand of DNA

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that forms a circle.

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And that's made up of thousands
of individual instructions

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and each one's called a gene.

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And those genes will
have to send out messages

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if the tools and machinery
that they code for

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are actually going to be made by the cell.

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So how does that work?

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Well, in front of each gene,

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there's a special sequence
of letters called a promoter

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that's like a start here sign.

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So that marks out where the gene starts,

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and at the end of the
gene, there's a stop sign.

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Now, if the start here sign
is exposed and visible,

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then that gene will be
able to send out messages.

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But if that sign is covered up,

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then the gene won't be able to.

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And this ability to
switch genes on and off

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is something called gene expression.

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And it's very, very
important as it allows cells

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to fine tune the machinery
and the spare parts

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and the tools that they
produce at any given time.

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Okay, let's take a look at how E. coli

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is going to make a change

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in response to a change
in its environment.

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E. coli's favourite food is glucose.

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That's a small sugar molecule

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and that's what it would like best,

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but sometimes that's not available.

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But there might be alternatives.

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For example, lactose,

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that's a sugar that's found
in milk and dairy products.

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And lots of humans
can't actually digest it

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and they're called lactose intolerant.

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And most of the time, E.
coli can't digest it either,

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but it does contain the
genes to digest lactose.

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It just doesn't turn them on.

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So normally the way that works

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is that there's a large repressor protein

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that's clinging tightly
to the start here sign

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at the start of the
genes to process lactose.

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And that means that those genes are off.

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But now look what happens
when lactose enters the cell.

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The lactose molecule combined
to that repressor protein

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and that changes the shape
of the repressor protein

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so that it falls off the start here sign.

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And now the gene is on

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and so the cell is going
to start making the tools

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and machinery that it
needs to process lactose.

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So this is a really clever response.

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It's using the presence of that
molecule in the environment

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to turn on the genes that it needs.

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And bacteria can do that
for all kinds of signals.

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So we've seen that E. coli
carries genes to process lactose,

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a sugar that it doesn't
really encounter very often.

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So we might wonder

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why cells don't just
carry thousands of genes

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to allow them to cope
with unlikely situations.

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Well, the problem is that
every time a cell adds a gene

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to a genome,

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then that molecule of DNA
becomes longer and longer.

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And when the cell comes to divide,

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the first thing it has to do
is to copy all of that DNA.

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And the more of it there is,

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the longer that's going to take.

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So that's going to sort
of slow an organism down.

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So it can't just have an
infinitely large genome.

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You know, what it would really like

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is to be able to have some
genes just when it wants them

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and then if those genes
are no longer useful,

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to be able to dump them again.

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And amazingly, bacteria
do have genes like that.

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So bacteria have extra bits of DNA,

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small circles of DNA, and
they're called plasmids.

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And they can pick them up

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and they can put them down quite easily.

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They can even pass them among themselves.

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And those are just genes
for special occasions.

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And one of the key genes
that are carried on plasmids

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are those for antibiotic resistance.

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Now, antibiotics are
drugs developed by humans

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to kill bacteria once
they're inside human bodies

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because they just damage bacterial
cells but not human cells.

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So it's not good for us
that bacteria are carrying

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all of these antibiotic-resistance genes.

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Why are they?

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We've said that bacteria
will only keep these plasmids

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if they're actually useful.

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And that implies that bacteria

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are often encountering
antibiotics in their environments.

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And the reason they're doing that,

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well, there's two reasons really.

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One is that antibiotics
are being prescribed

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to humans a lot even when
they don't really need them.

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And the second reason is that antibiotics

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are being given to animals
even when they're not even ill.

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Now, remember, we said
it's quite difficult

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for us to digest a
molecule called cellulose,

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which is found in plant cell walls.

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But imagine if you're an animal like this,

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and that's pretty much all you eat.

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Cows just eat grass, and
grass is mostly cellulose,

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so they have to have a very
special system of stomachs

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in order to allow them to digest it.

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And they have a big
stomach called a rumen,

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which is why they're
sometimes called ruminants,

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and that is absolutely
stuffed full of bacteria

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who are all busy digesting cellulose,

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which then allows the cow

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to get some nutrition from the grass.

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There is a bit of a
problem with that though,

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which is that because it's
quite low quality food

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and it takes a lot of digesting,

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then cows only grow quite slowly.

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And as humans who eat cows,

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we might want cows to grow faster.

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And in cattle feedlots in the US,

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they don't feed cows grass.

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Instead they give them pellets
of much easier to digest food

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and that means that the
cows can grow faster.

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The problem is that the bacteria

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can also eat that easy-to-digest food.

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But now the farmers don't
really want the bacteria there.

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They're not helping the cow.

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They're actually just taking
some of the food away.

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So what they do is feed
the cows antibiotics

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and they give them those on
a routine basis every day

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to suppress all of the
bacteria in their rumens.

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And that means that that's
tonnes of antibiotics going out

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into the environment all the time.

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So this is something that we might think

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is not a terribly good idea.

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So that's something we all
need to decide as a society.

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How do we use antibiotics safely

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so that we still have
antibiotics that work

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if one of us gets a nasty infection?

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So bacteria might be the
organisms that we love to hate,

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but we really shouldn't hate them.

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In fact, they're
incredibly important to us.

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For example, they help
us to digest our food.

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And you know what?

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They have been incredibly successful.

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They've been around on our planet

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since 3.8 billion years ago

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and they're still gonna be around

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when we are nothing more
than a few fossilised traces.

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However, there's one thing
they haven't been able to do,

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and that's build a true
multicellular being

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because to do that, you need
a very different type of cell,

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one we're gonna meet in the next episode.

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Well, I really hope you enjoyed that video

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and found it useful.

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And if you did, please do share
with friends and colleagues.

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There's a lot more information

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about bacteria in
chapter five of this book

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including other ways
that bacteria can respond

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to their environment.

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They've got some pretty cool
tricks up their sleeves.

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There's links to buying the
book in the description below.

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If you also want to watch more videos

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and there's another one coming,

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which is all about eukaryotes,

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which are these kind of monstrous cells

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that you and I are made from.

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(birds chirping)