How bacteria talk Bonnie Bassler
bacteria are the oldest living organisms
on the earth they’ve been here for
billions of years and what they are are
single-celled microscopic organisms so
they’re one cell and they have the
special property that they only have one
piece of DNA so they have very few genes
and genetic information to encode all of
the traits that they carry out and the
way bacteria make a living is that they
consume nutrients from the environment
they grow to twice their size they cut
themselves down in the middle and one
cell becomes two and so on and so on so
they just grow and divide and grow and
divide so kind of boring life except
that what I would argue is that you have
an amazing interaction with these
critters I know you guys think of
yourself as humans and this is sort of
how I think of you and so this man is
supposed to represent a generic human
being and all of the circles in that man
are all the cells that make up your body
so there’s about a trillion human cells
that make each one of us who we are and
able to do all the things that we do but
you have ten trillion bacterial cells in
you or on you at any moment in your life
so 10 times more bacterial cells than
human cells on a human being
and so of course it’s the DNA that
counts so here’s all the 80 G’s and C’s
that make up your genetic code and give
you all your charming characteristics so
you have about 30,000 genes well it
turns out you have a hundred times more
bacterial genes playing a role in you or
on you all of your life and so at the
best your 10% human we’re more likely
about 1% human depending on which of
these metrics you like so I know you
think of yourself as human beings but I
think of you as ninety or ninety two
point nine percent bacterial and these
bacteria are not passive writers these
are incredibly important they keep us
alive
they cover us in an invisible body armor
that keeps environmental insults out so
that we stay healthy they digest our
food they make our vitamins they
actually educate your immune system to
keep bad microbes out so they do all
these amazing things that help us and
keep in our vital for keeping us alive
and they never get any press for that
but they get a lot of press because they
do a lot of terrible things as well so
there’s all
insert bacteria on the earth that have
no business being in you or on you at
any time and if they are they make you
incredibly sick and so the question from
my lab is whether you want to think
about all the good things that bacteria
do or all the bad things that bacteria
do the question we had is how could they
do anything at all I mean they’re
incredibly small you have to have a
microscope to see one they live this
soaring sort of boring life where they
grow and divide and they’ve always been
to consider to be these a social
reclusive organisms and so it seemed to
us that they’re just too small to have
an impact on the environment if they
simply act as individuals and so we
wanted to think if there couldn’t be a
different way the bacteria live and the
clue to this came from another marine
bacterium and it’s a bacterium called
Vibrio fischeri
and so what you’re looking at on this
slide is just a person from my lab
holding a flask of a liquid culture of a
bacterium a harmless beautiful bacterium
that comes from the ocean named Vibrio
fischeri
and this bacterium has the special
property that it makes light so it makes
bioluminescence like fireflies make
light so we’re not doing anything to the
cells here we just took the picture by
turning the lights off in the room and
this is what we see and was actually
interesting to us was not that the
bacteria made light but when the
bacteria made light what we noticed is
when the bacteria were alone so when
they were in dilute suspension they made
no light but when they grew to a certain
cell number all the bacteria turned on
light simultaneously and so the question
that we had is how can bacteria these
primitive organisms tell the difference
from times when they’re alone and times
when they’re in a community and then all
do something together and what we
figured out is that the way that they do
that is that they talk to each other and
they talk with a chemical language so
this is now supposed to be my bacterial
cell when it’s alone it doesn’t make any
light but what it does do is to make and
secrete small molecules that you can
think of like hormones and these are the
red triangles and when the bacteria ours
alone the molecules just float away and
so no light but when the bacteria grow
and double and they’re all participating
in making these molecules the molecule
the extracellular amount
that molecule increases in proportion to
cell number and when the molecule hits a
certain amount that tells the bacteria
how many neighbors they are they
recognize that molecule and all of the
bacteria turn on light in synchrony and
so that’s how bioluminescence works
they’re talking with these chemical
words and the reason that Vibrio
fischeri is doing that comes from the
biology so again another plug for the
animals in the ocean the real fish fry
lives in this squid what you’re looking
at is the Hawaiian bobtail squid and
it’s been turned on its back and what I
hope you can see are these two glowing
lobes and these how’s the Vibrio
fischeri cells they live in there at
high cell number that molecule is there
and they’re making light and the reason
the squid is willing to put up with
these shenanigans is because it wants
that light and so the way that this
symbiosis works is that this little
squid lives just off the coast of Hawaii
so just in a sort of shallow knee-deep
water and the squid is nocturnal so
during the day it buries itself in the
sand and sleeps but then at night it has
to come out to hunt and so on bright
nights when there’s lots of starlight or
moonlight that light can penetrate the
depth of the water the squid lives in
since it’s just in those couple feet of
water and what the squid has developed
is a shutter they can open and close
over this specialized light organ
housing the bacteria and then it has
detectors on its back so it can sense
how much starlight or moonlight is
hitting its back and it opens and closes
the shutter so the amount of light
coming out of the bottom which is made
by the bacterium exactly matches how
much light hits the squids back so the
squid doesn’t make a shadow so it
actually uses the light from the
bacteria to counter illuminate itself in
an anti predation device and so it so
predators can’t see its shadow calculate
its trajectory and eat it and so this is
like the stealth bomber of the ocean but
then if you think about it this squid
has this terrible problem because it’s
got this dying stick culture of bacteria
and it can’t sustain that and so what
happens is every morning when the Sun
comes up the squid goes back to sleep it
buries itself in the sand and it’s got a
pump that’s attached to its circadian
rhythm and when the Sun comes up it
pumps out like 95% of the bacteria and
so now the bacteria are dilute that
little hormone molecule is gone so
they’re not making light but of course
squid doesn’t care it’s asleep in the
sand and as the day goes by the bacteria
doubled they release the molecule and
then light comes on at night exactly
when the squid wants it and so first we
figured out how and this bacterium does
this but then we brought the tools of
molecular biology to this to figure out
really what’s the mechanism and what we
found so this is now supposed to be
again my bacterial cell is that Vibrio
fischeri has a protein that’s the red
box it’s an enzyme that makes that
little hormone molecule the red triangle
and then as the cells grow they’re all
releasing that molecule into the
environment so there’s lots of molecule
there and the bacteria also have a
receptor on their cell surface that fits
like a lock and key with that molecule
these are just like the receptors on the
surfaces of your cells and so when the
molecule increases to a certain amount
which says something about the number of
cells it locks down into that receptor
and information comes into the cells
that tells the cells to turn on this
collective behavior of making light and
why this is interesting is because in
the past decade we have found that this
is not just some anomaly of this
ridiculous glow-in-the-dark bacterium
that lives in the ocean all bacteria
have systems like this so now what we
understand is that all bacteria can talk
to each other they make chemical words
they recognize those words and they turn
on group behaviors that are only
successful when all of the cells
participate in unison and so now we have
a fancy name for this we call it quorum
sensing they vote with these chemical
votes the vote gets counted and then
everybody responds to the vote and
what’s important for today’s talk is
that we know that there are hundreds of
behaviors that bacteria carry out in
these collective fashions but the one
that’s probably the most important to
you is virulence so it’s not like a
couple bacteria get in you and then they
start secreting some toxins you’re
enormous that would have no effect on
you you’re huge but what they do we now
understand is they get in you they wait
they start growing they count themselves
with these little molecules and they
recognize when they have the right cell
number that if all of the bacteria
launched their virulence attack together
they’re going to be successful at
overcoming an enormous host so bacteria
always control pathogenicity with quorum
sensing
and so that’s how it works we also then
went to look at what are these molecules
so these were the red triangles on my
slides before and so this is the Vibrio
fischeri molecule this is the word that
it talks with and then we started to
look at other bacteria and these are
just a smattering of the molecules that
we’ve discovered and what I hope you can
see is that the molecules are related so
the left-hand part of the molecule is
identical in every single species of
bacteria but the right-hand part of the
molecule is a little bit different in
every single species and what that does
is to confer exquisite species
specificities to these languages so each
molecule fits into its partner receptor
and no other so these are private secret
conversations these conversations are
for interest species communication each
bacteria uses a particular molecule
that’s it like its language that allows
it to count its own siblings and so once
we got that far we thought we were
starting to understand that bacteria
have these social behaviors but we
started what we’re really thinking about
is that most of the time bacteria don’t
live by themselves they live in
incredible mixtures with hundreds or
thousands of other species of bacteria
and that’s depicted on this slide this
is your skin so this is just a picture a
micrograph of your skin anywhere on your
body it looks pretty much like this and
what I hope you can see is that there’s
all kinds of bacteria there and so we
started to think if this really is about
communication in bacteria and it’s about
counting your neighbors it’s not enough
to be able to only talk within your
species there has to be a way to take a
census of the rest of the bacteria in
the population so we went back to
molecular biology and started studying
different bacteria and what we found now
is that in fact bacteria are
multilingual so they all have a species
specific system they have a molecule
that says me but then running in
parallel to that is a second system that
we’ve discovered that’s generic so they
have a second enzyme that makes a second
signal and it has its own receptor and
this molecule is the trade language of
bacteria it’s used by all different
bacteria and it’s the language of
interspecies communication and so what
happens is that bacteria are able to
count how many of me and how many of you
and they take that in for
Meishan inside and they decide what
tasks to carry out depending on who’s in
the minority and who’s in the majority
of any given population and so then
again we turn to chemistry and we
figured out what this generic molecule
is so that was the pink ovals on my last
slide this is it it’s a very small five
carbon molecule and what the important
thing is that we learned is that every
bacterium has exactly the same enzyme
and makes exactly the same molecule so
they’re all using this molecule for
interspecies communication so this is
the bacterial Esperanto and so once we
got that far we’ve started to learn the
bacteria can talk to each other with
this chemical language but what we
started to think is that maybe there’s
something practical that we can do here
as well so I’ve told you the bacteria do
you have all these social behaviors that
they communicate with these molecules
and of course I’ve also told you that
one of the important things they do is
to initiate pathogenicity using quorum
sensing so we thought what if we made
these bacteria so they can’t talk or
they can’t hear couldn’t these be new
kinds of antibiotics and of course
you’ve just heard and you already know
that we’re running out of antibiotics
bacteria are incredibly multi
drug-resistant right now and that’s
because all of the antibiotics that we
use kill bacteria so they either pop the
bacterial membrane they make that
bacterium so it can’t replicate its DNA
we kill bacteria with traditional
antibiotics and that selects for
resistant mutants and so now of course
we have this global problem in
infectious diseases so we thought well
what if we could sort of do behavior
modifications just make these bacteria
so they can’t talk they can’t count and
they don’t know to launch virulence and
so that’s exactly what we’ve done and
we’ve sort of taken two strategies the
first one is we’ve targeted the intra
species communication system so we’ve
made molecules that look kind of like
the real molecules which you saw but
they’re a little bit different and so
they lock into those receptors and they
Jam recognition of the real thing and so
by targeting the red system what we are
able to do is to make species specific
or disease specific anti quorum sensing
molecules we’ve also done the same thing
with the pink system we’ve taken that
universal molecule and turned it around
a little bit so that we’ve made in
goodness of the interspecies
communication system and these the hope
is that these will be used as
broad-spectrum antibiotics that work
against all bacteria and so to finish
I’ll just show you the strategy and this
one I’m just using the interspecies
molecule but the logic is exactly the
same so what you know is that when that
bacterium gets into the animal in this
case a mouse it doesn’t initiate
virulence right away it gets in it
starts growing it starts secreting its
quorum sensing molecules it recognizes
when it has enough bacteria that now
they’re going to launch their attack and
the animal dies and so what we’ve been
able to do is to give these virulent
infections but we give them in
conjunction with our anti quorum sensing
molecules so these are molecules that
look kind of like the real thing but
they’re a little bit different which
I’ve depicted on this slide and what we
now know is that if we treat the animal
with the pathogenic bacterium a
multidrug-resistant pathogenic bacterium
in at the same time we give our anti
quorum sensing molecule in fact the
animal lives and so we think that this
is the next generation of antibiotics
and it’s going to get us around at least
initially this big problem of resistance
so what I hope you think is the bacteria
can talk to each other they use
chemicals as their words they have an
incredibly complicated chemical lexicon
that we’re just now starting to learn
about and of course what that allows
bacteria to do is to be multicellular
right and so in the spirit of Ted they
are doing things together because it
makes a difference right so what happens
is that bacteria have these collective
behaviors and they can carry out tasks
that they could never accomplish if they
simply acted as individuals and what I
would hope that I could further argue to
you is that this is the invention of
multicellularity bacteria had been on
the year on the earth for billions of
years humans a couple hundred thousands
so we think bacteria made the rules for
how multicellular an organization works
and and we think by studying bacteria
we’re going to be able to have insight
about multicellularity in the human body
so we know that the principles and the
rules if we can figure them out in these
sort of primitive organisms the hope is
that they will be applied to other human
diseases and human behaviors as well
I hope that what you’ve learned is that
bacteria can distinguish self from
others so by using these two molecules
they can say me and they can say you and
again of course that’s what we do both
as Malec in in a molecular way and then
also in an outward way but I think about
the molecular stuff this is exactly what
happens in your body it’s not like your
heart cells and your kidney cells get
all mixed up every day and that’s
because there’s all of this chemistry
going on these molecules that say who
each of these groups of cells is and
what their tasks should be and so again
we think that bacteria invented that and
then you’ve just evolved a few more
bells and whistles but all of the ideas
are in these simple systems that we can
study and then the final thing is again
just to reiterate that there’s this
practical part and so we’ve made these
anti quorum sensing molecules that are
being developed as new kinds of
therapeutics but then to finish with a
plug for all the good and miraculous
bacteria that live on the earth we’ve
also made Pro quorum sensing molecules
so we’ve targeted those systems to make
the molecules work better and so
remember you have these ten times or
more bacterial cells in your on you
keeping you healthy what we’re also
trying to do is to beef up the
conversation of the bacteria that live
as mutualist with you in the hopes of
making you more healthy making those
conversations better so bacteria can do
things that we want them to do by in
here better than they would be on their
own and then finally I wanted to show
you this is my gang at Princeton New
Jersey everything I told you about was
discovered by someone in that picture
and I hope when you learn things like
about how the natural world works I just
want to say that whenever you read
something in the newspaper you get to
hear some talk about something
ridiculous in the natural world it was
done by a child so science is done by
that demographic there all of those
people are between 20 and 30 years old
and they are the engine that drives
scientific discovery in this country and
it’s a really lucky demographic to work
with I keep getting older and older and
they’re always the same age and it’s
just Anna crazy delightful job and I
want to thank you for inviting me here
it’s a big treat for me to get to come
to this conference
you