Bonnie Lynn Bassler is an American molecular biologist who revolutionized microbiology with her discovery of the use of chemical communication between bacteria known as quorum sensing, as well as the idea that disruption of chemical signaling can be used as an antimicrobial therapy.
Bonnie Bassler – TED-ED TRANSCRIPT
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 are one cell and they have this special property that they only have one piece of DNA. 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. They just grow and divide, and grow and divide — so a 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. So this man is supposed to represent a generic human being, and all of the circles in that man are all of the cells that make up your body.
There is 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 10 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 of course, it’s the DNA that counts, so here’s all the A, T, Gs and Cs 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 100 times more bacterial genes playing a role in you or on you all of your life.
And so at the best, you’re 10% human, but more likely about 1% human, depending on which of these metrics you like. I know you think of yourself as human beings, but I think of you as 90% or 99% bacterial. And these bacteria are not passive riders, 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 are 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 kinds of 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 for 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 sort of boring life where they grow and divide, and they’ve always been considered to be these asocial reclusive organisms.
And so it seemed to us that they are 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 that 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.
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.
What 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’ve 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 is 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 of that molecule increases in proportion to cell number.
And when the molecule hits a certain amount that tells the bacteria how many neighbors there 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. Again, another plug for the animals in the ocean, Vibrio fischeri lives in this squid. What you are 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 house 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, just in 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 is 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 that 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 squid’s 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 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, the squid has this terrible problem because it’s got this dying, thick 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.