Vibrio fischeri communicate using a chemical language by secreting small molecules that function like hormones. As the bacterial population increases and the concentration of these signaling molecules reaches a certain threshold, all the bacteria can detect it. This results in a coordinated behavior where they all turn on bioluminescence simultaneously.
"You have to have a microscope to see 1. They live this 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'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 that bacteria live. And the clue to this came from another marine bacterium, and it's a bacterium called Vibrio fischeri.
03::00And so what you're looking at on this slide is just a person from my lab holding 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 what was actually interesting to us was not that the bacteria made light, but when the bacteria made light.
2. Culture Of Bacteria
03::30What 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.
04::13And 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 they are, they recognize that molecule, and all of the bacteria turn on light in synchrony. And so that's how bioluminescent works. They're talking with these chemical words.
04::44And the reason that Vibrio fischeri is doing that comes from the biology. So again, another plug for the animals in the ocean. Vibrio fischeri lives in this squid. What you're looking at is the Hawaiian bobtail squid, and it's been turned on its back. What I hope you can see are these two glowing lobes, and these house the Vibrio fischeri cells.
05::03They live in there at high cell number. That molecule is there, and they're making light. The reason the squid is willing to put up with these shenanigans is because it wants that light. The way that this symbiosis works is that this little squid lives just off the coast of Hawaii, so just in 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.
05::28And 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. 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, 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.
06::13But then if you think about it, this 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, and so now the bacteria are dilute. That little hormone molecule is gone, so they're not making light, but of course the squid doesn't care. It's asleep in the sand.
06::42And as the day goes by, the bacteria double, they release the molecule, and then light comes on at night exactly when the squid wants it. And so first we figured out how 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 Vibrio fischeri has a protein. That's the red box. It's an enzyme that makes that little hormone molecule, the red triangle.
07::09And then as the cells grow, they're all releasing that molecule into the environment, so there's lots of molecules 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.
3. Studying Different Bacteria
07::48All 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."
How do the Vibrio fischeri communicate? What happens as a result of their communication? short answer
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