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.
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.
And 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 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 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 launch their virulence attack together, they are 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.