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Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate toward each other, and aggregate to form fruiting bodies up to 500 micrometres long. Photo: <a href="http://en.wikipedia.org/wiki/File:Myxococcus_xanthus.png">Ayacop</a>, Creative Commons, some rights reserved
Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate toward each other, and aggregate to form fruiting bodies up to 500 micrometres long. Photo: Ayacop, Creative Commons, some rights reserved

Natural Selections: Bacterial "quorums"

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Bacteria have an awareness of when they are part of a large population, and change their behavior as a result. In the sea, bioluminescence is governed by this phenomena, known as "quorum-sensing." In the body, it may trigger the disease-causing effects of large infections. Martha Foley and Curt Stager get together with microbial crowds.

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Martha: A listener sent me a newspaper clipping from The New York Times service. Weird thing- quorum sensing among bacteria. The basic jist was that one bacterium can’t really do much, but if there are enough of them then you get sick. But they have to have a quorum to accomplish this. They don’t do it on their own; it’s kind of a cooperative thing. Does that make sense to you?

Curt: Well of course it makes sense. If you have a few bacterial cells in your body, they can’t make you too sick. But it’s not just a dose thing; they’ll behave differently when they have a quorum.

Martha: It’s like the glowing bacteria in the ocean. They don’t really glow if it’s just one. But if you get them all together, I guess this is where scientists started noticing this quorum sensing thing. If you get a bunch of them together, then they start this glow thing.

Curt: Yeah, it would be different if you go out at night and you throw a stone in the ocean and it lights up, that’s usually another kind of organism. But there are bacteria that will actually glow too. Some of the marine fish that glow in the dark in the deep sea actually grow them on their skin to make the glowing patches. Apparently this came to public attention about 10 years ago in New Hampshire. A couple was at a seafood restaurant and the power went out, and they looked down and their fish was glowing.

Martha: From the bacteria? Eeew.

Curt: Yeah, they said it was bright enough that you could read a newspaper by it.

Martha: Wow, now did that indicate that it had been sitting around in the kitchen maybe just a little too long.

Curt: It wasn’t so much a decay thing; I guess those bacteria may often be there. I don’t know how they survived the cooking. But the context then is that these kinds of bacteria are widespread in the ocean but they only get together and glow visibly like that when there are large concentrations of them and they seem to “know” when there are enough of them there.

Martha: Now how would they know?

Curt: It would have to be a chemical signal. And in their case, I don’t know what benefit they would get from deciding to glow all of a sudden. But the reason this got wider attention among microbiologists is that a lot of other bacteria do the same thing with what they do, including bacteria that cause disease in humans.

Martha: So is it that there’s enough of them that they start releasing their poison or whatever it is that they do to you?

Curt: Yes, the analogy that folks use sometimes is that these kinds of bacteria act like cowardly bullies that lay low until there are enough of them to gang up on you. Which makes sense from their perspective; you have a pretty effective immune system really, and if there were just a few of them, your immune system would attack them and wipe them out.

Martha: Right, if they release the toxin then, they would be done for. But if there are enough of them to overwhelm the immune system, then they can do it.

Curt: And it’s not really clear; they’re probably not out there trying to poison you, it’s more like the toxins are just their waste that they make in large numbers. Though I guess the deal would be that they would be relatively inactive, trying to hide and not let their presence be known until there are enough of them and then they start to do their thing and one of the byproducts is a toxin that can make you really sick.

Martha: So theoretically if you mess with their chemical signal, if you can interfere with that I’m thinking in terms of not getting sick or getting better once they start to do this. If you interfere with them you could destroy their cooperative venture.

Curt: Yeah and people are thinking along those lines. You could make some pretty interesting new drugs with that. The typical way to fight a bacterial disease is you take an antibiotic which is meant to kill them off. Evolution is going to kick in then. It’s sort of a natural selection for the types that are immune or resistant to your antibiotic and it’s hard to keep antibiotics around. But if you mess with something that’s a built in part of their life cycle, like their chemical communication it’s going to be a lot harder for them to become resistant to that.

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