Public release date: 2-Apr-2013 [ | E-mail | Share ]

Contact: Jim Sliwa jsliwa@asmusa.org 202-942-9297 American Society for Microbiology

Glowing bacteria inside squids use light and chemical signals to control circadian-like rhythms in the animals, according to a study to be published on April 2 in mBio, the online open-access journal of the American Society for Microbiology. The Hawaiian bobtail squid, Euprymna scolopes, houses a colony of Vibrio fischeri bacteria in its light organ, using the bacteria at night as an antipredatory camouflage while it ventures out to hunt. The results of the study show that, in addition to acting as a built-in lamp, the bacteria also control when the squid expresses a gene that entrains, or synchronizes, circadian rhythms in animals.

"To our knowledge, this is the first report of bacteria entraining the daily rhythms of host tissues," says corresponding author Margaret McFall-Ngai of the University of Wisconsin - Madison. If bacteria can entrain daily rhythms in an animal, McFall-Ngai says, it's reasonable to think these influences will eventually be found in other animals. It's possible that microbial partners in the human gut, for instance, could similarly influence human daily rhythms through chemical signaling.

Like all animals, squids make proteins that set their inner clock to environmental light. E. scolopes produces two of these "light entrainment" proteins (cryptochromes, or CRYs), and one is regulated in the squid's head, just like every other animal. McFall-Ngai and her co-authors noticed that escry1, the gene that encodes the other protein, is most highly expressed in the light organ, where the squid houses its glowing bacterial symbionts. "The animal uses the luminescence in the evening, so the luminescence is greatest at night. The gene escry1 cycles with the bioluminescence of the animal and not with environmental light," says McFall-Ngai.

But is it the bacterial luminescence that synchronizes the cycling, or is it the bacteria themselves? It's both, says McFall-Ngai.

The bacteria are necessary for cycling, she says, since squid grown without their bacterial symbionts do not cycle their expression of escry1, and mimicking the bacterial light with a blue light did not induce the cycling.

And they showed that the light is also necessary, because squids grown with defective V. fischeri symbionts that lack the ability to luminesce didn't cycle their expression of escry1 either. With light-defective bacteria in their light organs, squids exposed to the blue light got back on track, cycling escry1 production as usual.

What is it about the bacteria that could be signaling to the squid? Long experience taught McFall-Ngai where to turn next: microbe-associated molecular patterns (MAMPs), molecules that signal the presence of microbes to other creatures. "In this system we have found again and again that bacterial surface molecules are active at inducing all kinds of cellular behavior in the host," says McFall-Ngai.

Her hunch was right. MAMPs plus light turned cycling on. In squid grown without symbionts, light, combined with MAMPs (either the lipid A component of lipopolysaccharide or the peptidoglycan monomer), could induce some degree of cycling. The squid did not respond fully, though, maybe because the MAMPs were only injected into their seawater habitat, not presented directly to the light organ.

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Symbiotic bacteria program daily rhythms in squid using light and chemicals