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.
The fact that a bacterium can control a daily rhythm in a squid is exciting for other areas of biology, says McFall-Ngai, because all animals, including humans, have clock genes like escry1.
"Recently, in two different studies, biologists have found that there is profound circadian rhythm in both the epithelium [of the human gut] and the mucosal immune system of the gut that is controlled by these clock genes. What are we missing? Are the bacteria affected by or inducing the cycling of the tissues with which they associate?" We don't know," says McFall-Ngai, but it's an area ripe for study.
|Contact: Jim Sliwa|
American Society for Microbiology