Symbiosis Between Squid And Bacteria Programs Daily Rhythms

Brett Smith for – Your Universe Online
A new study on glowing bacteria found within Hawaiian bobtail squid could change the way scientists look at symbiotic microorganisms.
The study, which was recently published in the journal mBio, showed the Vibrio fischeri bacteria in the squid´s light organ not only camouflage the animal at night — the bacteria also influence a gene that entrains, or synchronizes, the cephalopod´s circadian rhythms.
“To our knowledge, this is the first report of bacteria entraining the daily rhythms of host tissues,” said co-author Margaret McFall-Ngai of the University of Wisconsin – Madison.
According to McFall-Ngai, other animals, including humans, could also have symbiotic bacteria that influence circadian rhythms of their host.
The researchers first became interested when they found the expression of “light entrainment” proteins (cryptochromes, or CRYs) in the squid´s light organ. These proteins set the circadian rhythms of all animals based on the levels of daylight and are typically found in the head.
“The animal uses the luminescence in the evening, so the luminescence is greatest at night. The gene (entrainment protein) escry1 cycles with the bioluminescence of the animal and not with environmental light,” McFall-Ngai said.
The researchers had to determine if the bioluminescence was synchronizing the cycling or if the bacteria were responsible for the protein expression themselves. First, the team found that squid grown without the V. fischeri bacteria do not sequence their expression of escry1. The team also mimicked the bacterial light with a blue light, which did not provoke the cycling.
The team also looked at squids with defective V. fischeri that were unable to luminesce. These squids didn’t cycle their expression of escry1 either. However, the light-defective squids did begin to cycle their escry1 expression when exposed to the blue light.
To determine how the bacteria could be signaling protein expression to the squid, the team looked at the molecules that indicate the presence of microbes to other creatures, referred to as microbe-associated molecular patterns (MAMPs).
“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,” McFall-Ngai said.
In squid grown without V. fischeri, certain MAMPs could cause some cycling in the presence of light. The team theorized the squid´s response has hampered because MAMPs were only injected into their local environment and introduced directly to the light organ.
McFall-Ngai said the fact that a bacterium can control a daily rhythm in a squid is exciting for studying clock genes like escry1 in other animals, including humans.
“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.
She added that her lab will continue to further investigate the effects of escry1 on the squid and its metabolic functions.