Bacteria that makes the Hawaiian bobtail squid bioluminescent also dictate when it expresses a gene that encodes circadian rhythm-controlling proteins,according to a paper due to be published in mBio.

The squid has fascinated microbiologists for years because of its harmonious relationship with just one bacteria -- Vibrio fischeri. The bacteria does not express light when it is freely roaming in the ocean, but when housed in the squid's light organ (located in its underbelly) it will work with the animal to emit light according to how much

moonlight and sunlight is visible above. In doing so, the squid will glow a light blue to mimic the light from above, eliminating its shadow on the seabed and rendering it invisible to predators potentially lurking below. The two live a happy coexistence: the bacteria getting sustenance from the squid, the squid getting camouflage from the bacteria.

A cyclic daily routine enjoyed by the two had already been noted. For instance, the bobtail squid  expels 95 percent of the bacteria every morning when it's about to go to sleep in the seabed. In doing so the squid ensures infant squid have access to new bacteria and that it stops emitting light while it sleeps. The remaining bacteria repopulate everyday and are back to full capacity by nightfall. This process employs a type of cell-to-cell communication called quorum sensing. It's induced when the individual bacteria release chemical autoinducers to alert others to its presence, and when the level of autoinducers reaches a certain density the bacteria turns on genes that react with proteins to emit the light. In the bobtail's case, the bacteria produces the enzyme luciferas. It's why the bacteria doesn't emit light outside the squid's organ -- in the ocean the autoinducers never accumulate to a high enough density.

What has been unclear is how the squid and bacteria communicate with one another -- for instance, how does the squid recognise and translate light messages to good bacteria and not bad. The mBio paper has gone a way in providing some clues as to how this symbiosis works behind the scenes. It has revealed that escry1, one of two genes in the squid that encodes proteins that set its inner clock (similar to our light sensitive biological clock) is dominant in the light organ where the bacteria thrives.

Lead author on the paper Margaret McFall-Ngai of the University of Wisconsin found that the gene was not cycling with environmental light however, as is the norm among animals and humans, but with the bioluminescence dominant at night. The find is an exciting one because it is "the first report of bacteria entraining the daily rhythms of host tissues", according to McFall-Ngai, and could be replicable in other animals or even humans. It's been difficult to breakdown the relationship between human, their good bacteria and viruses because the system is so complex -- we have millions of "good bacteria" thriving within us. Conversely, there is just one bacteria dominant in the bobtail squid and the pair make excellent research subjects because each can thrive without each other. 

This means researchers can study bacteria-free squid in the lab to see what natural states the Vibrio fischeri are affecting -- which is how McFall-Ngai confirmed her suspicions about what was happening with the gene cycles.

She found that squid in the lab that lacked Vibrio fischeri bacteria could not luminesce and did not cycle their expression of the escry1 gene. Using a blue light to mimic the luminescence still did not induce gene cycling. However, if the bacteria was present but defective (unable to luminesce), gene cycling did kick in when the fake light was used. It proves that the bacteria and the light are together essential for controlling gene cycling in the squid.

Breaking the process down further McFall-Ngai found that microbe-associated molecular patterns (MAMPs), which alert animals to the presence of certain microbes, induced some cycling when combined only with light and not bacteria. Next up she and her team will investigate how the escry1 gene affects the squids metabolism, but the whole find could point us in the right direction to understanding how the millions of bacteria in our gut possibly regulate other processes in the body.

"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," McFall-Ngai said in a statement. "What are we missing? Are the bacteria affected by or inducing the cycling of the tissues with which they associate? We don't know."

Further studies of the processes could answer questions about how our body communicates with good bacteria, and how the Vibrio fischeri bacteria developed a symbiotic relationship -- considering it is closely relation to other Vibrio bacteria that are far from symbiotic, causing cholera and gastroenteritis.