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.
