Margaret McFall-Ngai, of the University of Wisconsin-Madison (UW-Madison), and colleagues, report how they discovered the symbiotic bacterium setting the biological clock of its host, in the 2 April online issue of the journal mBio.
New Insights Into MicrobiomeThe study is the first to show a microbe regulating the circadian rhythm of its host. It is important because it suggests a lot more is going on between living hosts and the microbes that inhabit them, than we may think: although other recent studies are also revealing some interesting new findings.
Take for example the "microbiome" that inhabits the human gut. This vast collection of microbial communities comprises thousands of bacteria, fungi and other organisms that carry out tasks essential to digestion, such as making vitamins and essential amino acids, breaking down toxins, and creating barriers against unfriendly invaders.
Recently, an international team of scientists established that at 3.3 million, the genes in the human gut's microbiome vastly outnumber the 23,000 or so genes in the human genome.
Another team of US researchers, reported finding that gut bacteria are part of a complex system that maintains the body's blood pressure, while a group from China and Europe said there is something very distinctive about the gut bacteria of people with type 2 diabetes.
Biological ClockHumans and other animals have a "biological clock" that regulates our circadian rhythm into a 24-hour cycle.
Our biological clock seems to be largely controlled by exposure to light and dark, and is responsible for patterns of sleep and wakefulness, and certain metabolic and physiological functions.
We are mostly unaware of this rhythm until it is disrupted, such as by jet lag, or when we work night shifts.
There is evidence that disrupting this daily cycle can seriously affect our health, giving rise, for example, to sleep and immune system disorders, and conditions like seasonal affective disorder.
Scientists suggest that at the molecular level, circadian rhythm is driven by a set of "clock genes" and their relatives.
"In humans, the genes expressed in the gut are on a profound circadian rhythm run by the clock genes," says McFall-Ngai in a statement.
The Hawaiian Bobtail SquidThe Hawaiian bobtail squid, Euprymna scolopes, is a small marine invertebrate that spends its nights foraging near the ocean surface.
It has many tricks for distracting or hiding from would-be predators: it can release ink as a decoy, it can bury itself in the sand, and it can change color.
But perhaps the most sophisticated tool in its arsenal is the one that contains a colony of bioluminescent V. fischeri carried in a special "light organ" within its mantle cavity.
The squid uses the light organ as a sort of cloaking device. It can direct and change the quality of the light, and it can also manipulate the intensity of the light to match down-welling moon and starlight, so as to mask its silhouette and evade predators.
At daybreak, the squid vents 90% of the glowing bacteria, burrows itself in the sand, and sleeps until nightfall, when, armed with a new crop of bacteria, it resumes its nocturnal foraging.
Light From Bacteria Triggers Genetic Cascade in the Squid's Light OrganIn their study, McFall-Ngai and colleagues found that the light generated by the colonizing bacterium Vibrio fischeri triggers a genetic cascade in the cells of the squid's light organ.
This cascade controls the daily cycle of activity that is normally synchronized by environmental cues such as sunlight.
"Instead of environmental light, this animal responds and cycles in response to the luminescence from its own light organ," says McFall- Ngai, a professor of medical microbiology and an authority on the bobtail squid and its luminescing symbiotic bacterium.
"This animal has a light-producing system in an organ in the middle of the body," she explains, "The bacteria in the light organ are luminous, and their luminescence affects the expression of a clock gene known as 'cry' in the cells of the light organ that are interacting with these luminous bacteria."
The team also looked at what happened to the cry gene with and without the bacteria present, and also what happened in the presence of mutant bacteria incapable of producing light.
They found that for the squid's clock genes to be expressed in a rhythm, they needed light from the bacterium.
Useful Model for Studying Interplay of Symbiont Bacteria and Host's BiologyThe researchers suggest the squid's light organ offers a useful model for understanding the interplay of a symbiont bacterium and the biological functions of its host.
McFall-Ngai says the human gut is also a system characterized by rhythm:
"Everything in the human gut is on a rhythm. Perhaps the thousands of bacteria there also govern the rhythms of the gut, just as the luminous bacteria partner of the squid sets the rhythms in the light organ."
She says the squid's light organ offers a simple model where you could manipulate just one microbe genetically and see how it affects the whole system.
This provides "much more resolution than can be done in studies of the human gut," says McFall-Ngai.
But perhaps the most astonishing part of this discovery, is as she puts it:
"We're beginning to realize that circadian rhythms are really important for health and that microbes are important for everything."
Funds from the National Institutes of Health and the National Science Foundation helped pay for the research.