Growing up in an antibiotic age has predisposed many of us to think of all bacteria as harbingers of death and disease. We see them as things to be wiped, washed and scrubbed away. But for the past decade or so, the research, consistently proving the essentiality of microorganisms to human life, has us changing our tune. Because the truth is, we live in a bacterial world. Microorganisms in and on our body outnumber human cells 10 to one, and it can be argued we are more bacteria than we are human. And instead of threatening us, they keep us healthy by supporting basic physiological processes from digestion to defense.
A new study, published in mBio, by a University of Wisconsin-Madison laboratory, presents the first discovery of a bacterium regulating the biological clock of its host. The group led by Margaret McFall-Ngai, professor of medical microbiology, showed that Vibrio fischeri, a bioluminescent microbe, influenced the circadian rhythms of its host, the Hawaiian bobtail squid.
The bobtail squid-V.fischeri system is the quintessential example of vertebrate-microbe symbiosis. Symbiosis is simply a “you scratch my back, I’ll scratch yours” harmonious relationship; both organisms benefit.
The V. fischeri colonize the “light organ” cavity inside the squid, and at night when the squid is active, emit a blue luminescence, mimicking the light from the moon and stars. The glowing squid, which forages near the surface of the water, is able to eliminate its shadow from the seabed and becomes invisible to its predators. In return for the camouflage, bacteria are provided with nutrients. At dawn, the squid expels 90 percent of its bacterial inhabitants, rids its luminescence and burrows into the sand.
Elizabeth Heath-Heckman, a graduate student in the McFall-Ngai lab and co-author of the published study, observed that the squid was subjected to two different cycles of light, a cycle of sunlight outside of the squid and an offset cycle of blue light emitted by the V. fischeri in the squid. Animals, like humans, often base their activity on how much sunlight is present.
“So the natural question was if the luminescence from the light organ had an influence on the squid’s daily activities,” Heath-Heckman said, “And we found that yes, it does.”
Heath-Heckman and her colleagues found two light receptor genes, cryptochromes or “cry genes” (escry1 and escry2), in the bobtail squid. Cryptochromes belong to a family of proteins that drive the circadian rhythm, or biological clock, of an organism. In both humans and animals, circadian rhythms largely affect sleep cycles, appetites and other daily physiological functions.
Both cry genes were turned on with sunlight in the squid’s head. However, in the squid’s light organ, only one of the cry genes, escry1, showed expression when presented with bioluminescence.
“I think the hardest challenge of this research is to know what questions to ask, and the best ways to ask them. Any system has limitations. It is important to recognize and appreciate those limitations,” Heath-Heckman said.
She found that the escry1 gene did not cycle in a bacteria-free squid or even when the bacteria-free squid was subjected to blue external light. However, production of the escry1 gene in a squid with mutant non-luminescing bacteria was initiated in response to blue external light.
This proved both bacteria and blue light were needed for transcription of the escry1 gene. Heath-Heckman and her colleagues theorized that microbe-associated molecular patterns (MAMPs) that the V.fischeri produced, a molecular form of communication between the microbe and host, also regulated the cry genes.
“The light organ needs something the bacteria are making. When we gave a bacteria-free squid the MAMPs, a form of lipopolysaccharide and peptidoglycan, the cry gene cycled,” Heath Heckman said.
She hopes to one day develop a behavioral assay to determine if the presence of bacteria changed how the squid acted and matured. For now, she will continue to study the squid’s circadian circuitry and metabolism.
“Everywhere we turn, we find that bacteria influence our basic physiological activities. In many animals, circadian rhythms can affect about 10 percent of all the genes being transcribed in the organism. It is possible the input of bacteria could be even bigger than we imagined.”