No animal is an island

Spotted hyenas. Image courtesy of Steve Jurvetson

By Caleb O’Brien

CHICAGO – Luminescent squid, hyena scent glands and autistic mice have more in common than you might think.

Research presented recently at an American Association for the Advancement of Science symposium suggest the three are linked by the role of the microbiome in human and animal behavior.

According to Kevin Theis of Michigan State University, 90 percent of the cells and 99 percent of the genes in a hyena are actually microbes. Humans, too, have 10 times as many microbial cells in their bodies as human cells. This population of bacteria is the micorobiome, the population of microorganisms that inhabit the bodies of humans and animals. Much of the research on microbes in the past relied on growing the bacteria in a laboratory culture. In recent years, however, the field has been transformed advances in DNA technology. Scientists can now analyze the DNA found in samples taken in the wild and get a fuller, more accurate picture of the microbes living in an animal. This new ability has begun opening scientists’ eyes to the role of bacteria in maintaining health and regulating behavior.

Consider the Hawaiian Bobtail Squid. When baby squid are born, they aren’t capable of bioluminescence. Instead, they strike up a symbiotic friendship with a species of bacteria called Vibrio fischeri. The bacteria live in tiny chambers within the squid’s bodies called crypts. There, in exchange for food they shed light that helps conceal the squid from predators. Margaret McFall-Ngai of the University of Wisconsin-Madison, said the bacteria are capable of influencing the squid’s internal clock, its circadian rhythm.

Bacteria have also been implicated the behavior of autistic mice. Elaine Hsiao of the California Institute of Technology has investigated the role of a specific bacteria, B. Fragilis, in regulating autism-like behaviors. Treatment with B. Fragilis adjusts the microbiome in a mouse’s gut, which in turn decreases the gut’s permeability. When the mouse guts are well-sealed, it keeps metabolites from leaking out of the mouse’s digestive system and into the bloodstream. One metabolite, called 4EPS, has been shown to cause some autism-like behaviors in mice.

And what about those mostly-microbial hyenas? Spotted hyenas have glands called scent pouches they use to communicate with other hyenas. The hyenas drag their scent glands along the grass in a process known as pasting. The substance secreted by their scent glands is called, appetizingly, hyena butter. It is largely composed of volatile fatty acids and other substances commonly produced by bacterial fermentation. (One such bacteria is the source of a scourge of humans everywhere: foot odor.) Pasting plays an important role in establishing and maintaining social dominance in spotted hyenas, and Thies’ research suggests that the hyena microbiome influences that process.

These three examples are merely the tip of the iceberg. Other presentations at AAAS discussed the role of the microbiome in human metabolic disorders, obesity and diabetes. “Our framework is changing quite dramatically,” Hsiao said. “I think we’re going to have to change our whole understanding of biology.”

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