Saturday, 06 August 2016 18:21

Hey Bear! Can I Sample Your Microbiota?

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Published in Microbial Sciences

Figure 1Figure 1. The moment we first spotted a sow grizzly bear with her cub, near Siyeh Bend in Glacier National Park. Photo by: Scott Chimileski

My brother and I were moments from our destination on a hike in Glacier National Park when a 500-pound Montana grizzly bear and her cub came into view. The bears were on the same trail as us, walking in our direction.

Glacier is the final national park we visited during our recent trip celebrating the 100-year anniversary of the National Park Service. We were there to photograph Precambrian stromatolite fossils the in the Altyn and Siyeh rock formations. These stromatolites are over one billion years old, much more ancient than the Jurassic stromatolites in Capitol Reef National Park. But unlike the challenge of 100-degree heat in Capitol Reef, this fossil hunt had one giant obstacle with sharp teeth and claws. Or more accurately, there was one giant obstacle and one tiny, adorable obstacle. So, since I wrote about fossils in part two of this series, I’m going to continue the encounter story and veer off onto this entirely different subject: bears.

Figure 2 bear microFigure 2. Seasonal change in brown bear microbiota. A) Cartoon of the bear’s seasonal cycle. B) Principal coordinate analysis of bear microbiota in summer and winter. C) Bacterial phyla detected within bear microbiota during winter and summer. Source: Sommer et al. 2016

We stopped immediately. There was a clear line of vision between us and the bears. They were at the other end of a tight sloping bend on the Going to the Sun Road, known as the Siyeh Bend. Though it was June, the road was still closed due to snow. We were two miles from our car and there were no other people in sight.

I thought, “What a lovely time to distract myself from my proximity to these apex predators, by thinking about the grizzly bear microbiota, all of the fascinating bear microbes helping to digest all of the bear foods they eat …”

The bears kept walking down the trail as we held our position. A few minutes later, they made a sudden turn and crashed down a hill, appearing for five very exciting seconds to head squarely in our direction. Then, they disappeared into some brush, right where the stromatolite fossils are. Of course. We had come all this way. Now what should we do? Turn back and go home without the stromatolite photographs, or continue, undeterred by the great wild beasts?

There’s actually a compelling reason to study the bear gut microbiota: they have one of the most variable omnivorous diets in the animal kingdom. Depending on the season, free-ranging grizzly bears eat live elk, dead elk, gophers, fish, moths, earthworms, mushrooms, and bark. They eat flowering plants, ants, roots, strawberries, huckleberries, and succulent grass shoots. Ideally, every summer day is all-you-can-eat buffet. This frenzied period of hyperphagia leads to a special physiological state: the bears become obese but remain metabolically healthy. Finally, they return to their dens and hibernate, eating absolutely nothing for five months.

Figure 3 bear hillFigure 3. The bears come crashing down a hill. Photo by: Scott Chimileski

However, the bears I had in mind at that moment were not hibernating—they were very much awake and possibly on a hunt of their own. My brother and I decided on a compromise between retreating and brashly carrying on. (I should mention here that as a bear enthusiast, I have read the book Bear Attacks: Their Causes and Avoidance, written by Stephen Herrero). We watched and waited. After 15 minutes without any further sightings, we continued to the rock formation, to the exact place where the bears stood when we first spotted them. Either they had moved on, or they were hidden a few feet away in the forest.

That was when the “Hey Bear!” shouts started. If you’ve ever hiked in bear country, you know this phrase. It’s a way to let bears know where you are. Because, as Stephen Herrero stresses in his book, bears are almost never out to eat people in a predatory-type attack. Most attacks occur when you surprise a bear. They act out of defense, trying to neutralize a threat—especially when it’s a mother with cubs.

Nevermind where the bears were. How were the microbial communities in their guts responding to what they ate that morning? And how will they respond to months of fasting this coming winter? Earlier this year, Sommer and colleagues answered this question by analyzing fecal samples collected from brown bears in winter and summer months. They found that microbial diversity surges during the summer as bears consume a varied diet, and reduces to a more stable core community during the winter. In all, 24 microbial phyla and over 4,000 potential species (as inferred by operational taxonomic units) were detected.

Figure 4 Siyeh BendFigure 4. A look back at Siyeh Bend on the walk to the car. Photo by: Scott Chimileski

Proteobacteria, Firmicutes, and Actinobacteria increased in abundance during the summer, while in the winter, Bacteroidetes species were dominant. Some individual species were tightly correlated with summer and winter samples. For example, Wolbachia species were present in June and nearly absent in February. This makes sense because Wolbachia are symbionts of the insects consumed by bears over the summer. And Bacteroides fragilis was the most common species in hibernating bear guts, reflecting a shift towards protein and fat metabolism, also seen in calorie-restricted humans.

The June and February bear microbiota was successfully transferred to germ-free mice and induced many of the same metabolic traits measured in bears. This suggests that the seasonal microbiota is not only present in the bear gut, but directly contributes to healthy obesity in summer and the ability to use fat stores during hibernation. Learning more about the bear microbiota may even lead to microbiome-based obesity therapies in humans.

Figure 5 black bearFigure 5. A beautiful cinnamon colored black bear near Jackson Glacier Overlook, feasting on dandelion flowers. Photo by: Scott Chimileskii

That bear biology might lead to advances in human health is all the more reason to appreciate these animals. As far as we were concerned back on Going to the Sun Road, the bears were gone. They were wild Glacier National Park grizzlies, probably off dining on a summer special of dandelion flowers by the time we got to the area, bellies filling with fiber and Firmicutes. It was a perfect wilderness grizzly sighting—not too close and not too far away. We found the stromatolites, I got the photos, and we hiked back through the majestic peaks safely.  

So if you find yourself in the vicinity of a lumbering grizzly bear, or if there aren’t many bears around but you are suffering from a severe case of bearanoia, go ahead and shout “Hey Bear!” And while you’re at it, why not add at the end, “Can I sample your microbiota?”



Further Reading

  • Sommer, F., Ståhlman, M., Ilkayeva, O., Arnemo, J.M., Kindberg, J., Josefsson, J., Newgard, C.B., Fröbert, O., & Bäckhed, F. (2016) The Gut Microbiota Modulates Energy Metabolism in the Hibernating Brown Bear Ursus arctos. Cell Reports, 14(7), 1655-1661. Full-text, open-access 


Last modified on Wednesday, 28 September 2016 15:45
Scott Chimileski

Senior Contributor Scott Chimileski is a Research Fellow in Roberto Kolter’s laboratory at Harvard Medical School and a member of the ASM Writer Team. Scott's research is focused on imaging biofilms and other microbial multicellular forms. He is a photographer, coauthor of the book Life at the Edge of Sight: A Photographic Exploration of Microbial World, and is currently spearheading several exhibitions on microbial life at the Harvard Museum of Natural History. You can find him on Twitter @socialmicrobes.