Dr. Sarah Knutie led a study to explore whether a commonly-used herbicide affects the gut microbes of frogs and if the gut microbes could mediate the effect of the herbicide on infection risk by the amphibian chytrid fungus. She conducted the work as a Post-doctoral Researcher at the University of South Florida and is currently an Assistant Professor at the University of Connecticut. Here, she shares her #StoryBehindThePaper for: “Do host-associated gut microbiota mediate the effect of an herbicide on disease risk in frogs?”

Cuban tree frog adult (Photo credit: Jeremy Cohen)

When I started my post-doctoral position at the University of South Florida in Jason Rohr’s lab, I had never worked with frogs, gut microbes, or skin pathogens, which are all the focus of my recent paper in Journal of Animal Ecology. Most of my past research explored the effects of novel parasitic nest flies on birds in the Galapagos Islands. However, I had a keen interest in understanding what factors affect animals’ ability to defend themselves against parasites. With all the new and exciting research on animal microbiomes and the need to understand what factors affect the decline in amphibians, I decided to start my new research program by exploring how gut microbes affect disease ecology in frogs.

Cuban tree frog adult (Photo credit: Jeremy Cohen)

I assembled an amazing team of undergraduate researchers and we looked at how early-life gut microbes affected later-life resistance to parasitic gut worms. This project started as a test of methods study but turned into ground breaking work, which was published last year in Nature Communications. We found that a disruption in the gut microbes of tadpoles increased infection risk in adult frogs, which had not been demonstrated previously. Furthermore, we suspected that a reduction in the diversity of gut microbes and abundance of bacteria from the phylum Fusobacteria in tadpoles could be responsible for this change in infection risk.

Left: Former undergraduate researchers from the University of South Florida that participated in the study. Right: Former undergrad Sahara Peters with the experimental frogs.

In the Nature Communications study, we used sterilized pond water to disrupt the gut microbes, which is not a natural disruptor in the environment. Therefore, we wanted to test whether “real-world” environmental insults, such as pesticides, would disrupt the gut microbes of frogs and if the gut microbes could be mediating the effect of the pesticides on infection risk. And thus began the conception of the study published in Journal of Animal Ecology; I was also honored to receive a British Ecological Society Large Research Grant for this study.

The two main questions from the Journal of Animal Ecology paper were: 1) does atrazine affect gut microbes of Cuban tree frogs? and 2) does this atrazine-related change in gut microbes relate to higher infection risk by the amphibian chytrid fungus? We designed an experiment in the laboratory with captive Cuban tree frogs. We decided to work with the herbicide atrazine for the study because recent work suggests that tadpoles exposed to atrazine have a higher risk of chytrid-related disease later in life. Additionally, chytrid is an important pathogen because it has contributed, in part, to the global decline of amphibians, which are the most threatened class of vertebrates in the world.

Graphical abstract for Knutie et al. (2017)

For the main experiment, we exposed tadpoles to atrazine or not then either infected them with chytrid or allowed them to metamorphose (become terrestrial) and infected them with chytrid as adults. We then characterized the gut microbes and quantified infection intensity of chytrid. The main finding was that atrazine did not affect the gut microbes of frogs and that the gut microbes did not mediate the early-life effects of atrazine on later-life disease risk. However, we did find that the gut microbe community in tadpoles predicted infection risk in adult frogs: 1) higher diversity of microbes in tadpoles related to lower infection intensity in adults, and 2) higher relative abundance of bacteria from the phylum Fusobacteria resulted in lower infection intensity in adults. We found no relationship between the gut microbe community and infection risk at the time of infection at either life stage (i.e., tadpole gut microbes did not affect infection risk of tadpoles).

These results were exciting because they corroborate our results from the Nature Communications paper but also show that the early-life gut microbes can affect later-life susceptibility to a skin pathogen. In other words, the gut microbes could be affecting the development of the immune system in a way that not only affects gut parasites, but skin parasites as well! Additionally, the results suggest that bacteria from the phylum Fusobacteria might be important in the development of the immune system, so increasing relative abundances of Fusobacteria might have lasting positive effects on amphibian health.

Although we did not find that gut microbes mediate the effect of atrazine on infection risk in this Journal of Animal Ecology study, we have exciting preliminary results to suggest that this is not the story for all other pesticides. Stay tuned for future papers from my lab group at the University of Connecticut.

Sarah Knutie with a Cuban Tree Frog (Photo credit: Laura Domine)

More Info:

Knutie, S. A., Gabor, C., Kohl, K. D., & Rohr, J. R. (2018). Do host-associated gut microbiota mediate the effect of an herbicide on disease risk in frogs? Journal of Animal Ecology, 87: 489-499.

Knutie, S. A., Wilkinson, C. L., Kohl, K. D., & Rohr J. R. (2017). Early-life disruption of amphibian microbiota decreases later-life resistance to parasites. Nature Communications, 8: 86.