Dissertation Research: Associations between Symbiotic Bacterial Communities and Infection by an Emerging Fungal Pathogen: Distinguishing Cause from Correlation

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Project Summary: Symbiotic microflora (communities of microbes living on or in multicellular organisms) are important players in the health of multicellular organisms. Variation in the microbial species inhabiting the human body has been linked to differences in susceptibility to diseases from obesity to salmonellosis. In contrast, little is known about the microflora of non-human animals or plants. This study assesses how symbiotic microflora affect disease resistance in the mountain yellow-legged frog (Rana sierrae). Previous results show that differences in the microbial communities found on the skin of these frogs are linked to variation in the severity of chytridiomycosis, a disease of the skin that results from infection by the fungal pathogen, Batrachochytrium dendrobatidis. The current study investigates the causes responsible for these patterns by experimentally manipulating the microbial communities living on the skin of R. sierrae, subsequently exposing frogs to B. dendrobatidis, and measuring resulting disease progression. This experiment addresses three questions: (1) Which factors control the identity of microbes that make up the normal microflora? (2) Do differences in the species composition of the microflora affect the ability of the frogs to resist infection by B. dendrobatidis? (3) When infection by B. dendrobatidis does occur, does it disturb the symbiotic microbial community, leading to changes in the microbial species that make up the normal microflora? Results will advance fundamental understanding of the factors shaping symbiotic microbial communities and their importance in health and disease. Broader impacts: Amphibians worldwide are undergoing mass declines and extinctions, and chytridiomycosis is one of the greatest threats to these animals. This study is relevant to the management of chytridiomycosis and the conservation of amphibians. Laboratory and field trials are currently underway to test the use of probiotic bacteria to protect amphibians from the disease, yet basic knowledge about the symbiotic communities naturally occurring on threatened amphibians and their function in disease resistance is limited. Such knowledge will be improved by this study. In addition, this project provides a critical training opportunity for the graduate student and for an undergraduate assistant who will gain research experience through integral participation in the study.