Batrachochytrium dendrobatidis, the "frog chytrid"

Joyce E. Longcore

Department of Biological Sciences, University of Maine, Orono, Maine 04469-5722, USA

"Chytrid" is the common name used for a phylum of Fungi that is characterized by asexual reproductive spores (zoospores) that swim by means of a posteriorly directed flagellum. The phylum currently contains a single class, 5 orders, and about 120 genera. Based on the ultrastructure of its zoospores and on sequences from its 18S rDNA, Batrachochytrium is classified in the largest order, the Chytridiales. It is the only genus of chytrids known to contain a species that is pathogenic to vertebrate animals.

On nutrient agar a zoospore of B. dendrobatidis retracts its flagellum and forms a wall. This encysted zoospore produces rhizoids, enlarges and within 4 days becomes a mature zoosporangium. The entire content of the zoosporangium cleaves into zoospores, which are released through one or more discharge papillae and continue the cycle. Alternatively, one to several septae may form in the young thallus; each cell thus formed continues as above to produce a zoosporangium. Stages found in pure culture are analogous to circular, vase-shaped and septate profiles that characterize the fungus in stained sections of amphibian skin.

Neither morphological nor cultural differences have been noted among isolates of B. dendrobatidis from 12 species of amphibians. Growth occurs slowly at 6°C, 23°C is the optimum growth temperature, and pure cultures in broth die after prolonged exposure to 30°C. No resting stages have been found.