Characteristics of monolayer formation in vitro by the chytrid Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis is a globally distributed generalist pathogen that has driven many amphibian populations to extinction. The life cycle of B. dendrobatidis has two main cell types, motile zoospores, and sessile reproductive sporangia. When grown in a nutrient-rich liquid medium, B. dendrobatidis forms aggregates of sporangia that transition into monolayers on surfaces and at the air-liquid interface. Pathogenic microorganisms use biofilms as mechanisms of group interactions to survive under harsh conditions in the absence of a suitable host. We used fluorescent and electron microscopy, crystal violet, transcriptomic, and gas chromatographic analyses to understand the characteristics of B. dendrobatidis monolayers. The cell-free monolayer fraction showed the presence of extracellular ribose, mannose, xylose, galactose, and glucose. Transcriptome analysis showed that 27%, 26%, and 4% of the genes were differentially expressed between sporangia/zoospores, monolayer/zoospores, and sporangia/monolayer pairs respectively. In pond water studies, zoospores developed into sporangia and formed floating aggregates at the air-water interface and attached film on the bottom of growth flasks. We propose that B. dendrobatidis can form surface-attached monolayers in nutrient-rich environments and aggregates of sporangia in nutrient-poor aquatic systems. These monolayers and aggregates may facilitate dispersal and survival of the fungus in the absence of a host. We provide evidence for using a combination of plant-based chemicals, allicin, gingerol, and curcumin as potential anti-chytrid drugs to mitigate chytridiomycosis.