Effect of captivity and water salinity on culture-dependent frog skin microbiota and Batrachochytrium dendrobatidis (Bd) infection

ABSTRACT The world is undergoing a sixth mass extinction, and the drastic decline of amphibians in the last several decades is a major contributing factor. The spread of the deadly Batrachochytrium dendrobatidis (Bd) fungus is a major causative agent, and captive breeding programs are under way to try and save endangered populations. However, how captivity affects Bd infection and the skin microbiota is not clear. We identified the skin bacteria of the South Australian frog Crinia signifera and showed that their culture-dependent skin microbiota is moderately stable in cohorts sampled from the same location in subsequent years. Their microbiota was also structurally different to that of a sympatric frog species, Limnodynastes tasmaniensis, indicating a specific microbial signature. Twenty-four C. signifera were placed into captivity for four weeks, exposed to a range of water salinity levels, during which time there was a significant reduction in identifiable skin bacterial species diversity, richness, and a change in microbial structure. The infection intensity of Bd was also significantly reduced in captivity, with some frogs becoming completely clear of infection, whilst water salinity level was not a significant determinant of Bd infection or skin microbiota.