The amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), has been linked to amphibian population declines. Bd has two life stages; a sessile zoosporangium and motile zoospore. The motile zoospores find a host, encyst in their skin, and develop into zoosporangia, which release new zoospores. Optical density is a fast and convenient method often used to measure Bd growth. Optical density measures everything in the samples including both life stages of Bd, dead or alive. This makes it difficult to understand Bd growth from optical density measurements. Other growth measurements include zoospore counts and MTT assays. Zoospore counts are labor-intensive but provide detailed data. MTT assays are similar to optical density but give a measure of Bd viability. With zoospore counts and MTT assays, we aim to further our understanding of Bd optical density measurements and how temperature affects optical density measurements. We grew Bd in the lab at 6 different constant temperatures (4C – 28C) and measured optical density daily and zoospore counts with MTT assays every 2-3 days. The optical density, MTT, and zoospore count data were then fit with Bayesian hierarchical models. We sampled the model parameters’ posterior distributions to examine correlations between and the temperature sensitivity of the parameters.
Results/Conclusions
We found that optical density measurements generally reflect zoosporangia growth patterns. Using the different types of growth data taken during the same treatments we were able to compare the methods to each other. The maximum zoospore counts occurred early on in the optical density growth data. The highest MTT measurements were after the peak zoospore counts and approximately midway through the optical density growth pattern. Some of these patterns were not conserved over temperature due to the temperature sensitivity of the zoospore and zoosporangia rates. As the optical density growth data reaches a stationary phase, indicating no more growth of Bd, we see MTT start to decrease. However, this drop in MTT is temperature dependent, indicating that at optimal temperatures it takes Bd more time to die off. Using zoospore counts and MTT assays to better understand optical density measurements provides more of an insight into this fast and easy methodology.