Development of an assay for testing the antimicrobial activity of skin peptides against the amphibian chytrid fungus (Batrachochytrium dendrobatidis) using Xenopus laevis

This report describes the preliminary characterization of a bioassay for testing the antimicrobial activity of amphibian skin peptides against the chytrid fungus, Batrachochytrium dendrobatidis. Peptide secretions from Xenopus laevis were induced by subcutaneous injections of norepinephrine. Partially purified secretions were quantified and incubated at various dilutions with 107 cells/mL of freshly isolated zoospores for 7 days. Peptide bioactivity was measured as cell growth inhibition over the incubation period. The concentration that inhibited growth by 80% or greater (IC80), based on the linear portion of the growth curve, averaged 457±158 μg/mL. Growth curve slopes of best-fit line equations for individual samples were less variable within control groups than the average IC80 value, and are viewed as a more reliable indicator of peptide mixture bioactivity. This assay may be useful for evaluating the impact of environmental chemicals on amphibian host resistance to potentially lethal skin infections.