Zoology/Biology session

Chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd for short), is a highly impactful amphibian disease that has already contributed to the extinction of at least 90 species in the last century. The severity of the epidemics varies from region to region, due to different virulence of locally distributed Bd lineages and differences in the immune defenses of amphibian populations. Chemical defenses are the first line of defense against pathogens in amphibians. A common method among toads of the family Bufonidae is the secretion of bufadienolide compounds. Significant differences in toxin production have been found not only between toad species, but also between populations. Bufadienolides may also be effective against Bd, but few studies have investigated this so far, and it is not known whether the toxin mix of toad populations coexisting with Bd is more effective against locally distributed Bd strains than against Bd strains found in other regions.

To answer this question, and to investigate the sensitivity of Bd to the toxins produced by toads, we examined the growth inhibition effect of toxin samples collected from common toad populations in 5 European countries on Bd strains isolated on the same localities in an in vitro experiment using a full factorial design. We determined the number of bufadienolide components and the total bufadienolide quantity in the toxin samples using HPLC-MS, then treated the liquid Bd cultures with the toxin samples at different concentrations and determined the extent of growth inhibition by measuring the optical density of the cultures.

Our results provide clear evidence that bufadienolide toxins inhibit Bd growth and show that the degree of inhibition depends on the origin of the toxin sample and the Bd isolate. We did not find a pattern that for a given toxin mix the inhibition was always the greatest against the Bd isolate from the same locality, nor that the tolerance of a Bd isolate was always highest against the toxin mix from the same region. However, we have shown that the inhibition is on average lower in case of toxin mix-Bd pairs of the same origin than in case of pairs not of the same origin. This may be due to the adaptation of Bd strains to better tolerate the toxins of local toad populations. We plan to investigate the effectiveness of the individual bufadienolide compounds to find out what causes the differences in the inhibitory capacity of toxin mixes and the sensitivity of Bd strains.