Mitigation of chytridiomycosis on <i>Bombina variegata</i> tadpoles with the antimicrobial metabolites of the bacterium <i>Xenorhabdus szentirmaii</i>

Survival of amphibian assemblages is threatened by many factors. Among them, chytridiomycosis, the disease caused by the chytrid fungus <i>Batrachochytrium dendrobatidis</i> (Bd), has great importance, also threatening populations of the yellow-bellied toad (<i>Bombina variegata</i>), which has a scattered, isolated distribution in Hungary. Treatment with secondary metabolites of the entomopathogenic bacterium <i>Xenorhabdus szentirmaii</i> is a promising method in the mitigation of chytridiomycosis, as recently demonstrated. To assess whether it may also be effective for lowering infection intensities in <i>B. variegata</i> tadpoles, we extracted cell-free culture media (CFCM) from liquid cultures of <i>X. szentirmaii</i> and tested their antifungal efficacy at a dilution of 0.1% (v/v), while also measuring possible short-term malign effects on tadpoles experimentally infected with Bd or sham infection. According to our results, CFCM treatment alone did not compromise tadpoles' survival probability, nor reduce the body mass and developmental rate of the individuals. At the same time, the treatment reduced the intensity and prevalence of Bd infection, but only in the case of one population out of the four tested. The antimicrobial metabolites produced by <i>X. szentirmaii</i> may therefore be suitable for the safe mitigation of chytridiomycosis in <i>B. variegata</i> tadpoles, but further studies are needed, aiming to validate and increase the efficacy of this method.IMPORTANCEThe importance of the study is that it offers a potential new microbial tool in the fight against chytridiomycosis, a major threat to amphibian populations worldwide, including the yellow-bellied toad in Hungary. The research tested a promising method using bacterial metabolites of the bacterium <i>Xenorhabdus szentirmaii</i> to combat the fungal chytrid pathogen. This treatment was safe for tadpoles and capable of reducing infection prevalence. While the results are encouraging, there is a need for further research to improve the treatment's effectiveness. This method could help protect vulnerable species like the yellow-bellied toad from the impacts of chytridiomycosis, which is among the most challenging issues in amphibian conservation.