The potential of using biosynthetic metabolites of entomopathogenic bacteria in the mitigation of the amphibian disease chytridiomycosis

Chytridiomycosis, caused by the chytrid fungus <em>Batrachochytrium dendrobatidis</em> (Bd), has caused extreme losses in amphibian biodiversity. Finding bacteria that produce metabolites with antifungal properties for the purpose of disease mitigation may turn out to be invaluable in the fight against chytridiomycosis. The entomopathogenic bacteria, <em>Xenorhabdus szentirmaii</em> and <em>X. budapestensis</em> produce secondary metabolites that are effective against a wide range of fungal plant pathogens. To assess whether they may also be effective against Bd, we extracted cell-free culture media (CFCM) from liquid cultures of <em>X. szentirmaii</em> and <em>X. budapestensis</em> and tested their ability to inhibit Bd growth <em>in vitro</em>. As a second step, we experimentally infected juvenile common toads (<em>Bufo bufo</em>) with Bd and also tested the <em>in vivo</em> disinfecting efficacy of <em>X. szentirmaii</em> CFCM diluted to 2 and 10% (v/v), while also assessing possible malign side effects on amphibians. Minimal inhibitory dilutions by CFCMs of both <em>Xenorhabdus</em> species were between 0.78 and 1.58% (v/v). The <em>in vivo</em> experiment showed that treatment with CFCM of <em>X. szentirmaii</em> applied at a dilution of 10% resulted in infection intensities reduced by ca. 73% relative to controls and to juvenile toads treated with CFCM applied at a dilution of 2%. At the same time, we detected no negative side effects of treatment with CFCM on survival, body mass, and skin microbial community of juvenile toads. Our results clearly support the idea that <em>X. szentirmaii</em> metabolites may prove safe and highly useful in the fight against chytridiomycosis, and that future research following this promising avenue has high potential of resulting in an effective, safe, and cheap method of Bd disinfection.