Amphibian microbiome: the first line of defense against chytrid infection

Abstract

Emergent infectious diseases, in particular those caused by fungi, have recently been recognized as a major threat to wildlife. Within this context, the global amphibian population declines and extinctions caused by the disease chytridiomycosis provide a great example of the devastating effects of a fungal emerging infectious disease. Chytridiomycosis is an amphibian skin disease that is caused by the fungi Batrachochytrium dendrobatidis (Bd), whose negative effects on amphibian populations have been caused by a single highly virulent genotype, the global panzootic lineage (Bd-GPL). Interestingly, recent studies have identified less virulent endemic genotypes of Bd with potentially restricted distribution ranges. For instance, in the Brazilian Atlantic Forest the endemic genotypes can co-occur and, in some cases, reproduce with the highly virulent Bd-GPL. Hence, the presence of multiple co-occurring genotypes of Bd with different degrees of virulence in the Brazilian Atlantic Forest provides a unique opportunity to determine the potential mechanisms allowing genotypes co-existence and their influence on disease dynamic. Recent work on host-associated bacterial communities have revealed that amphibians harbor diverse and host-specific communities in their skin, whose members, in some cases, can inhibit the growth of Bd in in vitro challenge assays. Consequently, interest has been generated in the development of a probiotic-based approach using amphibian skin bacteria to mitigate Bd infections in the wild. As a result of a substantial amount of research exploring the potential interaction between the amphibian skin bacterial communities and Bd, it has been determined that infections by Bd can alter the structure of these communities, and that these bacterial communities can influence the susceptibility of the amphibian host to Bd. Moreover, current evidence suggests that the ability of some bacteria to inhibit Bd growth in in vitro challenge assays can vary across Bd genotypes and strains of the Bd-GPL. However, studies assessing the variation in the interaction between the amphibian skin bacteria and distinct Bd-genotypes in the wild are still lacking. In the proposed study, using our current understanding on the genetic structure of Bd across the Brazilian Atlantic Forest, we aim to determine at a fine scale (i.e. site-level) the pattern of infection of the different co-occurring Bd genotypes (Bd-GPL, Bd-Brazil and hybrids) across habitats and amphibian host-species. In addition, considering the variation in virulence across the Bd genotypes, we aim to determine whether these genotypes differ in the degree to which they can interact with the amphibian skin bacterial communities.