Amphibians have diverse bacterial communities on their skins that can protect against a global and devastating fungal pathogen, Batrachochytrium dendrobatidis (Bd). Knowing how these communities are assembled, maintained and function is important to understanding amphibian disease dynamics and to developing disease mitigation strategies. Here we use experimentation to 1) determine how immigration affects diversity, 2) uncover the mechanisms behind disease protection, and 3) develop hypotheses on how these communities assemble, maintain and function. To investigate the importance of immigration of environmental bacteria onto amphibians’ skins we experimentally exposed red-backed salamanders to a bacterial reservoir or no reservoir and then observed changes in bacterial community structure. To determine which bacteria were over-represented or under-represented, and therefore possibly selected for or against, we compared bacterial community composition to a neutral model. Bacteria found to be over or under represented were compared to a database of antifungal bacteria. We expect that these communities function by undergoing interference competition, i.e., the production of secondary compounds. To determine if these bacteria use interference competition, we grew different strains in mono-culture and co-culture and tested if their products inhibited the chytrid fungus. Bacterially produced metabolites were identified and emergent metabolites were tested against the pathogen.
Results/Conclusions
The diversity and structure of salamanders’ bacterial communities were greatly influenced by the presence of bacteria to colonize the host. If environmental bacteria (immigrants) were no longer available to colonize a salamanders’ skin, bacterial diversity greatly decreased and community structure became uneven. The bacterial communities on salamanders with a bacterial reservoir were not reflections of the environmental reservoir; some bacteria are over or under-represented compared to their environment. Over-represented bacteria were highly similar to antifungal bacteria, whereas under-represented bacteria were not. This demonstrates that some bacteria may be selected for and/or filtered from the environment. Interference competition between bacteria can result in the secretion of inhibitory metabolites, which can inhibit Bd and protect the host. Indeed, bacteria grown in co-culture resulted in additive or synergistic inhibition to Bd compared to mono-cultures. Furthermore, bacteria in co-cultures produced emergent antifungal metabolites. We hypothesize that these communities are first assembled by species sorting based on host characteristics; then the bacteria undergo interference competition that indirectly protects some amphibian hosts from disease. Current work focuses on whether immigrating environmental bacteria induce greater competition and result in greater protection. A future challenge will be determining why some bacterial communities are protective whereas others are not.