Bacterial Biofilms and Bd. Biofilm thickness and inhibitory traits reduce pathogen success

Microbial symbionts can provide protection against pathogens by preventing their establishment on the host. In many host-microbe systems (including, but not limited to: humans, plants, insects, and amphibians), there is a recurring relationship between host-associated microbial diversity and pathogen infection risk. Diversity may prevent infection by pathogens through sampling effects and niche complementarity-- but an alternative hypothesis may be that microbial biomass is confounded with diversity in widely-used 16S amplicon sequencing approaches, and that host-associated biofilms are deterring pathogen establishment through space pre-emption. In this study, we use the amphibian system as a model for host-microbe-pathogen interactions to ask two questions: (1) is bacterial richness confounded with biofilm thickness or cell density, and (2) does biofilm thickness or cell density deter the establishment of the amphibian fungal pathogen, Batrachochytrium dendrobatidis? To answer this question, we built a custom biofiom microcosm that mimics the host-environment interface by allowing nutrients to diffuse out of a fine-pore biofilm scaffolding. This creates a competitive environment in which bacteria and pathogen compete for colonization space. We then challenge bacterial biofilms ranging in community richness, biofilm thickness, bacterial cell density, and Bd-inhibitory metabolite production with live Bd zoospores to determine how Bd establishment success on membranes vary. We find that biofilm thickness and Bd-inhibitory isolate richness work in complement to reduce Bd establishment success. Our finding highlights the fact that diversity, as measured through 16S amplicon sequencing, may obscure the true mechanisms behind microbe-mediated pathogen defence, and that physical space occupation by biofilm-forming symbionts may significantly contribute to pathogen protection.