Assessing the ability of environmental bacteria to inhibit the growth of Batrachochytrium dendrobatidis

Abstract

Amphibians have been facing devastating declines through the decades and it wasn’t until 1998 that the once enigmatic declines of amphibians were attributed to Batrachochytrium dendrobatidis. This lethal fungal pathogen is responsible for causing the disease chytridiomycosis and with its variation in virulence across lineages, environments and host susceptibility, scientists have been fighting to find a cure for this lethal disease. Through several defence mechanisms, including an adaptive and innate immune system, amphibians have been battling to protect themselves not only against chytridiomycosis but also against other pathogens including bacterial infection. Their adaptive immune response can clear an infection caused by a pathogen while the innate immune system provides a non-specific response against a pathogen and this includes antimicrobial peptides produced by the host as well as the colonization of protective microbes on the skin of amphibians. Research has proved that there is a wide range of bacteria genera that have various effects on the growth of B. dendrobatidis (Bd) by either inhibiting or promoting the growth of this pathogen. With two known Bd lineages in South Africa, namely the Global Panzootic Lineage (GPL) and the CAPE lineage, that differ in virulence, it was imperative to determine the relationship between bacteria and these lineages, but also to determine how bacteria colonize on the skin of adult frogs, and the mouthparts of tadpoles from various species from a wild frog community in a protected area. Molecular analysis revealed a uniqueness to the tadpole microbiome diversity with representative bacteria that was not found on corresponding adults as well as variation in microbiome structure across various species. A further variation in bacteria was observed between bacteria sampled from the environment and bacteria sampled from amphibians. A challenge assay study with 20, environmentally sampled, bacterial isolates’ cell-free supernatant were conducted to determine the effects that individual bacterial isolates have on the two Bd lineages known from South Africa. After it was discovered that individual isolates had either an inhibiting or augmenting effect on the growth of Bd a consortium of bacteria were assembled and challenged against Bd. Results obtained indicated that with a consortium of bacteria the effects on Bd are significantly different than when bacteria were challenged individually against Bd. The results obtained may explain why some amphibian species are more susceptible to diseases, but it also indicates that a lot of work is required to determine why there is a variation in virulence across Bd lineages.