Microbial associates living on multicellular hosts can strongly impact host health. The cutaneous microbial community can influence host health in amphibians exposed to Batrachochytrium dendrobatidis (Bd), a fungal pathogen that is a main cause of current amphibian declines. Bacterial species isolated from amphibian skin have been shown to confer disease resistance by inhibiting Bd growth. Resistance to Bd in amphibian populations is correlated with the presence of anti-Bd cutaneous microbes. Identifying anti-Bd cutaneous microbes for use in bioaugmentation strategies is increasingly important with the spread of Bd.
We aimed to determine if green frogs, an abundant species in New Jersey, harbored potential probiotic bacterial strains that inhibit Bd in vitro, and whether the presence of these microbes varied by site. We swabbed and cultured microbes from the skin of green frog adults and tadpoles at three sites that varied in pH. Environmental pH is known to be an important factor in structuring microbial communities, so we predicted that cutaneous microbial communities might also be affected by pH. Individual bacterial colonies were isolated and used in in vitro challenge assays to determine if bacteria isolated from green frog skin could inhibit the growth of Bd.
Tadpoles and adult green frogs at all sites harbored cutaneous microbes that completely inhibited Bd growth in vitro. Green frog adults and tadpoles at the near-neutral pH site harbored the largest number of unique isolates (ANOVA, p=0.08). At each site, tadpoles harbored a higher frequency of inhibitory isolates than adults, but the differences were not significant across sites (ANOVA, p=0.53). There was no difference in the frequency of inhibitory isolates among sites (ANOVA, p=0.11).
We were able to isolate cutaneous microbes from green frog skin with the ability to inhibit the growth of Bd, a deadly fungal pathogen. These bacteria could potentially be used as probiotics applied to the skin of susceptible individuals to help confer disease resistance. We also showed that sites that vary in pH do not differ in the frequency of inhibitory isolates present on amphibian skin. Green frogs may harbor cutaneous inhibitory isolates regardless of differences in environmental factors such as pH. Understanding how the cutaneous microbiome changes across sites and life stages will inform conservation strategies for protecting amphibians against Bd.