The community of bacteria that naturally occurs on the skin of frogs could help them battle a deadly fungal infection, according to new work published in PLOS One this month.
The fungus Batrachochytrium dendrobatidis (Bd) has caused the collapse of hundreds of species of frogs and salamanders. The pathogen causes a devastating infection called chytridiomycosis: The skin grows several times thicker than normal, affecting the animal’s ability to regulate water and ion exchange. However, amphibian skin hosts a diverse community of resident bacteria, and some of these may help inhibit Bd.
To see if skin microbiome alters disease outcome, a team led by Virginia Tech’s Jenifer Walke collected 60 juvenile bullfrogs (Lithobates catesbieanus) during August of 2010 from a pond in Giles County, Virginia, where the fungus had previously been detected. These bullfrogs showed low levels of Bd infection; this species was picked because they’re generally not considered susceptible to chytridiomycosis. Each bullfrog was swabbed in the field to see what their initial microbiota looks like.
Back in the lab, some of the bullfrogs were left alone (as controls), while others had their skin microbiota manipulated in one of two ways: Antibiotics reduced the resident microbes, while anti-chytrid probiotics (previously isolated from a four-toed salamander) augmented the microbes. The bullfrogs were then either exposed or left unexposed to a lethal strain of Bd from Panama. The team then tracked the responses of the host and skin bacterial community. While frogs exposed to Bd grew less than those who weren’t, no frogs died during the 42-day experiment.
Antibiotics didn’t significantly change the rate of fungal infection, they found, and probiotics didn’t decrease fungal infection either. The naturally occurring skin microbiota, on the other hand, responded to infection by adjusting community structure and function to compensate. These actions include the production of molecules necessary for metabolism (called metabolites) that likely inhibit the colonization and development of spores. "It turned out that in this experiment, it wasn't about a single bacterium being protective,” Walke said in a statement. “Rather the structure of the whole community was important in infection and frog health.”
Since each amphibian species harbors distinct bacterial communities, figuring out why some develop protective microbiota while others don’t will be the crucial next step. These factors can range from habitat and diet, to the availability of microbe species.