Frogs, toads, salamanders and other amphibians are disappearing as fast as -- or faster than -- any other class of animals around the world. One reason is the fungus known as Batrachochytrium dendrobatidis, or Bd, an infectious skin disease that has contributed to significant declines of amphibian populations around the world.
A new study shows amphibians now have to fight off an additional threat: antibiotic pollution in their waterways that increases the threat of Bd.
Normally, Bd doesn't thrive in ponds -- the fungus has to compete with microbes that are seeking their own ecological niches. Scientists have hypothesized that chemical mixtures produced by these microbes could act like a vaccine against Bd.
"When there's high competition, particular microbial species like these generate compounds that inhibit other bacteria and microorganisms from succeeding," said first author Jessica Hua, a UW-Madison professor of forest and wildlife ecology.
With antibiotic use and resistance growing, Hua and her colleagues wanted to examine the effect of polluted waterways on Bd.
For the study, published in Scientific Reports, the researchers exposed Bd to two separate chemical cocktails produced by the common bacterium Pseudomonas aeruginosa -- one that had adapted to living in water contaminated by antibiotic drugs; and one that had not encountered the fungus yet. They also raised tadpoles in water with Bd and both chemical cocktails.
The study results showed the chemical mix from Pseudomonas that hadn't been exposed to antibiotics was good for amphibians and bad for the fungus. The bacteria reduced the fungi's growth rate significantly in lower concentrations and entirely in large doses. Tadpoles were also less likely to be infected with Bd while living in this water.
On the contrary, chemicals from Pseudomonas that had adapted to antibiotic pollution helped Bd grow faster. Tadpoles in this water were 6x more likely to be infected with Bd than their clean water counterparts.
However, the results completely flipped when the researchers repeated their experiments with strains of Pseudomonas that clump together in colonies called biofilms -- much to the team's surprise. This time, the chemical cocktail from biofilm-forming Pseudomonas that had not encountered antibiotics was worse for the tadpoles and better for Bd. Meanwhile, the antibiotic-tolerant, biofilm-forming bacteria inhibited Bd growth and kept tadpoles healthier longer.
"This shows us that if we're going to develop some sort of treatment for an emerging disease this way, we have to consider the history of the microbes we're looking at," said Hua. "The traits, the effect on amphibians, could very easily change. Here we've seen them change from help to harm, especially in light of increasing pollution. There are so many considerations -- ignoring them may do more harm than good."