Skin secretions protect some salamanders from lethal fungal infection

Abstract

Amphibian populations are crashing worldwide and when the IUCN published the list of 100 most endangered species in 2012, nine amphibians were teetering on the brink of extinction. Threatened by loss of habitat, invasive species and toxins in the environment, amphibians are also at risk from a disease, chytridiomycosis, when infected by either Batrachochytrium dendrobatidis or Batrachochytrium salamandrivorans fungi. However, some species may be able to withstand the lethal fungi. ‘There is growing evidence that antimicrobial substances secreted onto the skin surface prevent chytridiomycosis by limiting the burden of infection’, says Kenzie Pereira from Duquesne University, USA. Yet, little was known about the vulnerability of aquatic salamanders to B. salamandrivorans, so John Pojman, Sr, from Louisiana State University, USA, went fishing for three-toed amphiuma (Amphiuma tridactylum) salamanders with bated minnow traps in a Baton Rouge bayou.Back in the lab, Pereira gently prodded the virtually legless amphibians to collect their skin secretions ready for Damien Wilburn at the University of Washington, USA, to analyse. Wilburn discovered that the major component of the secretions is made up of proteins, which are probably antimicrobial. Meanwhile, Pereira tested the effects of the secretions on the growth of the two fungi in test tubes and she was impressed to see that the secretions completely killed off B. dendrobatidis at concentrations of 75 μg ml−1, while the B. salamandrivorans fungus succumbed completely at 187 μg ml−1.Having confirmed the potency of the salamanders’ skin secretions, Pereira, Pojman and colleagues Brian Crother, David Sever and Clifford Fontenot, Jr, from Southeastern Louisiana University, USA, scrutinised the distribution of the skin glands that produce the antimicrobial and mucous skin secretions. Collecting small sections of skin from museum specimens – ranging from a 72-year-old preserved animal to more-recently collected samples – the team found that there were far more mucous glands than granular glands in the salamander skin, with the highest number of mucous glands occurring around the head. However, the granular glands, which produce the antimicrobial secretions, occurred mainly around the base of the salamander's tail, where they were larger than the granular glands found in other parts of the animal's body. In addition, the researchers found the largest granular glands in a male that had been caught during the mating season, suggesting that the glands may contribute to courtship and communication.Having confirmed that the skin secretions of three-toed amphiuma are capable of killing off lethal B. dendrobatidis and B. salamandrivorans fungi in the lab, Pereira says, ‘The big take-home message from this research is that antimicrobial skin secretions may protect some species of salamander from chytridiomycosis caused by B. salamandrivorans’. In addition, she and PI Sarah Woodley, also from Duquesne University, are keen to test the antifungal powers of the skin secretions of other salamanders to find out how well they may be able to protect themselves from these fungal scourges.