Salamanders wield microbial shield
Some salamander populations may have an invisible microbial shield protecting them from the deadly chytridiomycosis disease that is currently devastating amphibian populations worldwide, according to new research presented at the 2013 Ecological Society of America conference in Minneapolis, MN, in August.
Researchers at the University of Maryland hypothesized that salamander declines along the US east coast may be due to Batrachochytrium dendrobatidis, the fungus that causes chytridiomycosis. However, field surveys revealed that only 1% of the salamanders tested harbored B dendrobatidis, even though they are susceptible to the fungal infection in lab experiments. The researchers therefore shifted their focus to determining whether something in nature was protecting the salamanders from infection.
Carly Muletz, a PhD student at the University of Maryland, presented initial results comparing the skin microbiome composition – the cutaneous bacterial communities – of two species of terrestrial salamanders (Plethodon cinereus and Plethodon cylindraceus) in the Shenandoah, Catoctin, and Mt Rogers National Parks in the Appalachian Mountains of the eastern US.
To identify antifungal bacteria, Muletz pursued two approaches. First, she swabbed the salamanders' skin and cultured bacteria to determine which individuals could inhibit B dendrobatidis. Then she amplified the 16S rDNA sequences – the conserved genetic region of prokaryotic ribosomes – from the skin swabs to identify and characterize the entire skin bacterial community, a technique referred to as metagenomics.
In total, Muletz identified 131 strains of bacteria capable of inhibiting B dendrobatidis. Interestingly, she found that salamanders in Shenandoah National Park contained the most antifungal bacteria (on average, four strains each). Furthermore, 96% of Shenandoah salamanders had at least one B dendrobatidis-inhibiting bacteria, whereas less than 50% of salamanders at Catoctin and Mt Rogers harbored any. “Something is happening at Shenandoah National Park, but I'm not yet sure what”, admits Muletz.
It's not clear how the diversity of microbial communities relates to disease – does disease lower microbial diversity? Or does having lower diversity predispose a salamander to disease? These are questions increasingly being asked by wildlife biologists.
“Pairing culturing work with metagenomics is a spectacular way to get at how microbes are involved in disease resistance”, says Valerie McKenzie, an amphibian ecologist at the University of Colorado Boulder, who is also exploring the importance of amphibian skin microbes in fending off disease. “It could be that the environment is shaping the microbiome, which shapes the organisms' response to disease”, she suggests.