A new study suggests that naturally occurring bacteria on the skin of salamanders could help protect other amphibians, including some species of endangered frogs, from a lethal skin disease. The researchers from James Madison University, Harrisonburg, Virginia and Vanderbilt University, Nashville, Tennessee report their findings in the November 2009 issue of the journal Applied and Environmental Microbiology.
Batrachochytrium dendrobatidis is a fungal pathogen that can cause a lethal skin disease in amphibians, however, some species remain relatively symptom free during infection. Innate immune factors, antimicrobial peptides, skin-associated microbial species, and behavior are all believed to attribute to the survival of some species over others. Researchers have found antifungal microbes to be of particular interest because their presence suggests they are mutualistic associates of amphibian species, meaning that there is a mutually beneficial relationship between the two organisms.
In a prior study Janthinobacterium lividum was identified as a bacterium that produces the anti-B. dendrobatidis metabolite violacein. Violacien was found on three of seven wild-collected red-backed salamanders (Plethodon cinereus) at concentration levels capable of inhibiting B. dendrobatidis, indicating a mutualistic community of violacein-producing bacteria. In this study researchers added J. lividum to the same species of red-backed salamanders and then exposed them to B. dendrobatidis. Results showed that adding J. lividum to the skin of the salamander increased the concentration levels of violacein already present and contributed to survival following experimental exposure to the fungus.
"Our study suggests that a threshold violacein concentration of about 18 μM on a salamander's skin prevents mortality and morbidity caused by B. dendrobatidis," say the researchers. "We show that over one-half of individuals in nature support antifungal bacteria that produce violacein, which suggests that there is a mutualism between violacein-producing bacteria and P. cinereus and that adding J. lividum is effective for protecting individuals that lack violacein-producing skin bacteria.
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