The skin of amphibians is one of their most important organs. Many amphibians breathe through their skin (this is known as “cutaneous respiration”) as well as absorbing water and ions through pores in their permeable skin. Toads and newts also often contain toxins in the granular glands of their skin. These granular glands can also contain antimicrobial peptides designed to prevent the growth of bacteria and fungi on the moist surface of their skin. But the skin of amphibians serves another function – it provides a home to many different species of bacteria. Some of these bacteria have entered into a mutualistic relationship with their amphibian hosts, where they prevent the growth of harmful pathogens on their hosts’ skin. One of the most well-documented examples of this comes from the bacteria Janthinobacterium lividum, a purple bacteria common in the soil and water of temperate regions. J. lividum has been isolated from the skin of amphibians, and was found to produce the antifungal metabolite violacein in the presence of the amphibian chytrid fungus. This bacteria has been successfully used to protect frogs from chytridiomycosis in the United States. The process of applying natural mutualistic bacteria to the skin of amphibian hosts as a defense against Bd is often called “bioaugmentation”. Bioaugmentation can also refer to the use of microorganisms to treat contaminated soil and water. Another interesting technique of bioaugmentation proposed to help combat chytridiomycosis is the introduction of zooplankton that consume Bd zoospores into the waters of areas where the chytrid fungus is already present in the environment. The reduction of infectious zoospores in the environment might help frog populations that live in the presence of chytrid to bounce back in numbers and potentially develop resistance against the chytrid. However, zooplankton that are not native to the area that they are introduced into may either die, or they could potentially cause harmful changes to the native aquatic communities.
When looking at ecological relationships, it is important to take the context of an interaction into account. The success of techniques such as bioaugmentation is extremely dependent upon environmental conditions. Many microbes only produce antifungal or antimicrobial metabolites at a certain environmental temperature or cell density. For this reason, bacteria, such as J. lividum, that are isolated in North America may not work in other countries with warmer climates, like South America and Australia. It is important to isolate and analyze skin bacteria from amphibian species in all climates to gain an understanding of the effects that environmental conditions might have on the ability of mutualistic bacteria to prevent chytridiomycosis in their hosts. It has already been shown that amphibians are more susceptible to population declines due to chytridiomycosis in cooler, higher elevation areas. This seems to suggest that bacteria in these areas may not produce as many antifungal peptides. However, as with many other relationships in ecology, this remains to be examined, and further research into the role of environmental conditions on amphibian resistance to the chytrid fungus is required.
Frog: 2, Fungus: 3