Mutualism of the month: Frogs and a protective bacterial complement

The natural world is occupied by species that are in a constant battle for survival against their environment and one another. Although it’s a dog eat dog world out there, sometimes organisms evolve alternatives to competition and work together with other species towards a mutual benefit and, by extension, survival. Such mutualisms come in all shapes and sizes, and exist among all known domains of life. Each month, we showcase one of these unusual relationships as FTDM’s Mutualism of the Month. This month’s mutualism is between the Panamánian golden frog Atelopus zeteki and its skin bacteria.

Animals host a variety of symbiotic microbes and many of these are vital to the host. These microbial communities consist of bacteria, fungi, and viruses ranging from mutualistic to parasitic. The composition and structure of these communities can have major impacts on the host. For some really cool mutualism examples check out the relationship between humans and their cancer-fighting mouth bacteria, a bacteria-virus duo and aphids, or the Hawai‘ian bobtail squid and their bioluminescent bacteria.

The highly poisonous and beautiful Panamánian golden frog. Photo source.

The skin of an amphibian is inhabited by a diverse complement of bacteria, some of which produce antimicrobial compounds that defend against skin pathogens. A particularly nasty amphibian skin pathogen is Batrachochytrium dendrobatidis (Bd), also known as the amphibian chytrid fungus. Bd is the causative agent of chytridiomycosis, responsible for devastating amphibian populations around the world and bringing many species near or to complete extinction. Bd is thought to have originated in Africa and then subsequently brought around the world through the African clawed frog pet trade. American bullfrogs are another common vector for the disease, acting as asymptomatic carriers; they aren’t actually affected by the disease, but are capable of infecting susceptible frogs. They often escape captivity and these carriers bring the fungus to native amphibian populations.

The cloud forests of western Panamá, the home of the Panamánian golden frog. Photo source.

One of the many species that has suffered greatly after the introduction of Bd to South America is the emblematic Panamánian golden frog, which is actually a toad despite its common name. Historically, this species was present in the cloud forests of west-central Panamá until Bd arrived. While beautiful, these golden frogs are also extremely toxic, containing a particularly potent version of tetrodotoxin (produced by another mutualistic bacterium). Falling victim to Bd, the endemic (found nowhere else in the world) Panamánian golden frogs, went extinct in Panamá. Fortunately, several of these toads were captured prior to Bd’s arrival and are now kept in captive-breeding programs. Unfortunately, reintroduction of these highly susceptible amphibians remains difficult since Bd remains in Panama hitching a ride on less susceptible amphibians.

Recently, a new strategy for reintroduction has been proposed, involving inoculating Panamánian golden frogs with probiotics (beneficial microbes). Probiotic therapies prevent disease, improve growth, and increase survival and are used in many other systems such as agriculture and aquaculture (farming marine organisms). The addition of the bacterium Janthinobacterium lividum, a species commonly found on the skin of North American amphibians, has been successfully used to prevent chytridiomycosis in several North American amphibians. Unfortunately, this treatment did not prevent chytridiomycosis in captive Panamánian golden frogs as the bacterium couldn’t maintain a population on the skin (likely due to competition from resident microbes).

Mass mortality of frogs due to chytridiomycosis. Photo source.

Despite these results, probiotics still represent a promising answer to this fungal problem. Research teams based all across the United States collected antifungal bacterial species from amphibians in Panamá. They found four candidate species that could compete with or kill Bd on Petri dishes and then experimentally determined which could prevent chytridiomycosis in captive Panamánian golden frogs. They inoculated some of the frogs with the bacteria and then infected them with the fungus and watched to see if those that had the probiotic treatment survived.

None of the bacteria worked.

28 days after treatment, the therapy bacteria were undetectable. While this is certainly bad news, there was a silver lining. Surprisingly, several individuals cleared the fungal infection despite losing the experimental probiotics. These individuals’ microbial communities were significantly different from those that didn’t clear the infection; they had a collection of bacteria that could clear the fungus! The frogs, where the infection cleared, started with a high level of six particular bacteria species, while those that died had elevated amounts of 17 other bacteria species.

A heat map of the relative abundance of different bacteria on frogs. Each column is a different frog and each row is a different bacteria species. A more yellow box means that there is a high level of a particular species of bacteria in that frog  and more blue means a lower level. Six species (top six rows) of bacteria are associated with frogs that cleared the fungal infection while 17 are associated with death of the frog. Photo source.

Currently the bacteria that may help fight infection and those that may facilitate infection are unidentified. Also unknown is how these six species may be helping to fight the infection. It may be that these species are competitively dominant over Bd or that they produce an antifungal molecule, similar to how our healthy gut bacteria protect ourselves.

Hopefully soon scientists will be able to identify the bacteria that help these toads survive infection so they can be used in probiotic treatment for this charismatic critter.

To read the original scientific article from the journal Proceedings of the Royal Society B: Biological Sciences click here.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

Read more mutualisms here!