Picture an amphibian. Do you think of the wonderful
diversity of the class? Do you think of their unique colors? Or how about their
ability to respire through their skin? When I think of amphibians, I think of
all of these things. I also think of their vulnerability to endangerment.
Amphibians boast all kinds of cool features, but these features also contribute
to their vulnerability. Amphibians are among
some of the most vulnerable organisms to extinction for a variety of reasons.
Habitat fragmentation and destruction, invasive species, pollutants, and
over exploitation are some of the factors that are contributing to the worldwide
decline of amphibians (Bernstein and Chivian 205-206). Recently, estimates have
projected amphibian numbers to have declined by about one-third (Hussain and
Pandit, 2012)! Clearly, many factors play into their decline, but perhaps most
concerning is amphibians’ susceptibility to pathogenic infections
(Houston et al., 2005).
One pathogenic infection, is a fungal infection called
chytridiomycosis, is causing global decline in amphibians (Houston et al., 2005).
Chytridiomycosis is a dermal infection caused by the chytrid fungus Batrachochytrium dendrobatidis. The map
below shows the prevalence of chytridiomycosis in amphibians. The map marks,
with balloons, cases in which amphibians have been found to contain the fungus.
As you can see, the infection is found on a global scale. This suggests that B. dendrobatidis can infect almost all
amphibians around the world (Hussain and Pandit, 2012). Is there a way to
help stop the spread of fatal infections such as chytridiomycosis? What if we
could help amphibians mount a defense simply by examining the bacterial
communities that live on the surface of their skin? Well this may just be
possible!
I came across an interesting article while reading an issue
of American Scientist. The article detailed the research of Reid Harris, of
James Madison University, and Vance Vredenburg, of San Francisco State
University. Harris and Vredenburg discovered that a bacterium called Janthinobacterium lividum, which lives
on the skin of amphibians located in North America, Central America, and other
continents, may combat the chytrid fungus. Laboratory, and field trials, showed
that the amphibians inoculated with the J.
lividum bacterium produced a substance called violacein which helps
neutralize the growth of the chytrid fungus (Burke 190). I don’t know about
you, but this sounds like a great option to help reduce amphibian deaths from
chytridiomycosis! As you can imagine, there are some problems associated
inoculating amphibians with J. lividum.
As you can see from the map above, amphibian diversity is
vast. Found in almost all countries around the world, and many different
ecosystems, how do you choose what species to inoculate? For example, the species
of amphibians in the Amazonian Rainforest, around 60-140 species, might not
respond to the same inoculations as species found in Madagascar, around 15-30
species. What about the possible negative impacts that inoculating an ecosystem
with the J. lividum bacterium might
have? What about other probiotics that might have positive effects on other
kinds of amphibian based infections (Burke 190-91)? I think it’s critical to
keep questions like these in mind lest we exacerbate the problem. Luckily,
Harris and Vredenburg have thought about these questions. They propose that
ecosystems being inoculated with probiotics must have native species that
contain the probiotics in their microbiome. This would in turn help reduce the
risk of any ecosystem problems that might arise (Burke 191).
Luckily, many amphibians are able synthesize their own
defenses against pathogens. Amphibians have glands that create skin peptides
which serve as a natural immunity to these pathogenic bacteria (Houston et al.,
2005). So, if amphibians contain natural defenses against peptides, what is the
point of inoculating them with other probiotics to assist in defense? Well, the
figure below describes this.
Photo Credit: Houston et al., 200
The graph above shows a species of amphibian, Xenopus
laevis, which was exposed to norepinephrine. The norepinephrine stimulus
caused a release of the natural defensive peptides. The y-axis represents the
peptide concentration found on X. laevis, and the x-axis represents
the days following peptide depletion. As you can see, following exposure to
norepinephrine, peptide concentration levels remained low for a period of 3
weeks. This suggests that the amphibians’ natural defense is inhibited after a
defensive response to pathogenic bacteria. It takes the amphibians a period of
longer than 3 weeks to synthesize enough peptides to again mount a successful
defense against possible infections (Houston et al., 2005).
I
think that this provides good evidence that the inoculation of amphibians with
probiotics is a good idea seeing as how immune defense takes extended periods
of time to re-establish itself. Amphibians have a great initial resistance to
pathogens, but what about continued exposure? This is why the search continues
to find a suitable probiotic to administer to different ecosystems. A recently
published paper shows that the family of Plethodontid Salamanders
(shown below) of the genus Speleomantis have a natural
resistance to the chytrid fungus (Pasmans et al., 2013).
This
family of Speleomantis salamanders,
found in Europe, had a presence of B. dendrobatidis found in their natural
habitats. However, of the 921 skin swab samples, none of the salamanders were
affected by the chytrid fungus. Following this, the researches inoculated
individuals with the B. dendrobatidis
chytrid fungus. After a period of 2 weeks, no chytrid fungus was found to grow
on the salamanders (Pasmans et al., 2013). They found that the salamanders
actually contained the ability to kill off the introduced fungus.
Photo
credit: Pasmans et al., 2013
This graph shows the
salamanders’ innate ability to kill off the B. dendrobatidis fungus.
On the y-axis is reduction of the genomic equivalents (GE) of B.
dendrobatidis on a logarithmic scale. The x-axis lists the different
species of salamanders from the family Speleomantis that were
inoculated with the bacterium (Pasmans et al., 2013). Seeing as none of these
salamanders were affected by the chytrid fungus, I think that they may be a
viable candidate in which to synthesize a probiotic from.
If the microbiome of
these salamanders was further studied, I think it might be possible to isolate
the bacterium that can help these salamanders neutralize B.
dendrobatidis. This bacterium could then be used in a probiotic solution
that can be introduced to some amphibians. Proper precautions would need to be
taken in order to insure that there would be no negative effects associated
with the inoculation.
This kind of research gives
me hope to the future of amphibians. With such large declines, amphibians can
use any kind of assistance offered. I believe that by reducing the effects of
chytridiomycosis on amphibians, we can begin to focus on other causes of
decline like habitat destruction. Now let’s go save those amphibians!
Works Cited
"AmphibiaWeb." AmphibiaWeb. N.p., n.d. Web. 28 May 2013.
Burke,
Katie L. "Probiotics for Frogs." American Scientist May 2013: 190-92. Print.
Chivian,
Eric, and Aaron Bernstein. "Threatened Groups of Organisms Valuable to
Medecine." Sustaining Life: How Human Health Depends on Biodiversity. Oxford: Oxford UP, 2008. N.
pag. Print.
Houston,
Laura E., Louise A. Rollins-Smith, Laura K. Reinhert, and Chadrick J. O'Leary.
"Antimicrobial Peptide Defense in Amphibian Skin." Integr. Comp. Biol. 45 (2005): 137-42. Web. 27 May
2013.
Hussain,
Qazzi A., and Ashok K. Pandit. "Global Amphibian Declines: A Review." International Journal of
Biodiversity and Conservation 4.10 (2012): 348-57. Web. 27 May 2013.
Pasmans,
F., P. Van Rooij, M. Blooi, G. Tessa, and S. Bogaerts. "Resistance to
Chytridiomycosis in European Plethodontid Salamanders of the Genus
Speleomantes." PLOS ONE (2013): n. pag. PLOS ONE. 20 May 2013. Web. 27
May 2013.
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