Does Sloth Fur Fungus Hold the Next “Wonder Drug?”
Throughout human history, humans have included wild animals in their folklore, mythology, and daily vocabulary. Animals with especially distinctive traits have likewise become ensconced in modern popular culture and language. Leonine features are considered striking and sexy, outfoxing someone is a demonstration of cleverness, and it is obnoxious to parrot someone in a conversation. Being slothful is unlikely to gain you any respect.
Sloths spend the vast majority of their lives nearly sedentary, moving through the canopy at incremental paces. These animals are so famously lethargic that crops of algae are able to flourish in their fur, providing habitat for resident moths (Pauli et al. 2014). Our lack of respect for sloths is unfortunate, however. These Neotropical mammals essentially carry their own arboreal ecosystems around on their backs, and scientists are only just starting to scratch the surface of the complex dynamics between sloths, their microbes, and the other plants and animals that exist on their bodies.
A new paper in PLoS ONE suggests that the ecosystems of microbes living in sloth fur may soon become more than just interesting animal trivia. In fact, it appears that sloths may carry a variety of potential treatments for diseases and infections that kill millions of people every year (Higginbotham et al. 2014).
Despite the staggering amount of money spent on pharmaceutical research and development every year, millions of people are still dying of both communicable and non-communicable diseases in developing countries, and even the world’s affluent populations are increasingly at risk from antibiotic-resistant bacterial infections. Thus, both the under-medicated and the over-medicated are suffering.
The continuing problem of neglected tropical diseases, in addition to the increasing futility of much of western medicine’s antibiotic arsenal, has spurred researchers to cast broad nets in their drug discovery efforts. This has led to a recent trend for collecting bioactive substances from unconventional natural sources.
Which brings us back to the sloth. A team of researchers--led by Sarah Higginbotham of the Smithsonian Tropical Research Institute--were interested in whether any of the fungi found in the sloth’s hair could potentially lead to treatments for several of the world's top life-threatening diseases. They collected fungi from the fur of live three-toed sloths (Bradypus variegatus) in Panama’s Soberanía National Park, and ended up with samples of 84 different fungal isolates.
The scientists assessed the bioactivity of the fungal isolates against several critical disease agents: Plasmodium falciparum for malaria, Trypanosoma cruzi for Chagas disease, and MCF-7 breast cancer cells. A fungal extract was classified as “highly bioactive” against a parasite or cancer cell if it caused at least 50% inhibition of growth for those cells in the lab.
Finally, the team conducted antibiotic activity profile screening (BioMAP), which tested extracts from 50 of the isolates for activity against 15 different bacteria known to be serious pathogens in humans. These bacteria included the species that cause MRSA infections, cholera, salmonella poisoning, and a range of other nasty infections.
The results should inspire a whole new respect for the under-appreciated sloth. The bioactivity assays revealed a potential medicine chest riding around in that famously fungal fur. Of the 70 fungal isolates that were assayed for bioactivity, two of 70 (2.5%) were highly active against the malaria parasite (P. falciparum), eight of 62 (12.9%) were active against the Chagas parasite (T. cruzi), and 15 of 73 (20.6%) were active against MCF-7 breast cancer cells.
The BioMAP analysis also showed that 40% of the fungal extracts appeared to be active against at least one of of humanity’s most-feared bacteria. Interestingly, some isolates within the same genus had much different levels of bioactivity. This seems to affirm that it is useful to cast a broad net when seeking pharmaceutical solutions from nature.
How did the humble sloth come to be the carrier of such a promising array of fungi? The researchers determined that the assemblage fungi found in the sloth’s hair was consistent with that found in soil and plants. This indicates that not just any sloth can host this type fungal assemblage—it may turn out that only wild sloths roaming their natural habitat may be able to grow such an ecosystem on their backs. Future analyses of both captive sloths and wild sloths from different geographic regions will shine more light on that issue.
The discovery that 29 of the fungal isolates from the sloth hair were the same as fungi found on plants also led the researchers to suggest that these fungi may have a mutualism with the green algae that grow in the sloth’s pelt. This facilitation by algae could explain why a whopping 8% of the microbial communities on the sloth’s integument were fungal, compared to < 0.01% for humans.
Despite serving as the eponym for one of the seven deadly sins, the sloth may actually may provide humans with promising leads to the development of new, desperately-needed pharmaceuticals. Even if no drug is ever developed from the three-toed sloth’s resident fungi, however, this study also serves as a proof of point: there are innumerable organisms, habitats, and ecosystems in nature that we have yet to explore for potential drug compounds. If something as humble as the sloth could hold a clue to a new "wonder drug," that is all the more incentive to preserve as much of the world’s biodiversity as we possibly can. You never know what unlikely organism may end up holding the key to a world-changing cure.
Higginbotham, S., W. R. Wong, R. G. Linington, C. Spadafora, L. Iturrado, et al. 2014. Sloth hair as a novel source of fungi with potent anti-parasite, anti-cancer and anti-bacterial bioactivity. PLoS ONE 9(1): e84549. Doi:10.1371/journal.pone.0084549
Pauli, J. N., J. E. Mendoza, S. A. Steffan, C. C. Carey, P. J. Weimer, and M. Z. Peery. 2014. A syndrome of mutualism reinforces the life of a sloth. Proceedings of the Royal Society B 281(1778). 20133006
Sloth in tree: By Stefan Laube (Tauchgurke) [Public domain], via Wikimedia Commons
Mother & baby sloth via Dr. Carin Bondar - Biologist With a Twist.