When it comes to medicines, humans have been tapping the biological riches of the natural world for millennia. Herbal remedies are still used throughout the world, and many synthetic Western pharmaceuticals are based on compounds originally extracted from nature – just think of aspirin from the bark of the willow tree.
A study published last week in PLoS One takes bio-prospecting for new drugs to a new and unusual level. Researchers from the Republic of Panama and the US have surveyed fungi residing on the hair of the three-toed sloth, Bradypus variegatus, for potential pharmaceutical compounds.
Fungi have been a particularly rich source of medicinal compounds for a long time. Alexander Fleming accidentally discovered the first antibiotic, penicillin, in 1927 from the Penicillium mould, and lovastatin, the first in the widely prescribed statin class of cholesterol-lowering drugs was isolated from the soil fungus, Aspergillus terreus.
But a gradual decline over recent years in the number of bioactive compounds identified from fungi led Sarah Higginbotham and her colleagues to look for fungi further afield – they turned to the fungal residents of sloth fur.
Sloths are strange-looking creatures that spend their lives dangling by curved, scythe-like claws from branches in the tropical rainforests of South and Central America. All six species are incredibly slow-moving, managing a pace of just 4 meters per minute at full throttle. They amble to the bottom of their arboreal homes only once a week to urinate and defecate.
Even stranger than a sloth’s appearance and lifestyle is its fur. A sloth’s pelt is composed of two distinct layers. Near their skin is a layer of fine, downy hair. On top of this lies a layer of course hairs, each almost half a millimetre thick. Grooves and cracks that run lengthwise along the hairs provide the foundation for a unique ecosystem of organisms that live in and on the fur, from symbiotic algae and fungi, to moth larvae and cockroaches.
To see what fungal treasures lurk in sloth fur, the researchers clipped hair samples from nine three-toed sloths residing in the Soberanía National Park in the Republic of Panama. Back in their lab, they placed segments of sloth hair onto beds of gelatinous, nutrient-rich growth medium and watched as fungi blossomed from the hairs over a two-week period.
The team isolated 84 separate fungal types, or isolates. Some isolates were undocumented strains of known fungal species, while others were species that may be new to science.
To test whether the fungal isolates produced any bioactive compounds, the researchers cultivated the isolates in a liquid broth, then blended and filtered each to produce crude fungal extracts. These extracts were tested for their ability to inhibit the growth of a range of human pathogens.
Two fungal extracts were highly active against the malaria parasite, Plasmodium falciparum, and eight were highly active against the parasite that causes Chagas disease, Trypanosoma cruzi. The identification of extracts that can inhibit T. cruzi is a promising find, as microbes with such bioactivity are rare, and current treatments are highly toxic. Fifteen extracts also contained compounds that inhibited growth in breast cancer cells grown in vitro.
The team also tested the extracts for their anti-bacterial properties, and identified twenty extracts that were bioactive against at least one of fifteen pathogenic bacteria. Fourteen extracts were active against the superbug methicillin-resistant Staphylococcus aureus (MRSA).
One extract had particularly potent antibacterial activity against a division of bacteria known as Gram-negative bacteria. The unusual pattern of antibacterial activity that the researchers observed from this extract indicate that the compound responsible could be working through a previously unidentified mode of action.
Whether the fungi play an active role in the health of the sloths is unknown. And the possibility that they could play a role in human health is yet to be seen. Any development of medicines from the sloth hair fungi will require careful isolation of active ingredients and tests for activity in animals and then human patients.
As current antibiotics continue to falter amid growing resistance and rates of non-communicable diseases such as obesity and cancer continue to rise, scientists will no doubt be looking to ever stranger microbial niches for tomorrow’s medicines. But I, for one, will be keeping an eye out for the pill that has its origins in the matted mane of the three-toed sloth.
Higginbotham S, Wong WR, Linington RG, Spadafora C, Iturrado L & Arnold AE (2014) Sloth Hair as a Novel Source of Fungi with Potent Anti-Parasitic, Anti-Cancer and Anti-Bacterial Bioactivity. PLoS ONE doi:10.1371/journal.pone.0084549