I saw a ghost at the Vancouver Aquarium last summer. I was walking out of a room overlooking the main shark tank when I saw something in a glass cage embedded in the wall, something small, black and yellow. I mean Black and Yellow—colours so intense that you almost expect to turn the creature over and find a country of origin embossed on its underside. It was a Panamanian golden frog, and it is extinct in the wild. It only survives in zoos and aquariums. It is an ecological phantom, a ghost of nature.
Several factors took the frog to the edge of oblivion but the one that landed the most punishing blows was a chytrid fungus called Batrachochytrium dendrobatidis, or Bd for short. It is the same grim antagonist that has severely reaped the populations of some 200 amphibians and seems to be working its way through the rest. It is catholic in its choice of hosts and apocalyptic in its effects. The Panamanian golden frog is just one of its victims.
Conservationists have been incredibly successful at breeding the frog in captivity. But if they release these animals back into their native habitat, where Bd still persists, who’s to say they wouldn’t just die? Their ark is full, but there’s no Mount Ararat in sight.
In 2006, a team of researchers stumbled across a possible solution. They found that a few amphibians, including two salamanders and the mountain yellow-legged frogs, naturally carry a bacterium called Janthinobacterium lividum that stopped Bd from growing. It was an anti-Bd probiotic, a microbial shield that turned frogs into resistant fungus-fighters. And when the team applied the bacterium to yellow-legged frogs that didn’t already have it, those individuals also became resistant.
Could J.lividum protect other frogs too? To find out, Matthew Becker from James Madison University, who was part of the original team, teamed up with Brian Gratwicke from the Smithsonian Conservation Biology Institute, who had a group of lab-bred golden frogs. They soaked the frogs in a J.lividum bath and challenged them with Bd. If the approach worked and the frogs survived, perhaps they could be released into the wild, cloaked in their living armour.
It didn’t work. The probiotic microbe didn’t persist on the frogs’ skins, and it did nothing to save them from the fungus. “We thought maybe it wasn’t a good fit,” says Becker. “This bacterium was from California and these frogs are from Panama.” Perhaps frogs from different parts of their carry their own particular probiotic microbes that have adapted to thrive on their skins. If Becker was going to find a probiotic that could protect the golden frogs, he would need to go to Panama.
He went in 2011 and spent a week surveying the skin bacteria of local frogs, focusing on species that were as closely related to the golden frog as possible. Over a week, he collected 450 samples and found several microbes that stopped Bd from growing, at least in lab tests. He focused on four of these, and applied them to captive golden frogs, to see whether they could then survive a bout with Bd.
They couldn’t. On average, the treated frogs survived no longer than untreated ones. And once again, “nothing persisted,” says Becker. “Their existing microbial community didn’t even shift in response to [the new microbes].”
The same problem plagues human probiotics. When they’re swallowed, they don’t take up permanent residence in the gut and they don’t affect the make-up of the local bacteria communities (although they do seem to change the activity of certain genes). After all, a typical yoghurt contains several billion bacteria, whereas our gut contains tens of trillions. It’s like a raindrop falling into a lake. Perhaps this explains why probiotics can help with a small number of diseases, like diarrhoea caused by infections, but have largely failed to live up to the hype that surrounds them.
With the frogs, Becker wonders if he applied too many microbes rather than too few. “I think we may have activated the frogs’ immune systems and prevented the probiotics from establishing,” he says. Alternatively, we know that even closely related animal species can host distinctive microbiomes, so what persists on one frog may just not thrive on another. It’s also possible that the skins of captive golden frogs are already colonised by microbes that stop the bacteria of their former Panamanian neighbours from colonising.
In the midst of their disappointment, the team found a silver lining. Five of the frogs managed to clear the fungus on their own. “That’s pretty unheard of in golden frogs,” says Becker. When he focused on these animals, he found that they differed from those that died, in the groups of bacteria on their skin and the chemicals that those bacteria produced.
What are these microbes? Do they actually protect against Bd or are they indicators of some inherent resistance, perhaps some immune genes that both resist the fungus and select for specific skin microbes? If they do protect against Bd, would they do so in the wild? Are they part of a golden frog’s natural repertoire, or did they only start colonising these animals in captivity? The team is now working to answer these questions. Becker is sampling 200 of golden frogs at Maryland Zoo in Baltimore to see if he can find these potentially protective communities, and then apply them to other frogs to see if they also become Bd-resistant.
The concept of using probiotics to protect amphibians (and perhaps other animals at risk from widespread epidemics, like bats) makes sense. Many animals, from humans to corals, carry skin microbes that protect us from incursions by disease-causing species, by secreting natural antibiotics, mobilising our immune systems, and simply filling up niches that the invaders might otherwise exploit.
But our own experience with probiotics, and Becker’s frog experiments, tell us that deploying these seemingly beneficial bacteria is easier said (and marketed) than done. Probiotics may help to save the frogs but it’s unlikely that we’ll see a one-size-fits-all solution, and the same could be said for the use of microbes in human medicine.
There will be more about frogs and conservation probiotics in my book, I Contain Multitudes, out next year.
Reference: Becker, Walke, Cikanek, Savage, Mattheus, Santiago, Minibiole, Harris, Belden & Gratwicke. 2015. Composition of symbiotic bacteria predicts survival in Panamanian golden frogs infected with a lethal fungus. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2014.2881
More on Bd:
Update: The post originally said that the 200 golden frogs that will feature in upcoming experiments were at the Smithsonian; they actually live at Maryland Zoo in Baltimore.