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But koalas?

It starts with marsupials’ picky eating habits. Koalas survive on nothing but eucalyptus leaves. And though they have been known to consume 10 varieties of the plant, a species called manna gum is the koala’s lip-smacking favorite.

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This preference has led to some problems.

Thanks to habitat destruction by humans, koalas often find themselves in isolated islands of eucalyptus, said Ben Moore, an ecologist at the Hawkesbury Institute for the Environment at Western Sydney University. “And that sort of enables over-browsing to occur and the trees to get destroyed,” he said.

In 2013, a koala population in Cape Otway, Australia, plucked the leaves off every manna gum tree in the area, triggering a die-off of trees and koalas. Over the next two years, about 70 percent of the approximately 8,000 animals in the area perished. That was bad news for a species considered vulnerable to extinction by the International Union for Conservation of Nature.

The tragic thing is that near all the stripped manna gum trees, there was plenty of another kind of eucalyptus. Known as messmate, it’s considered less nutritious, but it’s eucalyptus all the same. “Some koalas eat nothing but messmate,” Moore said.

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Many of the Cape Otway koalas wouldn’t touch the stuff and starved to death, with the messmate trees in plain view. Moore thought it might have something to do with the marsupial’s microbiome.

To chip away at the mystery, Moore and his colleagues analyzed the feces of koalas that fed exclusively on manna gum and compared it with those that prefer messmate. This confirmed a hunch — that koala microbiomes vary by population and contain different species of bacteria depending on the kinds of eucalyptus they consume. (You are what you eat, it seems, even if you’re a koala.)

Next, the team captured 12 wild koalas from a manna gum forest in Cape Otway. A control group received fecal transplants — poop pills, basically — from animals that fed on manna gum. The others got fecal transplants from wild-caught koalas living in and dining on messmate. The payloads were delivered via specially designed capsules that could stay intact until they reached the koala’s hindguts, where their eucalyptus-munching microbes live.

After nine days of coaxing the koalas to swallow the pills and another 18 days monitoring them, the team found that the microbiomes of the animals in the control group didn’t really change. But the bacteria found inside those that received messmate microbes changed a lot.

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The presence of bacteria associated with messmate digestion skyrocketed, said Michaela Blyton, an ecologist at the University of Queensland and the lead author of a paper describing the findings. It was published in the journal Animal Microbiome.

Most important was that the koalas that had been seeded with the new bacteria actually went on to eat more messmate than the control group.

“We know through human studies and work in other animals that the diet influences the composition of the microbiome,” Blyton said. “And our evidence suggests that it may go the other way as well.”

Blyton and Moore’s study is the first to provide evidence that fecal transplants can establish a new bacterial regime in koalas, which in turn helps the animals exploit a different food source.

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In some ways, the idea of a fecal transplant for koalas isn’t all that extraordinary. In addition to breast milk, koala mothers produce a goopy, green poop that their joeys lap up. It may sound revolting, but scientists surmise that it’s nature’s way of making sure the next generation acquires the microbes it needs to survive.

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Zoos have also been gaming koala microbiomes for a while now.

Priya Bapodra-Villaverde, senior veterinarian at the Columbus Zoo and Aquarium in Ohio, said she and her colleagues often turn to fecal transplants to counterbalance the effects of antibiotics, which have a tendency to kill good bacteria along with the bad. It is especially important for koalas, who cannot digest their meals without their live-in digestion partners.

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“So one of the things that we’ve started doing is we’ll usually get a stool sample from a healthy animal and look at it under a microscope,” said Bapodra-Villaverde. “Does everything kind of look okay? Is there a good mix of fungal organisms and bacteria and protozoa moving around?”

If so, the team mashes up the feces with water and eucalyptus to make a probiotic milkshake of sorts. If the koalas are feeling picky, they let fecal pellets soak in water and then squirt some of the soup into the animals’ mouths.

Commercial probiotics are already available for many kinds of animals, from cows and goats to dogs, cats, birds and reptiles. However, these products encourage the growth of more general microbes, not the specific varieties known to inhabit koala guts. That matters, Blyton said, because “the microbes in a koala are quite different to those that are in a cow.”

The fecal transplants Blyton and Moore are pioneering might have helped wildlife managers save some of the Cape Otway koalas in 2013, and targeted probiotics clearly would be useful for zoos. But the technology may have wider applications.

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As human development encroaches on koalas’ natural habitat in Australia, more of the marsupials are being struck by cars or attacked by dogs and are ending up in veterinary clinics. About half of the nation’s koalas are also infected with a strain of chlamydia, which can cause blindness or genital tumors. In other words, koala hospitals never want for patients.

“And the first thing that happens when a koala is brought into a koala hospital is they get a big shot of antibiotics, and of course that disrupts their microbiome,” Moore said. “So the hope now is that by using a modified version of this approach, it will give us a way to improve the number of koalas that recover from these sorts of treatments and get them back out there, where they need to survive if populations are going to persist.”