It can be challenging to get a fussy eater to broaden their food choices, especially when they are a koala.

Key points: Koalas that eat different species of eucalypts have different gut microbes

Faecal transplants could help widen the diets of koalas with limited microbes

Scientists hope this could help koalas adapt to new environments or overcome some of the effects of chlamydia treatment

These iconic animals survive on a diet of eucalyptus leaves, but not all eucalypts are equal in the eyes — or the guts — of a koala, according to a new study by a team of Australian researchers.

The researchers found that some koalas may be pickier than others due to the mix of microbes in their lower gastrointestinal tract.

But the balance of microbes — and the koala's tolerance to a wider range of eucalyptus species — could be altered with faecal transplants, they report in the journal Animal Microbiome.

Not that the koalas themselves appreciated being fed a daily capsule laced with microbes sourced from poo, admitted Michaela Blyton who co-led the research with Dr Ben Moore at Western Sydney University.

"They're not particularly co-operative."

Dakar is a young adult male that received a faecal transplant from wild koalas feeding on messmate. ( Supplied: Michaela Blyton )

But the study's findings have important implications for the conservation of starving and sick animals.

Dr Blyton was inspired to conduct the research after witnessing a devastating drop in a population of koalas on Cape Otway in Victoria in 2013.

"It was quite distressing because we ended up with a 70 per cent mortality rate," said Dr Blyton, now at the University of Queensland.

This population of koalas had reached very high densities and overgrazed a species of eucalyptus known as a manna gum — a rich source of protein and lower in toxic compounds known as tannins.

"Interestingly, these koalas were not moving into another species of eucalyptus known as messmate, which was less preferred," Dr Blyton said.

"This was despite the fact there were koalas that lived their entire life in the messmate and were able to exist entirely on that food source."

The team suspected that gut microbes might play a role, based on previous research that showed the microbiome of manna-loving koalas differed from their messmate-munching mates.

To find out, the researchers captured 12 manna koalas and housed them at the Cape Otway Conservation Ecology Centre.

How do you give a koala a poo transplant?

While captive koalas were settling into their digs, the researchers used radio tagging to track wild koalas that exclusively ate messmate diets and placed mats underneath the trees they slept in.

"We came back the next day and collected the faecal pellets they'd left for us overnight."

The researchers also collected poo from the manna-eating captive koalas.

These little koala poo nuggets contain gut microbes. ( ABC Sunshine Coast: Harriet Tatham )

They then extracted microbes from the poo, turning them into a small acid-resistant capsule which could be given orally to the koala.

Half the captive koalas were given the messmate microbes for nine days, while the other half were given manna microbes.

"The whole process including capturing the koala from its enclosure can be done in five to 10 minutes, so it's not particularly invasive.

"Afterwards they were a bit like 'What was that all about? Oh well, I'll just go back to eating my eucalyptus leaves'."

For the next 18 days all the koalas were exclusively fed messmate leaves. During this time the researchers found the microbiomes of the koalas fed messmate microbes had changed.

"We were able to change the microbiome of those manna gum-feeding koalas to resemble the microbiome of the messmate-eating animals."

And this change in microbiome also resulted in an apparent increased liking for messmate.

"Over the period we had them in captivity they increased the amount of messmate they ate."

Whether or not they continued to have a taste for messmate when they were released back into the wild is unknown.

"I suspect that the koalas in our study primarily went back to feeding on manna gum in the wild as it was available to them and is a comparatively nutritious species," Dr Blyton said

However, she said, the finding could have implications for the conservation of koalas that need to be translocated to new areas with few or no koalas.

"We think that giving these animals these capsules could help them to adapt to a new habitat they're being introduced in to."

Will it work in the wild?

Celine Frere, an animal ecologist at the University of the Sunshine Coast, said the study had significant implications for the translocation of koalas.

"We've always advocated that gut microbiome is really important factor to take into consideration when moving animals around," Dr Frere said.

"This study presents strong scientific evidence of the limitations of a koala gut microbiome to adapt to changes in its diet."

Some translocations of koalas have been more successful than others. ( ABC News: Margaret Paul )

After a month of captivity, the study demonstrated that the koala's microbiome only changed if they were given the messmate poo transplants, enabling the animals to eat more of the nutritionally lower messmate.

But housing koalas in captivity to change their microbiome before translocation was neither practical, or guaranteed that they would actually eat the leaves in the wild.

"Even if you could shift the koala's microbiome, then you release it there's a lot of added stress. It's an unknown environment, it might then seek its preferred food source rather than the most common one."

And even if it did start eating different leaves there's no guarantee the koala will get the nutrition it would need to thrive.

"How much a koala eats does not necessarily mean how much nutrition it's getting from its food," Dr Frere said.

Translocation is becoming increasingly common in habitats where koalas are under stress such as south-east Queensland, northern New South Wales, Victoria and Canberra.

"One of the problems is it's not current practice to actually track the koalas post-translocation and to see how they're going."

But, Dr Frere said, this study could inform practices before koalas were moved.

"Mapping the gut microbiomes across the landscape might actually help us understand barriers to translocation."

Rebecca Johnson, a koala geneticist at the Australian Museum, said taking an evidence-based approach to conservation was critical.

"These kinds of intensive studies are so valuable because so many decisions are made on hunches, or seeing a koala," Dr Johnson said.

Although translocation should be "an absolute last resort", she said there is an increasing possibility that we will be moving animals.

"As we're going out there to try to identify habitats to conserve, or have a negotiation about conserving, this kind of information is extra important," she said.

Dr Johnson said the study's findings could also help with the rehabilitation of koalas that have been affected by chlamydia.

"Koalas are notoriously difficult to treat with antibiotics because we suspect the drugs really interfere with their microbiome which can then lead to very bad condition and weight loss," she said.

"The possibility of being able to do a dual therapy is really exciting."

But faecal transplant capsules will need to be much more robust before they can be used for conservation or rehabilitation, said Dr Blyton.

And making the pellets is currently too intensive.

"We have to make them up fresh ... which takes about two to three hours and then give them to the koala while the microbes are still healthy.

"What we'd like to do is to develop them into something more like you'd get at the chemist, something that's a freeze-dried capsule in a bottle sitting on the shelf and the wildlife carers can give it to the koala when necessary."

"But not all microbes can survive that sort of treatment so we need to test whether the important microbes for the koala can be preserved in that way."