There is growing evidence faecal transplants could be causing some patients to take on the physical and mental traits of their donors, including body shape and even symptoms of depression, an expert in infectious diseases says.

What is a faecal transplant? Faecal matter, or stool, is collected from a donor

Faecal matter, or stool, is collected from a donor The matter is mixed with a solution, strained, and placed in a patient by colonoscopy, endoscopy or enema

The matter is mixed with a solution, strained, and placed in a patient by colonoscopy, endoscopy or enema The aim is to replace the recipient's "bad" gut bacteria with the donor's "good" bacteria

The aim is to replace the recipient's "bad" gut bacteria with the donor's "good" bacteria The procedure is commonly used to treat complications from antibiotic therapy

The procedure is commonly used to treat complications from antibiotic therapy It is being used experimentally for conditions like IBS, MS, autism and Parkinson's

It is being used experimentally for conditions like IBS, MS, autism and Parkinson's Its use has been documented in 4th Century China

Faecal transplants are becoming an increasingly popular treatment for conditions like chronic fatigue, Parkinson's, autism and irritable bowel syndrome.

More commonly, they are used to replace the gut bacteria of people who have complications after prolonged use of antibiotics.

The treatment involves transplanting a donor's faeces into a patient's bowel to improve the balance of "good" and "bad" bacteria.

Associate Professor Patrick Charles, from the Department of Infectious Diseases at Austin Health, has presented evidence of the unusual side-effects to specialists at the Royal Australasian College of Physicians congress in Melbourne.

Dr Charles said the treatment had been used in modern medicine since the 1950s, but doctors are still learning about its effects.

"It's a very successful way to fix … an overgrowth of the bad bacteria causing terrible diarrhoea," he said.

"What we're learning about this now is the change in the mix of bacteria when you get this transplant can alter the person who is getting it to take on some of the characteristics of the donor sometimes.

"There have been people who have taken on the shape of the donor, such as if the donor is either overweight or underweight they've become more like that.

"There's even been reports of some people who have never been depressed getting a transplant from someone who's had depression and ending up with their first episode of depression after that."

He said while the science is not yet at the stage where doctors could treat conditions like depressive and weight disorders with faecal transplants, it was only the tip of the iceberg.

"We're just getting to the early stages of understanding this, but it's something we need to learn a lot more about because it could have a role in the future," Dr Charles said.

There's more to poo than we know

Dr Charles said the growing research was changing the way doctors look at faecal matter.

"We know it's very complicated. Before, we've had a little bit of knowledge and we've been able to grow different bugs in poo samples, but now with DNA technology we're finding out there's a whole lot more in there that we didn't know about, and a lot of bacteria that don't actually grow in the laboratory so that we couldn't identify them before," Dr Charles said.

But he said faecal transplants are currently not a common treatment.

"If someone gets a course of antibiotics and ends up with an overgrowth of bad bacteria in their bowel, sometimes we try their normal treatment, which is antibiotics, to fix that, but sometimes that's not working anymore," he said.

"Occasionally we have to resort to doing a faecal transplant to fix this problem."