Not all poo is created equal — especially when it comes to faecal transplants.

Key points: Poo transplants to treat infections or diseases rely on donations of faecal matter

Poo transplants to treat infections or diseases rely on donations of faecal matter Recent studies have uncovered "super donors", whose stool samples seem to have better results

Recent studies have uncovered "super donors", whose stool samples seem to have better results Teasing out why transplants from super donors are so effective could help better treat inflammatory bowel disease and other conditions

"Super donors" — people whose stool donations have strikingly high success rates in poo transplants — have turned up in multiple studies, according to a review paper published in Frontiers in Cellular and Infection Microbiology.

And while a large part of the super-donor effect is likely down to the community of bugs dwelling in a person's gut, known as the gut microbiome, it's certainly not the be-all and end-all.

Faecal microbiota transplants, also known as FMTs or poo transplants, take a sample of faeces from a healthy person and "transplant" it into the intestines of a person with an infection or disease.

The idea is that the introduced bugs will "take hold" and restore the host's microbiota to a healthy state.

While it might seem a relatively modern clinical treatment, the first poo transplants were documented by Chinese doctors as far back as the fourth century.

More recently, poo transplants have been used since the 1950s to treat Clostridium difficile infections.

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

Faecal matter, or stool, is collected from a donor Stool is mixed with a solution, strained and introduced to a patient's gut via colonoscopy, endoscopy, enema or in pill form

Stool is mixed with a solution, strained and introduced to a patient's gut via colonoscopy, endoscopy, enema or in pill form A faecal transplant can be administered once or in multiple doses

A faecal transplant can be administered once or in multiple doses The aim is to replace the recipient's "bad" gut bacteria with the donor's "good" bacteria

Such infections are bacterial overgrowths in the gut that are often resistant to antibiotics and can cause severe diarrhoea, said Justin O'Sullivan, University of Auckland microbiologist and paper co-author.

By introducing a diverse dose of microbes via poo transplant, C. difficile must compete for gut real estate. Symptoms can clear up within a few hours.

"It's an over-90-per-cent cure rate. It's incredible," Dr O'Sullivan said.

But using poo transplants to treat inflammatory conditions such as ulcerative colitis and Crohn's diseases have yielded mixed results.

Even though inflammatory bowel diseases are characterised by shifting microbiota populations, exactly how our gut bugs affect symptoms isn't well known, said Hannah Wardill, a gut microbiologist at the University of Adelaide who was not involved in the paper.

So finding out more about these diseases, as well as the donors who have therapeutically potent poo, could provide patients with a better chance of recovery.

What makes a super donor so super?

What constitutes a "good" or "healthy" gut microbiota isn't entirely clear. Nor what makes a poo donor "super".

There's a general idea that a super donor's microbiota has more bacterial diversity as well as a few "keystone" species that seem to benefit gut health.

Then there's all the other stuff in the gut.

"Bacteria aren't the only things there," Dr O'Sullivan said.

"You have fungi, protozoa and viruses there as well and they can change the bacterial populations."

A virus that attacks one species of bacteria, for instance, will modify the gut microbe community.

Scanning electron micrograph of various bacteria found in a sample of human faeces. On the whole, the more diversity, the healthier the microbial community. ( Getty Images: Steve Gschmeissner/ Science Photo Library )

What's floating around the microbes' environment, such as bits of bacterial debris, DNA and proteins, seems to help restore gut health too.

Researchers strained stools and dispensed the filtered fluid, or filtrate, in patients with C. difficile.

"Filtrates don't have the actual bacteria in them, but they also see clinical resolution," Dr O'Sullivan said.

Still, no matter how "super" a donor's stool sample is, its success is also highly dependent on the recipient.

What they eat, for instance, might help or hinder a poo transplant.

Transplanted microbes also need to be able to integrate with the existing residents.

If a recipient's immune system attacks the introduced species, they may not see as much benefit.

There's probably a genetic component too, but as Dr O'Sullivan and his colleagues point out in their paper, this is hard to tell yet as few studies collected genetic information.

So you think you're a super donor

If you think you have "super poop", how can you put it to good use?

Bit of bad news for you: it turns out that only a select few are allowed to donate their dung.

Selection criteria vary between clinics and research teams, but Chris Leung, a gastroenterologist at Austin Health in Melbourne, said potential donors must tick quite a few boxes before they're able to contribute a sample.

"Donors must be between 18 and 55 years old, be in good health, have no gastrointestinal illness, not travelled outside Australia or taken antibiotics in the past six months and must have nice firm stools."

If they pass that checklist, they then undergo thorough blood and faeces screenings.

"We test blood for infections like hepatitis A, B and C, Epstein-Barr virus which causes glandular fever, cytomegalovirus, syphilis and HIV," Dr Leung said.

"In stool samples, we look for parasites and bugs like C. difficile."

So it's no surprise that OpenBiome stool bank in the US, for instance, accepts just 3 per cent of applicants.

"It's not like you can just do a poo and get paid for it," Dr Wardill said.

But if you have a healthy gut, you might be a super donor — for yourself.

Treatments such as chemotherapy can decimate a person's gut colonies.

So for someone about to start cancer treatment, researchers are investigating if it's worth collecting and banking their poo to re-administer afterwards, Dr Wardill said.

"Obviously for someone with inflammatory bowel disease, it would be impossible to do this because you'd be collecting their already-damaged microbiome."

And as microbiologists uncover the mechanisms behind the super-donor phenomenon, they might find a whole range of super donors specific to certain diseases.

"Perhaps there are different microbiomes that have effects specifically on the immune system, or an effect on the brain, or somewhere else," Dr Wardill said.