A new study, led by researchers from the University of Auckland, has described the phenomenon of "super donors," people who contribute stool samples for use in trials whose poop seems to be significantly more effective in leading to clinical improvements for fecal transplant subjects.

Despite a long history of anecdotal use, the science behind fecal transplantation is still in its infancy. Altering a person's gut microbiome via a fecal transplant has proved mildly successful across a variety of different trials, but results have proved frustratingly inconsistent. The mixed results have led some researchers to try to understand whether there are particular fecal donors whose poop is more effective than others. A new study has investigated this "super-donor" phenomenon, suggesting that it does indeed exist.

"We see transplants from super-donors achieve clinical remission rates of perhaps double the remaining average," says Justin O'Sullivan, senior author on the new study. "Our hope is that if we can discover how this happens, then we can improve the success of fecal transplantation and even trial it for new microbiome-associated conditions like Alzheimer's, multiple sclerosis and asthma."

Homing in on exactly what specifically constitutes super poo has presented a complicated challenge for researchers. One of the most fundamentally significant features of good donor stool seems to be a broad microbial diversity – the larger the variety of species in the stool, the more effective the outcome when delivered via fecal transplant. The study also suggests high levels of what are referred to as "keystone species" are important in the efficacy of a fecal transplant.

"In inflammatory bowel disease and diabetes for example, keystone species that are associated with prolonged clinical remission produce butyrate – a chemical with specialized functions in regulating the immune system and energy metabolism," says O'Sullivan.

Interestingly, however, research has shown that when these beneficial keystone species are isolated and administered individually, for example in the form of a probiotic, they are not as effective as when delivered as part of a whole stool sample. The researchers hypothesize the failure of this kind of precision medicine implies that microbial structure as a whole plays a greater role in the success of fecal transplants than simply the actions of a single microbial species.

"For example, the success of fecal transplants has been associated in some studies with the transfer of viruses which infect other gut microbes," explains O'Sullivan. "Some cases of recurrent diarrheal infection have even been cured with transplants of filtered stool, that has had all the live bacteria filtered out but still contains DNA, viruses and other debris."

Ultimately, the researchers conclude that while it may be incredibly difficult to clearly characterize what makes an effective fecal super-donor, it is vital for future fecal transplant research to take into account the specificities of individual donor microbiomes. On a general level, a broad diversity of gut microbiota is the best metric to identify a super-donor but it is also suggested that supporting the transplanted microbiome through an adjusted diet in the recipient may be fundamental to a fecal transplant's success.

The study was published in the journal Frontiers in Cellular and Infection Microbiology.