Faecal transplants are becoming more common in the treatment of human gut problems, but there is still some mystery as to what the wondrous poop pills actually contain.

A review of evidence by US scientists suggests that while most studies on poop transplants focus on the bacteria that is transferred, there are other travellers that come along for the ride that could also be players in a transplant's success.

The study found that healthy human faeces contains on average 100 billion bacteria per gram.

However, the detailed analysis found it also contains 100 million viruses and archaea per gram (archaea are single-celled organisms that were classed as bacteria until the 1970s).

In addition, there are about 10 million human skin cells from inside the colon and a million yeasts and other single-celled fungi per gram.

“This research is just getting started,” says Seth Bordenstein, associate professor of biological sciences and pathology, microbiology, and immunology at Vanderbilt University.

“It is being driven by the new paradigm of the microbiome which recognises that every plant and animal species carries a collection of fellow microbial travellers that have significant and previously unrecognized effects on their host’s health, performance and behaviour.

“There is no doubt that poo can save lives,” he continued.

“Take the case of the use of faecal transplants to treat Clostridium difficile infections. According to the literature, it has a 95% cure-rate.

“Right now faecal transplants are used as the treatment of last resort, but their effectiveness raises an important question: When will doctors start prescribing them, or some derivative, first?”

Faecal transplantation has been practiced since at least the 4th century by Chinese doctors, and was popular enough to get the nickname “yellow soup.”

But interest in the West was low until about 2010, when an exponential rise in faecal transplant studies began appearing.

This has seen a major increase in animal experiments involving faecal material as well, including one in which researchers found that faecal transplants from lean mice turned sterile mice into lean mice, while faecal transplants from fat mice turned sterile mice into fat mice.

“While bacteria in faeces can contribute to the outcomes of faecal transplant experiments in certain cases, there are many other cases where we simply do not know. Bacteria are not the only potential player in the donor’s stool,” Bordenstein said.

“Faeces is a complex material of various biological and chemical entities that may be causing or assisting the effects of these treatments.

“When scientists identify the specific cocktails that produce the positive results, then we can synthesize or grow them and put them in a pill. That will go a long way to reducing the ‘icky factor’ that could slow public acceptance of this new form of treatment,” said Bordenstein.

His latest study is accessible here.