Gastroenterologists in the US have developed a tracking model that can predict which bacterial strains will engraft successfully after faecal microorganism transplant.

Using genetic sequencing in 20 patients with C. difficile infection who received therapeutic FMT, they analysed the strains of bacteria in the gut microbiota of donors and recipients before and four months after a transplant.

They found that after FMT about 30% of the donor bacteria engrafted in the recipient, with the most abundant strains more likely to engraft.

“That’s important to know when designing a microbiome-based therapeutic like this,” said co- author Dr Ramnik  Xavier, Chief of the Division of Gastroenterology at Massachusetts General Hospital.

“If a drug only colonises 30% of the patients you put it in, then the maximum efficacy of your drug is 30%,” he said.

Their analysis, published in in Cell Host & Microbe also revealed an “all or nothing” pattern in the 30% of engrafted strains. If the faecal donor had five different strains of a bacterial species, for example, all five strains transferred into the patient.

The strain tracking also found that, if the recipient already had some of the strains found in the donor, the probability of those strains engrafting was higher.

However some previously undetected strains frequently colonised patients receiving FMT, suggesting that the post-FMT microbiome is a mixture of bacterial strains from the donor, recipient, and the environment.

The researchers said their Strain Finder method was the first predictive strategy for developing a synthetic probiotic.

“This paper provides a context for understanding how to make these live biological therapeutics as an alternative to transferring raw faecal matter,” they concluded