A woman in the US has rapidly gained weight to the point of being obese, following a faecal microbiota transplant (FMT) from an overweight donor. 

The woman received the transplant to treat a recurring  Clostridium difficile infection, which causes diarrhoea, nausea, and sometimes colitis - inflammation of the inner lining of the colon that can lead to rectal bleeding. Because this is the kind of infection that tends to return more often than not, treatment usually involves a prolonged use of antibiotics. But increasingly, doctors have been prescribing faecal microbiota transplants as another way to restore the balance of 'good' bacteria in the gut to fight the infection.

For the woman in question, antibiotics weren't working, after weeks and weeks of treatment, so in 2011, she received a faecal microbiota transplant from her daughter, via a colonoscopy. The woman was 32 years old at the time and had been maintaining a stable, weight of 61 kg (136 pounds) and a BMI of 26. Her 16-year-old daughter, though generally healthy, was overweight, at 63 kg (140 pounds) and with a BMI of 26.4.

Sixteen months later, with no changes to her diet or exercise levels, the the woman had gained 15 kg (34 pounds), and her BMI increased to 33. With those stats, she was now classified as 'obese'. Even after embarking on a medically supervised liquid protein diet and exercise program, the weight kept piling on. Three years following the transplant, the woman had grown to 80 kg (177 pounds), with a BMI of 34.5, and she remains obese even now.

So what actually is going on here? 

"We're questioning whether there was something in the faecal transplant, whether some of those 'good' bacteria we transferred may have had an impact on her metabolism in a negative way," Colleen R. Kelly from Warren Alpert Medical School of Brown University - one of the researchers who wrote the case report - said in a press release. If they do find this link between gut bacteria and weight gain, it would actually correlate with research done on mice, where gut bacteria transfer from obese to healthy mice would sometimes lead to a significant increase in fat levels. 

The team reports that they haven't quite figured out what happened, due to the range of factors at play. One particularly intriguing factor is that when the woman was going through initial testing, it was found that she also had a Helicobacter pylori infection. This bacteria makes its way into a person's digestive system, and after many years there, can suddenly cause ulcers in the lining of the stomach and small intestine, and in the worst cases, can bring on the development of stomach cancer. The bacteria is found in around two-thirds of the world's population - Michael Specter at The New Yorker says it "may be the most successful pathogen in human history" - but for most people, its presence will go unnoticed due to a total lack of symptoms.

Previous studies have suggested that the eradication of H. pylori could actually lead to weight gain, with a 2002 study done on over 300 people showing that their body weight and BMI increased significantly 12 months after the eradication of their H. pylori infections. Could this be the case here? The team discusses the possibilities in the journal Open Forum Infectious Diseases:

"There are several possible contributions to the weight gain, including the resolution of [the] Clostridium difficile infection (with subsequent increased appetite) and concurrent treatment of H. pylori. There is a known association between H. pylori treatment and weight gain, especially in children, thought to be due to restoration of ghrelin levels ['hunger hormones'] after eradication of the bacteria.

However, it is notable that she was never obese prior to FMT and that the stool donor similarly experienced significant weight gain, raising the possibility that the obesity was at least in part a consequence of FMT. The hypothesis of FMT triggering or contributing to obesity is supported by animal models demonstrating that an obese microbiota can be transmitted. An important limitation in our case is that the microbiome sequencing comparing the patient and the donor is not known."

The case remains unsolved for now, but the researchers recommend that perhaps for now, faecal microbiota transplants should not be received from overweight donors. "Careful study of FMT will advance knowledge about safe manipulation of the gut microbiota," they conclude in the paper. "Ultimately, of course, it is hoped that FMT studies will lead to identification of defined mixtures of beneficial bacteria that can be cultured, manufactured, and administered to improve human health."