A new case report published in the journal Open Forum Infectious Diseases reveals that a woman who was treated for a recurrent Clostridium difficile infection with the gut bacteria of an overweight donor quickly and unexpectedly gained weight herself following the procedure. The authors say the case suggests doctors should avoid using bacteria from overweight donors for such treatment.

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A woman who received gut bacteria from an overweight donor – for the treatment of recurrent C. difficile infection – rapidly gained 34 pounds in the 16 months after the procedure.

Infection with C. difficile can cause inflammation of the colon, leading to diarrhea, fever, loss of appetite, abdominal pain and nausea.

According to the Centers for Disease and Prevention (CDC), C. difficile is responsible for around 337,000 infections and 14,000 deaths in the US each year. Elderly individuals and people with certain medical conditions are most at risk of C. difficile infection.

For primary C. difficile infection, antibiotics are the first port of call when it comes to treatment, and for most patients this is effective. Some patients, however, may experience recurrent C. difficile infection. In such cases, fecal microbiota transplantation (FMT) may be recommended.

FMT is a procedure in which a stool is collected from a donor free of C. difficile, and “friendly” gut bacteria are isolated from the fecal matter. This bacteria are then transferred to the recipient’s gut, with the aim of replacing the good gut bacteria that may have been suppressed by overpopulation of C. difficile.

The 32-year-old woman described in the case report underwent FMT at Newport Hospital, RI, in 2011 after experiencing recurrent C. difficile infection.

According to the report authors, including Colleen R. Kelly of the Warren Alpert Medical School at Brown University in Providence, RI, the woman weighed 136 pounds and had a body mass index (BMI) of 26 at the time of the procedure.

The donor bacteria used for transplantation came from the woman’s 16-year-old daughter, who was healthy but overweight – with a weight of 140 pounds, which later rose to 170 pounds, and a BMI of 26.4.

The report states that 16 months after undergoing FMT, the patient reported a weight gain of 34 pounds, taking her weight to 170 pounds and her BMI to 33, which is classed as obese.

Despite undergoing an exercise program and a supervised liquid protein diet, the woman continued to gain weight; 3 years after the procedure, she weighed 177 pounds and had a BMI of 34.5. Today, the woman remains obese.

Prior to undergoing FMT, the woman tested positive for Helicobacter pylori infection alongside C. difficile infection. This was treated with amoxicillin, clarithromycin and proton pump inhibitor, and such treatment has been associated with gain, the authors note.

“However,” they add, “it is notable that [the patient] 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.” They add that previous studies in animal models have shown it is possible that obese microbiota can be transmitted.

As such, the authors say this case report indicates that doctors should avoid using obese patients as donors for FMT:

The untoward consequences of using non-ideal FMT donors are important, because patients may prefer to use a family member rather than an unrelated or unknown stool donor due to the perception that these sources are safer. However, studies have shown that FMT using a frozen inoculum from unrelated donors is effective in treating relapsing CDI.

This case serves as a note of caution when considering the use of non-ideal donors for FMT, and we recommend selecting non-overweight donors for FMT.”

In an editorial linked to the study, Dr. Ana A. Weil and Dr. Elizabeth L. Hohmann – both of the Massachusetts General Hospital – say this case report emphasizes the importance of investigating the long-term effects of FMT and raises a number of questions about the donor selection process for the procedure.

“FMT has not been studied in large-scale controlled trials, and we have much to learn about the effects of this treatment beyond the intended restoration of a diverse microbiota,” they add.

“Careful study of FMT will advance knowledge about safe manipulation of the gut microbiota. 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.”

In November 2014, Medical News Today reported on a study suggesting that people who have a class of bacteria in their gut called Christensenellaceae – the presence of which is mainly influenced by genes – are more likely to have a lower body weight.