(CNN) The US Food and Drug Administration issued a safety alert on Thursday over the use of fecal transplants after one recipient died.

The investigational treatment has not been approved by the FDA. Known as fecal microbiota for transplantation, it has been used to treat cases of Clostridium difficile, or C. diff, that haven't responded to traditional antibiotic treatments. C. diff is a bacterium that causes diarrhea and inflammation of the colon.

Fecal transplant utilizes the stool of a healthy person to transfer good bacteria into the intestines of an infected person.

The FDA said that two people with weakened immune systems who received transplant samples from the same donor developed another antibiotic-resistant infection, a form of E. coli. The donor sample was not tested for the bacteria before use.

Get CNN Health's weekly newsletter Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

The FDA will now require that all stool samples used in transplants be tested for drug-resistant microorganisms. All donors will also need to be screened for potential drug-resistant infections.

Read More