According to FDA, the yogurt contained the Mucor circinelloides mold.

Randy Worobo, a professor of food science at Cornell University and a leading expert in food spoilage, says the mold is commonly associated with fruits, vegetables and dairy and is not considered a disease-causing foodborne microorganism.

“This mold should not pose a health risk to most consumers,” he says. “Very rarely, it can act as an opportunistic pathogen, but not through food and usually through inhalation only for people with compromised immune systems. The organism is regularly used for the production of natural flavor compounds that are widely used in the food industry.”

But according to Duke study author Joseph Heitman, the research indicates the particular strain of fungus on the yogurt may pose more serious risks than previously thought.

“Typically when people think about food-borne pathogens, they think about viruses or bacteria; they don’t think of fungi,” says Soo Chan Lee, a senior research associate at Duke who led this study. “Our research suggests it may be time to think about fungal pathogens and develop good regulations to test for them in manufacturing facilities.”

A couple who became sick after eating the yogurt turned the remaining yogurt over to the researchers for it to be analyzed.

After testing the samples, the Duke researchers isolated Mucor circinelloides from the yogurt container and applied DNA barcoding—analyzing standardized regions of the fungal genome—to further identify its exact subspecies. The researchers found it belonged to the most virulent subspecies, Mucor circinelloides forma circinelloides (Mcc).

“There are three closely related species, and one of them we typically find infecting humans,” Heitman says. “There was some chance this yogurt isolate would be the human pathogenic form, and we found it was.”

Spores from the fungus were then injected into the bloodstreams of mice, where it was found to produce a lethal systemic infection in four out of five diabetic mice. Mice that were fed the spores experienced less dramatic reactions, with severe weight loss reported in one case.

The Duke researchers found the fungus survived passing through the gastrointestinal tract of the mice, suggesting it could opportunistically colonize in an immune-compromised host.

“We still don’t know if the fungus infects the gastrointestinal tract, or if it produces some sort of toxin that makes people sick,” says Heitman.

None of the 16 other samples of Chobani yogurt tested at the lab contained Mucor circinelloides.

The company responded last week with the following statement:

“Chobani conducted an aggressive, statistically significant series of tests of the products voluntarily recalled in September 2013, with third—party experts confirming the absence of foodborne pathogens. Chobani stands by these findings, which are consistent with regulatory agency findings and the FDA’s Class II classification of the recall on October 30, 2013,” said Alejandro Mazzotta, Chobani vice president of global quality, food safety, and regulatory affairs.

“In regard to this specific study, we were just made aware of it and want to take more time to review its methodology and assertions,” Mazzotta continued. “To our knowledge, there is no evidence, including the assertions presented in this publication, that the strain in the recalled products causes illness in consumers when ingested. Food quality and safety have always been and always will be paramount to Chobani.”

 The company has added equipment for microbiological testing and conducts more than 500 tests a day.