Fungus has been indispensable to cheese ever since the moldy milk that sounds dreadful but tastes delicious was accidentally discovered around 7,000 years ago. In the millennia that followed, different species of fungi and bacteria have been purposely used to create different cheeses, such as Penicillium roqueforti, which gives Roquefort its blue veins and pungent aroma, or Penicillium camemberti, the reason why Camembert overflows with gooey indulgence. Unfortunately, not all fungi crawling in cheese enhance the sensory experience. Some of them can be poisonous.
When a panicked cheesemaker approached Tufts University microbiologist Benjamin Wolfe, the cheese in question was rotting instead of ripening, its rind pocked with patches that were visibly disintegrating. The timing was no coincidence. Wolfe and his research team had already been investigating an outbreak among artisan cheeses in the northeastern US, one that left flaking black patches on goat cheese and other varieties aging in cheese caves. Analysis of the samples revealed the culprit: Aspergillus westerdijkiae, a mold that was hardly new to science, but had never before made itself at home in cheese. As it feeds, the fungus secretes dangerous toxins—and a species that had never troubled cheesemakers before has suddenly become their bane.
“Over the past decade, disruptive contamination events of Aspergillus spoilage molds have occurred at cheese production facilities in Massachusetts, Connecticut, and Vermont in the United States, causing aesthetic, flavor, and potential safety issues,” Wolfe said in a study recently published in Applied and Environmental Microbiology.
Aged cheeses owe their distinct flavors to an entire microbiome thriving on their surfaces, and which microbes take up residence depends on the variety the cheese is destined to become. Now, however, those beneficial bacteria and fungi are facing an unwelcome intruder. Spoilage molds can muscle out the communities that belong on the rind, leaving behind cheese that looks unappetizing—and, worse, may be unsafe to eat. Aspergillus species in the Circumdati group are known to produce ochratoxins and other harmful mycotoxins. Ochratoxins, in particular, can damage the kidneys. A. westerdijkiae compounds the problem by secreting antimicrobial metabolites such as penicillic acid, which keep beneficial bacteria and fungi from colonizing the rind in the first place.
Until now, though, the genomes of these molds—and how they interact with their surroundings—had gone unexamined, and that gap is what Wolfe and his team set out to close. What little was known pointed in a strange direction. Spoilage Aspergillus species tolerate higher temperatures and thrive in warmer conditions, and while A. westerdijkiae had already been linked to the spoilage of foods other than cheese, the fungus had drawn little scientific scrutiny. It also doesn’t occur naturally in the region it’s now plaguing, which led Wolfe to suspect it had arrived from somewhere else—possibly hitching a ride on imported ingredients like salt. To find out, he isolated Aspergillus strains from contaminated rinds across multiple facilities. The damage, he found, looks different on every cheese: black flakes on goat cheese, fuzzy white and dark gray splotches or dusty black patches on block cheddars.
Comparing those genomes turned up a surprise: A. westerdijkiae wasn’t working alone. A second species, Aspergillus ostianus, was infesting rinds alongside it, and each species was represented by several distinct strains across cheeses aged in different locations. Sometimes they overlapped. One sample harbored two strains of A. ostianus plus a strain of A. westerdijkiae. The genomes also revealed that strains within a species leaned on the same molecular machinery—metabolism, along with core cellular processes like DNA replication and repair. More than a thousand of those genes ramped up once A. westerdijkiae was placed on cheese, a sign that they’re what allow the fungus to take hold on a rind specifically. A. westerdijkiae also carried genes for a wider range of mycotoxins than A. ostianus did.
Where it came from is still an open question. When Wolfe searched the GlobalFungi database, A. westerdijkiae turned up nowhere in the northeastern U.S. That leaves two possibilities: either it was imported, or it’s been there all along and simply went unidentified. The one thing every affected facility had in common was salt. While Wolfe keeps hunting for a natural reservoir in the region, he has a practical fix for cheesemakers in the meantime: hold cheese caves at or below 50°F, a temperature at which the fungus slows dramatically. It works. Cheesemakers who brought their caves into that range told him the rot stopped. The salt, though, still has his attention.
“We are unaware of studies demonstrating the presence of A. westerdijkiae in sea salts,” he said, “but this may be a fruitful area of future research for understanding how these spoilage molds may spread around the world and into food production environments.”
Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.