In the early 2000s, a deadly gut infection began to surge. After decades of lurking in intestines and hospitals—more opportunistic nuisance than lethal threat—the bacterium Clostridium difficile abruptly exploded, spreading rapidly and causing more severe diarrheal disease than ever before. By 2011, the Centers for Disease Control and Prevention estimated that C. diff infected nearly half a million people in the US that year, killing approximately 29,000.

Two strains led the deadly reign: RT027 and RT078 (named based on the genetic code of their ribosomes, or “ribotype”). But scientists could only speculate as to why this duo was suddenly so menacing. At least one of them turned up with resistance to a class of antibiotics called fluoroquinolones, which contains ciprofloxacin among other common antibiotics. This fact led some researchers to suggest that the bacteria’s rise may have been linked to development of that drug resistance.

But scientists had identified fluoroquinolone resistance in C. diff back in mid-'80s. Why would it suddenly matter? There was another, cryptic factor at play, it seemed.

With a study published in Nature recently, scientists think they’ve finally figured out what that enigmatic element was—and it’s even more obscure than anyone may have guessed. It wasn’t some new weapon the bacteria acquired or a waning antibiotic. It was a boring, harmless sugar—one often found in ice creams. And its part of the story started back in the ‘90s in Japan.

At the time, food scientists were trying to come up with an inexpensive way to make that sugar, called trehalose. It’s a disaccharide made up of two glucose molecules linked with a sturdy α,α-1,1-glucoside bond, and it’s naturally found in low levels in some bacteria, fungi, plants, and invertebrates. Its chemistry made food scientists drool. Trehalose’s strong bond means it’s resistant to breaking down in high temperatures and acidic conditions. It also seems to have a gel phase that stabilizes and protects cells from extreme dryness and cold. In foods, it can be used as a mild sweetener, moisture-preserver, thickener, and stabilizer. The trouble was, making it in large quantities was expensive—about $700 per kilogram.

With tinkering, syrup scientists at Japan’s Hayashibara chemistry company finally figured out a novel enzymatic method to make it on the cheap from starch. The method brought costs down to just $3 per kilogram. By 2000—just before the rise of C. diff.—the company got approval from the US Food and Drug Administration to use it as an additive in food. Approval for use in Europe came the following year. Manufacturers started pouring trehalose into a variety of foods, from pasta to ground beef to ice creams.

To us, trehalose is an indistinguishable sweetener. It’s about 45 percent as sweet as table sugar (sucrose) and breaks down to simple glucose. But, according to the authors of the new Nature study, we’re not the only consumers. In a sugar-eating screen, the study authors noted that two C. diff. strains (out of 21) could happily survive on just a dash of trehalose. Those strains were RT027 and RT078.

Cloying killer

It’s not uncommon for C. diff to carry the genetic blueprints for trehalose digestion. But, the study authors, led by microbiologist Robert Britton at Baylor College of Medicine, found that the two epidemic strains had genetic tricks to live on just tiny amounts. RT027 had a genetic mutation that made it more sensitive to trehalose concentrations. RT078, on the other hand, had gotten hold of a cluster of four new genes for trehalose metabolism.

The sweet genetics helped the strains cause trouble, Britton and colleagues found. In one experiment, they infected 55 mice with either RT027 or a genetically engineered version that couldn’t metabolize trehalose. When the mice drank trehalose-laced water, RT027 killed nearly 80 percent of them. But, the trehalose mutant only killed 33 percent of the mice. In a second experiment, the researchers infected another 55 mice with RT027 and gave them either trehalose-laced water or plain water. The trehalose-laced water increased mortality three-fold between the groups.

Looking closer at the rodents’ infections, they found that trehalose didn’t make RT027 grow more—rather, it produced more toxins, leading to more severe disease. Next, they pitted RT078 against a genetically engineered version that couldn’t metabolize trehalose. In a mini-bioreactor that simulates C. diff infection in the bowels, the RT078 elbowed out the mutant when trehalose was present at low doses.

Together, the data suggests that trehalose metabolism gave the epidemic C. diff strains an advantage over their relatives and made them more deadly in the gut. Last, the researchers collected intestinal juices from three anonymous donors and found they contained enough trehalose to get RT027 to switch on trehalose metabolism.

“On the basis of these observations, we propose that the widespread adoption and use of the disaccharide trehalose in the human diet has played a significant role in the emergence of these epidemic and hypervirulent strains,” Britton and colleagues concluded.

Jimmy Ballard, a microbiologist at the University of Oklahoma Health Sciences Center, noted that there are some catches. In an accompanying editorial, he pointed out that researchers need more data to know for sure if the trehalose metabolism could explain the higher death rates in people. He also noted that the gut juices the team tested were from the small intestine, not the colon where C. diff causes mayhem.

That said, he concluded that the study is “compelling.”

“It is impossible to know all the details of events surrounding the recent C. difficile epidemics,” he wrote. “But the circumstantial and experimental evidence points to trehalose as an unexpected culprit.”

Nature, 2018. DOI: 10.1038/nature25178 (About DOIs).