One of the most terrifying things about cholera is its lethal speed. A victim can consume contaminated food or water, come down with diarrhea a day later and, if untreated, be dead a day after that—having inadvertently spread the microorganism to friends, neighbors and family members in the meantime. Hence cholera’s reputation for tearing explosively through populations, mostly recently in Haiti beginning in 2010 and Yemen in 2016.
Two major challenges—one diagnostic, the other preventive—make it difficult to stop cholera epidemics. A simple field test can distinguish cholera from other forms of diarrhea, but only after symptoms have already appeared. And although existing vaccines can prevent the disease, they require two or three weeks to elicit protective immunity. Neither diagnosis nor vaccination is fast enough for public health workers racing to stop the first few cases of cholera from breaking out into an epidemic.
Two new studies published Wednesday in Science Translational Medicine could change that, although both are still in preliminary testing on animal models of cholera. In the first study researchers at Massachusetts Institute of Technology wanted to know “whether we could engineer a bacterium that could serve both to diagnose and prevent cholera,” says senior author James Collins. The researchers focused on Lactococcus lactis, which people have routinely consumed for thousands of years incultured dairy products like yogurt and sour cream.
The initial plan for treatment was to genetically engineer the bacterium “to produce and secrete antimicrobial peptides specific to cholera,” Collins says. But on attempting to culture L. lactis in a laboratory dish together with the cholera pathogen, he says, the researchers found to their “pleasant surprise” that no such engineering was needed: L. lactis “was either inhibiting or killing off the cholera” on its own. This was apparently because the lactic acid it secretes creates an inhospitable environment for the cholera pathogen in the petri dish—as it also presumably does in the small intestine. In testing on laboratory mice 84.6 percent of those fed L. lactis and the cholera pathogen together survived, compared with 45.7 percent of those fed the cholera pathogen alone. When the researchers experimentally altered L. lactis to stop it from producing lactic acid, this protective effect disappeared. It was, Collins says, the first time anyone has demonstrated the mechanism by which a probiotic—that is, a bacterium that confers a health benefit on its host—can prevent disease.
To turn the probiotic into a diagnostic tool, the researchers spliced in genes enabling L. lactis to detect the presence of cholera in the gut and “report” it, after relatively simple processing in the laboratory, by changing the color of the host’s stool sample. Collins conjectures that further refinements could bypass the need for laboratory processing. In the 47 countries where cholera is endemic—and in places like Haiti, where seasonal outbreaks are likely—people could in theory take the genetically altered probiotic for prevention, and could also get an early alert to cholera infection when the probiotic changes the color of their feces. Collins says his lab (which specializes in basic synthetic biology research) is now looking to work with other partners focused on applied research to address questions of safety, effectiveness and cost.
The second Science Translational Medicine study describes a vaccine that appears to function unexpectedly—and almost immediately—as a probiotic against cholera. Researchers in Matthew Waldor’s laboratory at Harvard Medical School were developing a live, attenuated (deliberately weakened) vaccine based on the highly virulent strain of cholera responsible for the Haiti outbreak. Turning the pathogen into the so–called HaitiV vaccine involved making nine different genetic alterations to minimize potential adverse reactions, as well as remove the pathogen’s ability to produce the cholera toxin and prevent the organism from becoming toxic again.
At that point, Waldor says, the standard practice would have been to give the vaccine to test animals, wait the usual two or three weeks for immunity to develop and then challenge the vaccine by injecting cholera. Instead, his doctoral student and co-author Troy Hubbard asked if he could try the cholera challenge just 24 hours after vaccination. He had a hunch the altered cholera organism in HaitiV might colonize the small intestine as rapidly as did the original cholera pathogen on which it was based. Hubbard was proposing, in effect, to defeat cholera by taking advantage of its own lethal speed. “That’s the great thing about having students,” Waldor says. “They come up with these new ideas.”
It appears to have worked, with the live weakened HaitiV bacteria colonizing the small intestine and thereby crowding out the wild cholera. In testing on rabbits, unprotected individuals injected with cholera were all dead after 18 hours—but those that had received the HaitiV vaccine 24 hours earlier were alive and well. More than half the vaccinated animals in a subsequent test showed no ill effects from cholera after 40 hours. The rest were slower to develop cholera symptoms, and the symptoms were milder. In human terms, Hubbard says, that translates into more time to get to the hospital. And because the medical treatment of cholera—replacing lost fluids—is extremely effective, that extra time can make the difference between life and death. “If there’s a vaccine that provides both short-term protection via one mechanism and also provides long-term adaptive immunity, that would be a significant advance,” says Edward Ryan, an immunologist and cholera specialist at Massachusetts General Hospital, who was not involved in either study. “This paper answers the first question, but it doesn’t say anything about the second.” Even so, he called both studies “very exciting.”
In a commentary published in Science Translational Medicine, Robert Hall, a microbiologist at the National Institute of Allergy and Infectious Diseases who did not take part in the new research, remarked that both studies “draw attention” to the intestine “as a competitive environment that can be manipulated to increase resistance to infection.” This could potentially reduce the need for antibiotics, Hall says, and thus minimize the risk of causing increased antibiotic resistance in cholera. The new idea of probiotic vaccines and carefully targeted probiotics could potentially lead to new ways of managing other bacterial infections as well, including Escherichia coli and Clostridium difficle, without relying on antibiotics.
The work on HaitiV also raises the question of whether Vaxchora, an existing live vaccine against cholera approved by the U.S. Food and Drug Administration in 2016, might likewise produce an immediate probiotic resistance to cholera. If so, that might expand the use of the existing vaccine for short-term protection against cholera in an outbreak. Vaxchora has never been tested for that rapid effect because no one ever thought to test a vaccine in the days before immunity develops, Hall says. He cautions, however, that Vaxchora is based on an older cholera strain that may colonize less aggressively, possibly reducing its ability to outcompete wild cholera in the small intestine.
“The history of cholera is one of amazing scientific advances and great optimism,” Hall says. “But cholera is vicious and really punishes complacency. So it is really excellent that these research groups come up with creative new ideas. Having contingency plans for dealing with cholera is something we will be profoundly grateful for as cholera continues to spring its nasty surprises on us.”