Whenever anyone from Gerry Wright’s laboratory goes on vacation, they come back with a small plastic bag of dirt. Usually these muddy mementos yield nothing of interest. But one graduate student’s soil souvenir from a hiking trip in Eastern Canada produced a compound that could hold the key to combating one of the world’s most insidious groups of superbugs.

Gerry Wright and colleagues are purifying a potential antibiotic adjuvant called aspergillomarasmine A from Aspergillus fungus (pictured). Image courtesy of Andrew King.

The compound has no bacteria-killing activity itself. Rather, the agent found within the teaspoon of soil from the Kejimkujik National Park in Nova Scotia is what Wright calls an “antibiotic adjuvant.” And much like an adjuvant used for boosting the immune response to a vaccine, the compound can enhance the potency of antimicrobials by, for example, blocking enzymes that serve as the linchpins for bacterial resistance.

Wright’s adjuvant helped restore the killing power of the antibiotic meropenem when given to mice infected with a form of Klebsiella bacteria that was difficult to treat, he and his colleagues from Canada’s McMaster University reported a couple years ago in Nature (1). The researchers have since tested the compound in rats and dogs to make sure it’s safe. And a group of Boston-area entrepreneurs is now raising money to move the agent into human clinical trials. “We continue to work really hard to turn this into a drug,” Wright says.

With the rise of resistance to every drug in the antibiotic arsenal, and fewer than 40 new agents in clinical development, experts have warned of a coming postantibiotic crisis in which 10 million people could die globally each year from untreatable infections by 2050, unless action is taken in the very near future (2).

Finding new antibiotics is essential. But as a stopgap, it’s important to “do our best to preserve the ones we have already, and the adjuvant idea is a really good one in that way,” says Wright.

Rescued From Resistance The adjuvant approach actually dates back to the mid-1970s with the discovery of the molecule clavulanic acid; several more adjuvants have followed, all of which restore sensitivity to what are known as β-lactam antibiotics. Such drugs—which include penicillins, cephalosporins, and carbapenems—contain a four-member ring called a β-lactam in their chemical structures. The adjuvants work by blocking the β-lactamase enzyme that drug-resistant bacteria use to break apart these molecular motifs. However, these and all other β-lactamase blockers on the market suffer from a common limitation: none are effective against a particular form of the enzyme that has recently emerged as a major public health concern. This new form, known as metallo-β-lactamase, has no proven drug inhibitors. And one particularly nasty version of the enzyme—the New Delhi metallo-β-lactamase-1, or NDM-1—often spreads in tandem with other resistance mechanisms that can render bacteria impervious to nearly all available antimicrobial agents. NDM-1 arose in India in the late 2000s and has since been found in bacteria across the world. Most of the bugs that harbor NDM-1 so far are actually not that good at causing disease, notes Timothy Walsh, a medical microbiologist at Cardiff University in Wales, who helped discover NDM-1. But he adds that it’s “only a matter of time before they pass their genes on to the nastier types of E[scherichia] coli and Klebsiella. And then we’re in trouble.” With soil samples from Nova Scotia and elsewhere in hand, Wright and his team set out on a mission to find inhibitors of NDM-1. The researchers engineered a strain of E. coli to express the enzyme, grew the bacteria on plates containing the β-lactam meropenem, and added extracts derived from environmental microorganisms that laboratory members had collected on their holidays. Was there anything in the soil that could render bacteria susceptible again to the antibiotic? The idea of looking to nature for drug leads has a long and proven track record in the fight against bacteria. Penicillin was discovered in the 1920s in a fungus; streptomycin in the 1940s in a bacterium. The approach may not have yielded any new broad-spectrum bacteria-killers for decades, but as Christian Melander, a chemist at North Carolina State University, points out: “We haven’t looked for this kind of adjunctive activity a lot with natural products.”

Hunting Down Inhibitors As a first pass, Andrew King, a graduate student in Wright’s laboratory, screened a small collection of 500 soil extracts. To everyone’s surprise, King got a hit right off the bat. “I thought the screen had gone awry,” recalls Wright. “We’d been screening tens of thousands of molecules for 15-plus years, and you never find something in the first 500.” But sure enough, from a strain of Aspergillus fungus collected in Kejimkujik, King had stumbled upon a molecule called aspergillomarasmine A (AMA). This fungal product neutralized NDM-1 and restored the killing power of meropenem, both in cultured NDM-1–expressing E. coli and in Klebsiella-infected mice. To see how broad-acting AMA might be, King shipped off a sample to Walsh, who had amassed a global collection of more than 200 metallo-β-lactamase–expressing superbugs from Russia, India, Pakistan, Australia, North Africa, and South America. Among these laboratory-cultured strains, Walsh found that the compound suppressed resistance in 88% of NDM-1+ bacteria and in 90% of bacteria with another metallo-β-lactamase called Verona integron-encoded metallo-β-lactamase, or VIM. AMA is a potent inhibitor and, assuming that its safety profile is all good, it “has a place in the antimicrobial arena,” says Walsh, a coauthor of the Nature study (1) that reported the compound’s discovery. Actually, as it turns out, it was more of a rediscovery. Chemists in France had first described AMA more than 50 years ago, detailing the compound’s structure and its ability to wilt plant leaves. A Japanese drug company later investigated AMA as a possible treatment for high blood pressure. Although King confesses he would have preferred to uncover something new, there was an upside to AMA’s research history: the Japanese scientists had shown that AMA had a low-toxicity profile in mice. That was a relief, given AMA’s mechanism of action, and it helped bolster King’s confidence that the compound could prove safe in people.