A Chestnut Tree Extract Shows Promise of Nature-Based Solutions to Curb Life-Threatening Staph Infections -- Without Antibiotics

The leaves of the European Chestnut Tree. Photo: Emory University

The researchers dubbed the molecule Castaneroxy A, after the genus of the European chestnut, Castanea. The use of chestnut leaves in traditional folk remedies in rural Italy inspired the research.

Methicillin-resistant Staphylococcus aureus (MRSA) causes infections that are difficult to treat due to its resistance to antibiotics. It is one of the most serious infectious disease concerns worldwide, labeled as a "serious threat" by the Centers for the Disease Control and Prevention. In the United States alone, nearly 3 million antibiotic-resistant infections occur in the U.S. each year, killing more than 35,000 people.

Antibiotics applied to staph infections may kill some of the bacteria. But overuse of antibiotics — a major crisis worldwide — has led to greater resistance among those few bacteria that survive, spawning "super bugs."

The Quave lab has identified compounds from the Brazilian peppertree, in addition to the European chestnut tree, that simply neutralize the harmful effects of MRSA, allowing cells and tissue to naturally heal from an infection without boosting resistance.

First author of the Frontiers in Pharmacology paper is Akram Salam, who did the research as a PhD student in the Quave lab through Emory's Molecular Systems and Pharmacology Graduate Program.

Quave is a medical ethnobotanist, researching traditional plant remedies to find promising leads for new drugs. Although many major drugs are plant-based, from aspirin (the bark of the willow tree) to Taxol (the bark of the Pacific yew tree), Quave is one of the few ethnobotanists with a focus on antibiotic resistance.

The story behind the current paper began more than a decade ago, when Quave and her colleagues researched written reports and conducted hundreds of field interviews among people in rural southern Italy. That pointed them to the European, or sweet, chestnut tree, native to Southern Europe and Asia Minor. 

Quave took specimens back to her lab for analysis. By 2015, her lab published the finding that an extract from the leaves disarms even the hyper-virulent MRSA strains capable of causing serious infections in healthy athletes. Experiments also showed the extract did not disturb normal, healthy bacteria on skin cells.

Finally, the researchers demonstrated how the extract works, by inhibiting the ability of MRSA bacteria to communicate with one another, a process known as quorum sensing. MRSA uses this sensing signaling system to make toxins and ramp up its virulence.

For the current paper, the researchers wanted to isolate these active ingredients from the plant extract. The process is painstaking when done manually, because plant extracts typically contain hundreds of different chemicals. Each chemical must be separated out and then tested for efficacy. Large scale fraction collectors, coupled to high-performance liquid chromatographic systems, automate this separation process, but they can cost tens of thousands of dollars and did not have all the features the Quave lab needed.

Marco Caputo, a research specialist in the lab, solved the problem. Using a software device from a child's toy, the LEGO MINDSTORMS robot creator, a few LEGO bricks, and some components from a hardware store, Caputo built an automated liquid separator customized to the lab's needs for $500. The lab members dubbed the invention the LEGO MINDSTORMS Fraction Collector. They published instructions for how to build it in a journal so that other researchers can tap the simple, but effective, technology.

The Quave lab first separated out a group of molecules from the plant extract, cycloartane triterpenoids, and showed for the first time that this group actively blocks the virulence of MRSA. The researchers then dove deeper, separating out the single, most active molecule from this group, now known as Castaneroxy A.

Tests on mouse skin infected with MRSA, conducted in the lab of co-author Alexander Horswill at the University of Colorado, confirmed the molecule's efficacy at shutting down MRSA's virulence, enabling the skin to heal more rapidly.

Co-author John Bacsa, director of Emory Department of Chemistry's X-ray Crystallography Center, characterized the crystal shape of Castaneroxy A. Understanding the three-dimensional configuration of the crystal is important for future studies to refine and optimize the molecule as a potential therapeutic.