New molecule found in chestnut leaves disarms dangerous staph bacteria
Scientists isolated
a molecule, extracted from the leaves of the European chestnut tree, with the
power to neutralize dangerous, drug-resistant staph bacteria. Frontiers
in Pharmacology published the finding, led by scientists at 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.
“We were able
to isolate this molecule, new to science, that occurs only in very tiny
quantities in the chestnut leaves,” says Cassandra Quave, senior author of
the paper and associate professor in Emory’s Center for the Study of Human
Health and the School of Medicine’s Department of Dermatology. “We also
showed how it disarms Methicillin-resistant Staphylococcus aureus by
knocking out the bacteria’s ability to produce toxins.”
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 work by
killing staph bacteria, which can lead 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.
“We’re trying
to fill the pipeline for antimicrobial drug discovery with compounds that work
differently from traditional antibiotics,” Quave says. “We urgently
need these new strategies.” She notes that antimicrobial infections kill an
estimated 700,000 globally each year, and that number is expected to grow
exponentially if new methods of treatment are not found.
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. “In Italian
traditional medicine, a compress of the boiled leaves is applied to the skin to
treat burns, rashes and infected wounds,” Quave says.
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.
“Our homemade
piece of equipment really helped accelerate the pace of our discovery,”
Quave says. “We were able to isolate this molecule and derive pure
crystals of it, even though it only makes up a mere .0019 percent of the
chestnut leaves.”
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.
“We’re laying
the groundwork for new strategies to fight bacterial infections at the clinical
level,” Quave says. “Instead of being overly concerned about treating
the pathogen, we’re focusing on ways to better treat the patient. Our goal is
not to kill the microbes but to find ways to weaken them so that the immune
system or antibiotics are better able to clear out an infection.”
Emory co-authors of
the paper also include graduate students Caitlin Risener and Lewis Marquez;
post-doctoral fellow Gina Porras; and former staff scientist James Lyles.
Additional authors from the University of Colorado are Young-Saeng Cho and
Morgan Brown.
The work was funded
by the National Center for Complementary and Integrative Health, Emory’s
Department of Dermatology, the National Institute of Allergy and Infectious
Diseases and the National Institute of General Medical Sciences.