Ceramic Industry

MATERIAL INNOVATIONS: Killer Clay

May 1, 2010
Research has advanced our understanding of the antibacterial activity of clay minerals and their ability to kill what the best antibiotics on the market can't touch.

ASU School of Life Sciences undergraduate Jenny Koehl and microbiologist Shelley Haydel investigate the chemistry and killing power of clays with antibacterial activity.


Alternative approaches to medicine are stock-in-trade in the Arizona State University (ASU) laboratory of microbiologist Shelley Haydel. When ASU senior Jenny Koehl joined Haydel's investigative team seeking firsthand knowledge of how basic research is done, drugs are tested and potential cures are produced, she found it and much more.

With the guidance of Tanya Cunningham, a graduate student mentor, Koehl has helped advance our understanding of the antibacterial activity of clay minerals and their ability to kill what the best antibiotics on the market can't touch. Haydel's group collaborated with Jack Summers, an inorganic chemist at Western Carolina University, to uncover two factors that control the antibacterial activity. Their work was published in the March 1 issue of Public Library of Science (PLoS) ONE.

"This work sets a baseline from which to look for potential mechanisms of antibacterial action," said lead author Cunningham, who is now a research technician with the Fred Hutchinson Cancer Research Center in Seattle.

"We need helpful alternatives, natural approaches to antibacterial cures, because there is bacterial resistance to drugs," Koehl said. "Knowing the mechanisms of action will help us develop our own topical treatments."

Understanding Antibacterial Activity

Clay has had a role in human health as ancient as man. However, specific identification of the mechanisms underlying this antibacterial activity has been elusive. The Haydel-Summers collaborative has added clarity to these distinctly muddy waters by screening more than 50 mineral mixtures and leachates that are marketed as health and cosmetic products using pathogens such as Escherichia coli, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Pseudomonas aeruginosa. Only two mineral mixtures of significantly different compositions (and their leachates) were discovered to possess antibacterial traits.

Understanding clay's structure is integral to answering questions about the mechanisms behind its antibacterial activity. Negatively charged surfaces attract positively charged elements, such as iron, copper, silver and other metals. In turn, water is absorbed between layers of the crystal structure, creating a cation sandwich with an aqueous filling (or interlayer). When extracted from the mineral mixtures, antibacterial activity in leachates confirms that the antibacterial activity is chemically based and not a result of physical interactions with microbes.

Because of the tendency of clay to attract multivalent ions, particularly metals, the scientists next examined the leachates' chemistry and antibacterial activity in the presence of chelators, which bind metals. The researchers also used thiourea, a hydroxyl radical scavenger, at various pH levels. Chelation of the minerals with ethylenediaminetetraacetic acid (EDTA) or desferrioxamine eliminated or reduced toxicity, respectively.

Further testing of the mineral leachates confirmed that there are higher concentrations of chemically accessible metal ions in leachates from antibacterial samples than from non-bactericidal mineral samples. In addition, acidic conditions were found to increase the availability of metal ions and their toxicity. Overall, these findings suggest a role of an acid-soluble metal species, particularly iron or other sequestered metal cations, in mineral toxicity.

Continued Studies

However, whatever advances the study puts forward also present researchers with further challenges. Acidity may complicate the development of topical treatments, if neutral pH, which is least damaging to skin and tissue, also reduces the mineral's antibacterial action. Another complicating factor is that chemical environments under which any particular clay can emerge can greatly influence its toxicity, adsorptive qualities and, according to the researchers' findings, its antibacterial effects.

"Because natural mineral mixtures can be variable, both mineralogically and chemically, we must continue to define specific chemical properties that influence the antibacterial effectiveness," Haydel said. "Our goal is to understand the details so we can, in the future, perhaps generate mineral mixtures that mimic the chemical compositions and environment so that the antibacterial activity can be controlled and ensured."

"This work is about eliminating the unknowns," Koehl said. "We have more analysis to do, looking at the leachate composition, the action of the chelators and activity of the iron scavengers." Studies are moving forward in other laboratories to develop structured clays for slow-release topical medical treatments, but there may be chemical schemes that come from Haydel's research that enhance their effectiveness.

"This study has given me an idea of how things move from idea to shelf," Koehl said. "One day, when I am a pharmacist, maybe I'll be selling this!"

This work is supported by the National Institutes of Health. Haydel's group is part of the School of Life Sciences, in the College of Liberal Arts and Sciences, and the Biodesign Institute at ASU.

For more information, visit www.asu.edu.

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