Antibacterial clays could fight superbugs

by Sara E. Pratt
Monday, December 8, 2014

Clays with antibacterial properties found near Crater Lake in Oregon could eventually lead to new agents in the fight against antibiotic-resistant superbugs like methicillin-resistant Staphylococcus aureus, better known as MRSA, according to a new study.

“As antibiotic-resistant bacterial strains emerge and pose increasing health risks, new antibacterial agents are urgently needed,” said Lynda Williams, a biogeochemist at Arizona State University (ASU) and co-author of the new study in Environmental Geochemistry and Health, in a statement.

The team, led by ASU graduate student Keith Morrison, recovered red, white and blue clays from a hydrothermally altered volcanic deposit near Crater Lake. The blue and white clays came from reduced, or oxygen poor, mineral zones in the deposit, while the red clays were sampled from oxidized mineral zones.

The clays were then incubated with different strains of E. coli and Staphylococcus epidermidis, which causes skin infections. The team found the blue clays to be the most antibacterial, while the white clays reduced the E. coli populations by about half and S. epidermidis populations by roughly a quarter. The red clays did not show an antibacterial effect. In the presence of the reduced blue and white clays, E. coli cells also precipitated intracellular nanoparticles of iron oxide.

Mineral analysis, including X-ray diffraction, revealed that the volcanic deposit is composed mainly of illite-smectite clays, with the blue clays containing more pyrite, a source of iron. The team suggested the clays kill bacteria by reducing the pH of the environment. Under acidic conditions, iron is more soluble. When bacteria rapidly take up an overdose of soluble iron, it impairs cell metabolism, Williams says. Figuring out the antibacterial mechanism of each clay — and each appears to have a different kill mechanism — “has been the focus of our decade-long research program,” Williams says.

In previous studies, the Crater Lake clays also proved effective against other strains of bacteria, including MRSA and beta-lactamase-resistant E. coli, which are resistant to pharmaceutical antibiotics. In 2006, Williams and colleagues found that a suspension of 100 milligrams of clay per millimeter of water killed 100 percent of MRSA cells in lab samples.

“Minerals have long had a role in nontraditional medicine. Yet there is often no understanding of the reaction between the minerals and the human body or agents that cause illness,” said Enriqueta Barrera, a program director in the National Science Foundation’s Division of Earth Sciences, which funded the research, in a statement. “This research explains the mechanism by which clay minerals interfere with the functioning of pathogenic bacteria.”

Historically, French green clays have been used to treat Mycobacterium ulcerans, which causes skin ulcerations called Buruli ulcers that are common in Africa. These ulcers can linger without healing, sometimes leading to permanent damage or becoming further infected, leading to death.

Normal skin is slightly acidic, which fends off bacteria. Chronic wounds, however, are often more alkaline, which allows certain bacteria to thrive. “The ability of antibacterial clays to buffer pH … in chronic nonhealing wounds to conditions of healthy skin appears key to their healing potential and viability as an alternative to conventional antibiotics,” the team wrote.

The deposits of French green clays have since been depleted. However, other deposits of antimicrobial clays are being sought. The most effective antibacterial clays found so far are the Crater Lake clays, Williams says.

The Crater Lake clay deposits are roughly 30 million years old, and were likely altered by thermal fluids from volcanic eruptions of silica-rich magmas within the last 70,000 years. The Crater Lake caldera formed when Mount Mazama erupted 7,700 years ago. Knowing the geologic conditions under which antibacterial clays have formed could help researchers find new deposits, Williams says.

The results of this study “have the potential to lead to the wide use of clays in the pharmaceutical industry,” Barrera said.

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