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[Image above] An open clay pit near Crater Lake, Oregon, holds some surprising antibacterial properties. Credit: Stan Williams; NSF

We here at ACerS are used to touting all the useful uses of ceramic materials. We’re champions, cheerleaders, and crusaders for ceramics—but the newest use of ceramic materials to come across my desk still surprised me.

An NSF Discoveries story reports that clay deposits may be key to stemming worldwide increases in deadly antibacterial resistance—the ability of bacteria to avoid death at the hands of standard antibiotic treatment.

Antibiotic resistance—which by itself is a normal phenomenon of bacterial evolution—is a growing worldwide problem because of misuse and overuse of antibiotics. The bugs acquire random mutations that allow them to shrug off antibiotic treatments—they evolve, they don’t die, and your infection doesn’t clear up. CDC director Tom Frieden recently commented that antibiotic resistance could become the “next pandemic.”

There’s currently no great alternative treatments when bacteria go rogue and stop responding to antibiotics. Doctors try administering different antibiotics, but those options can quickly run out also. As an alternative, some scientists are turning to nature and history to find better solutions.

“Minerals have long had a role in non-traditional medicine,” Enriqueta Barrera, earth sciences program director at the National Science Foundation, says in the Discoveries report. “Yet there is often no understanding of the reaction between the minerals and the human body or agents that cause illness.”

While that’s not entirely true (read here; and, if you’re not squeamish, check out these graphic images), it is true that mineral clays have long been used for their healing properties. In the new study, Arizona State University and US Geological Survey scientists probed a bit deeper to characterize just why some clays exhibit exceptional antibacterial properties.

The scientists tested different clay samples from an Oregonian mine pit, known to be rich in ash from ancient volcanic activity, for their bug-busting ability. They sampled three regions of different oxidation conditions within one clay pit—oxidized white and red clays from the upper part of the pit, and unoxidized blue clays from deeper levels.

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Nodule of Oregon blue clay, coated with red clay and sulfur crystals encased in white clay. Credit: Lynda Williams; NSF

Taking the samples back to the lab and cozying them up to bacterial samples of common pathogens Escherichia coli and Staphylococcus epidermidis showed that the clays were indeed anti-bacterial, but not all were created equal. The blue samples were the most effective, white clays were moderately effective, and red clays had no bug-busting power.

Although the report doesn’t detail the compositional differences between the different colored clays, the mineral makeup, which includes color-contributing impurities in the clay, must be biologically important. Previous research by Arizona State colleagues Shelley Haydel and Caitlin Otto shows that Fe+2, Cu+2, and Zn+2 ions in particular are important for antibacterial activity. 

To further understand why the clays were effective, and thus ultimately help determine other anti-bacterial-rich deposits, the scientists turned to the bacteria themselves and took a closer look. Ultimately, they determined that the clays stop bacteria dead in their tracks largely because they rapidly uptake iron, which interferes with the bugs’ metabolism. According to the NSF report, “Cells were flooded with excess iron, which overwhelmed iron storage proteins and killed the bacteria.”

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Biogeochemist Keith Morrison sampling an outcrop of blue clay in the Oregon deposit. Credit: Lynda Williams; NSF

The paper describing the research, published in Environmental Geochemistry and Health, further elaborates that the iron-flooded cells filled up with precipitated iron oxide nanoparticles, which might further add insult to injury by generating cell-damaging reactive oxygen species.

The clays may also lend some of their antibacterial properties to their ability to buffer pH changes in wounds. Because chronic wounds are more alkaline than slightly acidic healthy skin, clay can help tip the balance back in favor of healing. “Antibacterial clays can buffer wounds to a low [more acidic] pH,” biogeochemist and senior author Lynda Williams says in the report. “The clays may shift the wound environment to a pH range that favors healing, while killing invading bacteria.”

The paper is “Mineralogical variables that control the antibacterial effectiveness of a natural clay deposit” (DOI: 10.1007/s10653-013-9585-0).

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Blue and white clay zones in the Oregon deposit, separated by a vein of rock containing sulfur. Credit: Lynda Williams; NSF