Folklore has often preached clay as a medicinal cure-all. Whether the clay is from the depths of a bubbling hot spring or the banks of a winding river, people have used it for centuries to heal.

Emerging science now shows that these cultural practices may have medical significance.

People have studied how clay heals wounds for a long time, “but they don’t understand why” clay is antibacterial. Lynda Williams, a clay chemist with Arizona State University, has been researching clay’s antibacterial properties for over a decade. Williams explains that people have studied how clay heals wounds for a long time, “but they don’t understand why” clay is antibacterial, she said. Williams presented her recent research results on 10 December at AGU’s Fall Meeting 2018 in Washington, D. C.

Williams found the inspiration for her work through an unexpected email. The message was from Line Brunet de Courssou, wife of a French diplomat to the Ivory Coast, who found villagers suffering from Buruli ulcers, an infection caused by Mycobacterium ulcerans, which is prevalent there. She treated the villagers with a family remedy: green French clay.

Miraculously, their infections healed. Courssou emailed a group of clay researchers, including Williams, seeking answers as to why the clay cured the bacterial infection.

Not All Clay Is Created Equal

To investigate, Williams and her team sampled two types of the green clay Courssou used to treat the villagers, as well as two other samples found in hydrothermal volcanic environments in Oregon and Nevada.

Only 5%–10% of natural clays have antibacterial properties, Williams explains. One key way to spot them, she says, is their color. Blue and green clays contain reduced iron, a less positively charged counterpart to oxidized iron, a known bacteria-fighting ingredient.

In the lab, Williams and her team heated the blue-green clays to purify them of natural microbes and then mixed dust mite–sized samples of the clay with distilled water. Adding water, she explains, releases elements such as iron and aluminum from the clay into the water-clay mixture, which needs to be acidic (with a pH of less than 5) or alkaline (pH above 9) to attack the bacteria.

The team then placed the clay-water mixture into test tubes with healthy bacteria. After a day of incubation, the researchers put samples into petri dishes, waited another 24 hours, then counted the number of bacterial colonies that grew or were killed. Using a microscope, the team also observed the chemical interaction between the bacteria and the clay.

The researchers found that aluminum and iron work together to kill the bacteria. Clay minerals are ideal delivery vehicles for this chemical tag team: The minerals form stacked crystalline layers with spaces in between that can store and release ions. “The clay absorbs the iron into [its] interlayer and can put it out slowly over time; it’s like a time release capsule of the active ingredients,” Williams explains. Aluminum attacks the bacteria’s cell wall, creating an entryway for iron. Iron enters the cell through the newly opened holes, oxidizing the bacteria and slowing down its growth.

“I often refer to it like a Trojan horse, because bacteria like reduced iron; they respire reduced iron and use it for metabolism,” Williams says. However, when the bacteria encounter the iron-rich clay, “it’s too much of a good thing; they don’t have the mechanism to shut off the flow of iron because normally they’re scavenging iron, and all of a sudden they have an ample supply of it.“

Of the four clays investigated, three killed bacteria within 4 hours to 1 day. The Oregon blue clay killed 100% of bacteria, including one strain of antibiotic-resistant E. coli and one strain of antibiotic-resistant Staphylococcus. The Walker clay from Nevada successfully killed 99%–100% of all bacterial species, and the Argicur French clay killed 84%–100% of bacteria. One French clay wasn’t effective because its pH was 8.

Steps Toward a Medical Treatment

In July, Williams published results from her collaboration with the Mayo Clinic in the International Journal of Antimicrobial Agents. The team investigated how the clay interacts with biofilms, which are coatings that colonies of bacteria use to protect themselves from potential threats. Biofilms make it harder for antibiotics—and ions from clay—to infiltrate. This study brings us one step closer to understanding exactly how the interaction between the clay solution and the bacteria would happen in the body.

The study reports that the blue-green clay-and-water solution significantly inhibits common bacterial strains by way of the same mechanism that occurred in the test tubes: Aluminum breaks down the cell walls, and iron rushes in to oxidize. Of nine bacterial species tested, seven were antibiotic-resistant and were killed completely by the clay mixture within 24 hours. Williams explains that the flesh-eating bacteria were not included in this research because the aim was to investigate whether clay killed bacteria that commonly infect hospital wounds in the United States.

“The most exciting thing is that this is clearly working by a different mechanism than antibiotics.” The next step in the work, Williams says, will be to test a clay topical cream on animals to confirm that trace metals like arsenic, mercury, and lead, which are common in clay, are present in small enough doses not to cause harm. Another challenge in making the clay marketable will be the small size of the clay particles. All the particles used in previous studies have been smaller than 200 nanometers across, and particles that small can lodge in human cell membranes and cause health issues.

“I want to understand the mechanism so we can make synthetic clay—we can control the particle size so it won’t be taken into the bloodstream, and we can make sure the toxic trace elements won’t be taken up either,” says Williams.

The applications of antibacterial clay range from agricultural to ointment, but ultimately, “the most exciting thing is that this is clearly working by a different mechanism than antibiotics, which are a problem for humans right now,” says Williams. People “are exposed to too many antibiotics, and bacteria are figuring out ways to resist those.”

Ironically, she noted, dusting off this ancient folk remedy may be the best way to circumvent bacteria’s defenses to 20th century antibiotics.

—Hannah Hagemann ([email protected]; @hannah_hagemann), University of California, Santa Cruz

Correction, 26 December 2018: The article has been updated to provide Lynda Williams’s correct affiliation.