The antibiotic properties of certain clays have been explained, hopefully leading the way to the production of more reliable antibiotic-resistant treatments for wounds.

Clays have a long tradition of medicinal use, including to treat wounds. The success of such folk medicines is mixed, but scientific testing has shown that some clays do have powerful antibiotic properties. With so many different sorts of clay in the world, however, just going into your back garden and digging some up is not a wise way to prevent infection.

Professor Lynda Williams of Arizona State University is identifying which clays work, and shedding light on why.

Sixteen years ago Williams got a message from Line Brunet de Courssou, who described her success in treating residents of Ivory Coast suffering from Buruli ulcers using French green clay.

Courssou's marriage to a French diplomat had brought her to Africa, and while there she became disturbed at the frequency of the ulcers, caused by Mycobacterium ulcerans. Given the limited access to modern antibiotics in one of the poorest countries on Earth, Courssou turned to a family remedy. The clay worked, but Courssou didn't know why and emailed clay researchers around the world in the hope of getting an explanation.

After years of research, Williams concluded Courssou's success was part luck. When Williams tested two French green clays she found one was indeed a potent killer of M. ulcerans, but the other actually promoted the growth of the bacteria. If Courssou had used the wrong clay she could have made things significantly worse.

This inspired Williams to test a range of clays from around the world, reporting in 2013 that blue clays collected from Crater Lake, Oregon, were the most effective she could find. At the time she attributed the effectiveness to a combination of the clay making the wound environment too acidic for the bacteria, and iron oxidation.

In a presentation at the American Geophysical Union Fall Conference, Williams revealed the advances a further five years of research have brought.

Williams mixed a wide variety of clays with water and reported that around 10 percent of those she tested reduced bacterial populations by a factor of 1,000 or more. One sample proved effective against 32 different species, some of them highly resistant to antibiotics. She found the aluminum in clay is as important as the iron, attacking the cell wall and allowing iron to enter. “It’s too much of a good thing," Williams told Eos. “[The bacteria] 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."

Williams has also published work on how clays interact with the biofilms that bacteria colonies use to protect themselves.

Williams wants to understand the processes because simply digging up some clay and slapping it on a wound or ulcer, as Courssou did, carries considerable risks. Even if you make sure to choose an antibiotic, rather than a bacteria-enhancing clay, there is the fact clays often contain arsenic and lead, sometimes in high enough concentrations to be dangerous for the patient.

Clays are defined as being made of particles less than 2 micrometers (0.00008 inches) across, but the ones Williams tested were less than a tenth this size, small enough to infiltrate the body with serious health consequences. 

Williams hopes synthetic clays, purified of heavy metals and with safe particle sizes could help us avoid these dangers.