In the 1990s French humanitarian Line Brunet de Courssou began treating Buruli ulcer—violent, flesh-eating eruptions of Mycobacterium ulcerans—with two imported French green clays. The application of one type of clay to such lesions produced a pain that some patients equated to childbirth and, after several days, purplish skin tissue surrounding the open wound. A subsequent application of a second variety, this one mixed with shea butter (a fat), produced no pain and helped heal the oozing wound, replacing it with flexible scar tissue over the course of several months.

For centuries the French have used such green clays, rich in iron, for healing wounds. And the clays have proved capable of treating these M. ulcerans outbreaks, for which the only other cure is surgical removal or amputation. But scientific proof was lacking, so Brunet de Courssou enlisted the aid of mineralogist Lynda Williams of Arizona State University (A.S.U.) in Tempe to take pictures of the microscopic structure of the clays and try to figure out the source of their healing powers.

"This clay was unique in that they were very small particle size, 200 nanometers," or one four-hundredth the width of a human hair, Williams says. Their suppliers—French companies Agriletz and Agricur—could not say where the clays came from, but French mineralogists believe they might have formed from volcanic ash deposits in the Massif Central, an upland area in south-central France . "The suppliers themselves either don't know where they come from," Williams says, "or won't tell us."

Intrigued, Williams arranged to test the clays against Escherichia coli, the ubiquitous food pathogen. "That's when we first discovered that the first clay promoted bacterial growth and the second killed it," though she declines to identify either clay specifically. "The suppliers do not have any more antibacterial clay, we tested all of their supplies, so naming them is irrelevant."

But E. coli cultures mixed with the original batch of one of the clays disappeared entirely. "Something in this clay, whether it be physical or chemical, is killing this bacteria," says A.S.U. microbiologist Shelley Haydel, who was enlisted by Williams to help probe both clays' properties.

Haydel then tested the germ-killing clay against a variety of bacteria, ranging from Salmonella to antibiotic-resistant Staphylococcus aureus. "The strains that we are using are the same ones that pharmaceutical companies use to test their antibiotics," Haydel says. "All of the gram-negative pathogenic bacteria to humans that we've tested, we can kill completely."

Further, as with certain antibiotics such as tetracycline, the clay inhibited the growth of the bacteria that it could not destroy. "The number of cells we start with is 107 Staph. aureus," Haydel says. "After 24 hours, that level reduces tenfold."

How the clay does this remains a mystery, however. Chemical testing revealed no particular minerals or metals in the clay that might explain its antibacterial properties, and even after leaching with water it retains its power. In fact, the clay kills pathogens even when heated to more than 1,000 degrees Fahrenheit (550 degrees Celsius), though it loses its germ-fighting ability when heated to more than 1,650 degrees F (900 degrees C).

Such heating destroys the clay's structure and leaves behind only the heaviest elements, such as radioactive cesium and selenium, along with poisonous arsenic. "But they are all below the minimum inhibitory concentration for E. coli," Williams notes. "They can tolerate 200 parts per million and we're talking about 50 ppm."

Williams says the clay's antibacterial effect appears to be chemical rather than physical, because its strength diminishes as it loses positively charged molecules and it does not smother the bacteria or cause its cell walls to burst. "After six hours, you can see a [bacterial cell] surface that is kind of wormy or grainy," Williams says. "It doesn't look like something is precipitating at the surface. Maybe something is going into the cell and damaging it that way."

It is also possible that the clay, which is, after all, a mix of many different elements in a malleable mass, combines several different properties to fight bacterial infection. "Is it a couple of things acting synergistically?" Haydel asks. "We just don't know."

Adds Williams: "I think these clays are buffering water to keep whatever's toxic to bacteria in there. If we remove the clay from water, I think it's not going to work."

The original batch of clay has already lost its curative power. "We went back and got some from the same batch and it didn't kill," Williams says. "This clay has been sitting outside in a pile for 10 years. It could have oxidized and maybe the oxidation state has affected the antibacterial properties."

Many more years of research will be required to determine what, if anything, gives the clay its ability to curb bacteria, and the researchers have yet to publish their results in a peer-reviewed journal. But they say there's clearly something to the ancient remedy: Two other clays with similar properties have already been identified. "They're all different mineralogically, though they have a general structure in common," Williams says. "We're trying to compare the properties of these antibacterial clays and see what's going on."

In the meantime, the clay treatment continues to be the only one available for flesh-dissolving M. ulcerans infections in Africa, a disease the World Health Organization has identified as an emerging health threat. And it is possible that the clays simply prompt the human immune system to respond to the infection, a response M. ulcerans normally suppresses.

Even so, the clays may well prove more useful than previously thought. "Even if you removed the antibacterial properties, do the clays have any effect on wound healing? What is the body's response? Are you stimulating tissue regeneration?" Haydel asks. "We already use maggots and leeches in hospitals. Why don't we use clays?"