Clay That Kills: Ground yields antibacterial agents

A fistful of slimy green clay may be just what the doctor ordered. Researchers studying a special type of French clay found that it smothers a diverse array of bacteria, including antibiotic-resistant strains and a particularly nasty pathogen that causes skin ulcers in some parts of the world.

Anecdotal accounts of clay’s medicinal value, particularly in cleansing and protecting the skin, date back millennia, says geochemist Lynda Williams of Arizona State University in Tempe. In 2002, Williams was approached by a French humanitarian who had been using the special clay to treat Buruli ulcer, a disfiguring illness caused by Mycobacterium ulcerans. This ailment leads to many amputations in Central and Western Africa.

After seeing the clinical data on the clay’s effectiveness against Buruli ulcer, Williams established a multicenter, interdisciplinary team of researchers to study the clay.

The researchers found that the clay, which they refer to as CsAg02, is strongly alkaline, with pH ranging from 9.4 to 10. It’s also rich in a chemical form of iron that gives it a characteristic green color. But many other clays have similar properties, says Williams.

To assess the effects of the clay on different microbes, the scientists incubated a variety of bacterial cultures with either CsAg02 or a similar clay. CsAg02 completely stopped the growth of Escherichia coli and Salmonella typhimurium, common causes of food poisoning, and of various strains of mycobacterium that lead to skin infections and ulcers.

In addition, CsAg02 inhibited the growth of both normal and antibiotic-resistant strains of Staphylococcus aureus, the most common cause of staph infections. The other clay enhanced the growth of some bacteria and had no effect on others. The researchers presented their findings in Denver on Oct. 29 at the annual meeting of the Geological Society of America.

To get at the root of how CsAg02 kills bacteria, the researchers treated the clay with a concentrated salt of potassium, which draws out charged molecules. This blocked the antibacterial function of the clay, suggesting that one or more of these charged molecules may normally disrupt bacterial metabolism. Williams has planned further studies to nail down what minerals in particular supply the crucial molecules. She says that the clay’s alkalinity may well be crucial to making the minerals toxic to bacteria. At a neutral pH of 7, the mineral concentrations found in CsAg02 would not be toxic, but the high pH may change that, she says.

Ulli Limpitlaw of the University of Northern Colorado in Greeley says that understanding how clays kill the potent bacteria is a big step in fighting bacterial infections that have no known cures.

“The ramifications of finding something this new and this powerful against some of the microbes they are looking at are huge,” she says. “It could really change medicine on some level.”

Researchers working with Williams intend not only to encourage the use of CsAg02 itself as a remedy but to promote use of the clay minerals in sewage treatment and as antibacterial agents in air filters.

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