Antibacterial Clays Help Combat Bacterial Infections

A new study shows that clays that contain Iron (Fe) in one or more mineral phases, including expandable phyllosilicates, can exhibit antibacterial activity.

Researchers at Arizona State University (ASU; Tempe, USA), the Mayo Clinic (Rochester, MN, USA), the U.S. Geological Survey (Reston, VA, USA), and other institutions conducted a study to examine healing clays, industrial clays, and standards from worldwide sources so as to identify the mechanisms by which they kill antibiotic-resistant pathogens. They found that about 10% of clays demonstrate a higher than 3 log10 reductions in bacterial populations. One such clay, rectorite, was bactericidal against 32 Gram-positive and Gram-negative bacterial pathogens, and even against antibiotic-resistant strains such as MRSA.

The general antibacterial mechanism appears to involve reactions with soluble redox active trace metals (e.g., Fe2+), with a significant role for Aluminum (Al3+) implicated in the binding to phospholipids in cell membranes, causing protein misfolding and poration, which may enhance the intracellular influx of reduced metals. In addition, intracellular Fenton reactions produce hydroxyl radicals that exist for nanoseconds, reducing nearby biomolecules--such as DNA and proteins--during metal oxidation. This antibacterial mechanism affects both planktonic and biofilm states of bacteria.

Secondary ion mass spectrometry analyses confirmed that both Fe and Al are transferred from the clay to the bacteria, olymerase chain reaction (PCR) gel electrophoresis showed DNA base pair destruction, and chemical testing verified intracellular protein oxidation. An ongoing in-vivo study on mice infected with S. aureus is also examining potential uptake of toxic quantities of metallic nanoparticles from aluminosilicates. The study was presented on December 10, 2018, at the American Geophysical Union (AGU) Fall meeting, held during December 2018 in Washington (DC, USA).

“Only 5-10% of natural clays have antibacterial properties; one key way to spot them is their color. Blue and green clays contain reduced iron, a less positively charged counterpart to oxidized iron, a known bacteria-fighting ingredient,” said lead author biogeochemist Lynda Williams, PhD, of ASU. “I often refer to it like a Trojan horse, because bacteria like reduced iron; they respire reduced iron and use it for metabolism. 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.”

Medicinal clays have been used both internally and topically by indigenous cultures worldwide. Some clays promote healing by element exchange through skin pores or hair follicles; others reduce edema by fluid adsorption; some line the stomach to alleviate indigestion or adsorb toxins; and some, such as French green clays, historically used in mineral baths, have been used to treat Mycobacterium ulcerans, the pathogen that causes Buruli ulcers.

Related Links:
Arizona State University
Mayo Clinic
U.S. Geological Survey