WHAT MAKES CLAY MINERALS ANTIBACTERIAL?

Early this century, the healing properties of some clay minerals were photographically documented to cure Buruli ulcer, a mycobacterial skin infection afflicting food gatherers from tribes in the Ivory Coast. The clays came from hydrothermally altered volcanic sources in France. Since then a variety of similar clays collected worldwide were tested in vitro against a broad-spectrum of human pathogens. These studies found that only 10% of the clay deposits tested have antibacterial properties, and their effect is variable on different pathogens. The research goal is to identify and compare the physical and chemical properties of antibacterial clays, in order to evaluate the bactericidal components and test for potential harmful side effects. Understanding natural antibacterial minerals may promote their safe use in appropriate medicinal applications.

Comparison of three antibacterial clays shows that the bactericide is a chemical rather than physical process. Aqueous leachates of the clays kill both Gram-positive and Gram-negative bacterial species to varying degrees. The clays are dominated by smectite and contain a variety of Fe-bearing minerals (e.g., pyrite, magnetite, hematite, goethite) with average crystal diameter <100nm. The nano-crystal size of these deposits may enhance the solubility of toxic elements such as Fe(II) that can generate reactive oxygen species (ROS) known to degrade biomolecules. The role of clay minerals is unproven, but due to the large relative surface area of smectites (>100m²/g), the clay minerals may buffer the water chemistry to a pH and oxidation state that increases the solubility of bactericidal compounds.

Elemental analyses by direct injection ICP-MS of E.coli treated with antibacterial clay (OMT) leachate from a sulfide rich hydrothermal system, showed elevated uptake of Al, P, V, Fe, Cu and Pb on the whole cell, compared to controls. Cell interior metal contents were measured on bacteria washed with EDTA-oxalic acid solution to remove exterior metals. Elevated Fe and P inside the cells are likely reactants in the bactericide. TEM imaging of the E.coli treated with OMT leachate showed formation of voids indicating that cells attempt to remove the toxins by efflux, but perhaps cannot accommodate the rapid chemical changes induced by the clay leachate.