What Makes a Natural Clay Antibacterial?

• Published in: Environmental science & technology Vol. 45; no. 8; pp. 3768 - 3773
• Main Authors: Williams, Lynda B, Metge, David W, Eberl, Dennis D, Harvey, Ronald W, Turner, Amanda G, Prapaipong, Panjai, Poret-Peterson, Amisha T
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.04.2011
• Subjects: Aluminum Silicates - chemistry; Aluminum Silicates - toxicity; Anti-Bacterial Agents - analysis; Anti-Bacterial Agents - chemistry; Anti-Bacterial Agents - toxicity; Antibacterial agents; Antibiotics. Antiinfectious agents. Antiparasitic agents; Biological and medical sciences; Cells; Clay; E coli; Ecotoxicology and Human Environmental Health; Environmental science; Escherichia coli - drug effects; Escherichia coli - metabolism; Escherichia coli - ultrastructure; Hydrogen-Ion Concentration; Hydroxyl Radical - chemistry; Iron; Iron - metabolism; Medical sciences; Microscopy, Electron, Transmission; Minerals - analysis; Minerals - chemistry; Minerals - toxicity; Oxidation; Oxidation-Reduction; Pathogens; Pharmacology. Drug treatments; Phosphorus - metabolism; Silicates - analysis; Silicates - chemistry; Silicates - toxicity; Clay; Pharmacological activity; Antiseptic; Natural product; Physicochemical properties activity relationship; Antibacterial agent; Biological activity
• DOI: 10.1021/es1040688
• ISSN: 0013-936X

Natural clays have been used in ancient and modern medicine, but the mechanism(s) that make certain clays lethal against bacterial pathogens has not been identified. We have compared the depositional environments, mineralogies, and chemistries of clays that exhibit antibacterial effects on a broad spectrum of human pathogens including antibiotic resistant strains. Natural antibacterial clays contain nanoscale (<200 nm), illite-smectite and reduced iron phases. The role of clay minerals in the bactericidal process is to buffer the aqueous pH and oxidation state to conditions that promote Fe2+ solubility. Chemical analyses of E. coli killed by aqueous leachates of an antibacterial clay show that intracellular concentrations of Fe and P are elevated relative to controls. Phosphorus uptake by the cells supports a regulatory role of polyphosphate or phospholipids in controlling Fe2+. Fenton reaction products can degrade critical cell components, but we deduce that extracellular processes do not cause cell death. Rather, Fe2+ overwhelms outer membrane regulatory proteins and is oxidized when it enters the cell, precipitating Fe3+ and producing lethal hydroxyl radicals.