title: What makes a natural clay antibacterial?
abstract: 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 Fe(2+) 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 Fe(2+). Fenton reaction products can degrade critical cell components, but we deduce that extracellular processes do not cause cell death. Rather, Fe(2+) overwhelms outer membrane regulatory proteins and is oxidized when it enters the cell, precipitating Fe(3+) and producing lethal hydroxyl radicals.
authors.affiliation: School of Earth & Space Exploration, Arizona State University , Tempe, Arizona 85287, United States. Lynda.Williams@asu.edu
authors.forename: Lynda B
authors.initials: LB
authors.lastname: Williams
authors.forename: David W
authors.initials: DW
authors.lastname: Metge
authors.forename: Dennis D
authors.initials: DD
authors.lastname: Eberl
authors.forename: Ronald W
authors.initials: RW
authors.lastname: Harvey
authors.forename: Amanda G
authors.initials: AG
authors.lastname: Turner
authors.forename: Panjai
authors.initials: P
authors.lastname: Prapaipong
authors.forename: Amisha T
authors.initials: AT
authors.lastname: Poret-Peterson
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry* / Q000633:toxicity; D000900:Anti-Bacterial Agents / Q000032:analysis* / Q000737:chemistry / Q000633:toxicity; D000077215:Clay; D004926:Escherichia coli / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D006863:Hydrogen-Ion Concentration; D017665:Hydroxyl Radical / Q000737:chemistry; D007501:Iron / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D008903:Minerals / Q000032:analysis / Q000737:chemistry / Q000633:toxicity; D010084:Oxidation-Reduction; D010758:Phosphorus / Q000378:metabolism; D017640:Silicates / Q000032:analysis / Q000737:chemistry / Q000633:toxicity
chemical_list: D000538:Aluminum Silicates; D000900:Anti-Bacterial Agents; D008903:Minerals; D017640:Silicates; C099089:illite; D010758:Phosphorus; D017665:Hydroxyl Radical; C033214:Smectite; D007501:Iron; D000077215:Clay