What makes a natural clay antibacterial?

Area of Science:

• Geochemistry
• Microbiology
• Materials Science

Background:

• Natural clays have historical and current medicinal uses.
• The specific mechanisms of clay-mediated bacterial killing remain largely unknown.
• Antibiotic-resistant pathogens necessitate novel therapeutic strategies.

Purpose of the Study:

• To identify the mineralogical and chemical properties of clays with antibacterial activity.
• To elucidate the mechanism by which certain clays kill bacterial pathogens, including antibiotic-resistant strains.
• To compare antibacterial clays from different depositional environments.

Main Methods:

• Comparative analysis of clay depositional environments, mineralogy, and chemistry.
• Investigation of clay leachates against a broad spectrum of human pathogens.
• Chemical analysis of bacterial cells (E. coli) exposed to antibacterial clay leachates.
• Assessment of iron (Fe) and phosphorus (P) intracellular concentrations.

Main Results:

• Antibacterial clays contain nanoscale illite-smectite and reduced iron phases.
• Clay minerals buffer pH and oxidation states, enhancing Fe(2+) solubility.
• Bactericidal clays increase intracellular Fe and P concentrations in E. coli.
• Iron (Fe2+) enters cells, precipitates as Fe(3+), and generates lethal hydroxyl radicals via intracellular oxidation.

Conclusions:

• The bactericidal effect of clays is primarily an intracellular process driven by iron.
• Clay-induced iron overload and subsequent intracellular oxidation are responsible for bacterial cell death.
• This mechanism offers a potential avenue for developing new antibacterial agents against resistant strains.