ANTIBACTERIAL MINERALS: ESTABLISHING AN ANTIBACTERIAL MECHANISM

Recent evidence reveals that certain clay mineral deposits rich sub-micron iron sulfides have the ability prevent pathogenic bacterial growth. Current results from Williams et al., 2011 showed elevated levels of intracellular iron which may be oxidizing and forming reactive oxygen species (ROS) upon cell death. The generation of ROS and the redox state of the iron interacting with the bacteria have yet to be determined. This research will test the hypothesis that mixed-layered clay minerals containing sub-micron iron sulfides release Fe²⁺ at low pH, while generating ROS resulting in lipid peroxidation of bacterial membranes.

To test this, model gram-negative and gram-positive bacteria (Escherichia Coli ATCC 25922 and Staphylococcus Epidermidis ATCC 14990, respectively) were reacted with aqueous suspensions of antibacterial clays and clay leachates (aqueous solutions equilibrated with antibacterial clays for 24 hrs). The geochemistry of the mineral leachates was measured using, inductively coupled plasma mass spectrometry (ICP-MS), ferrous/ferric iron assay, hydrogen peroxide (H₂O₂) assay, thiobarbituric acid aldehydes assay (TBARS) and scanning transmission X-ray microscopy (STXM).

Antibacterial susceptibility testing and ICP-MS elemental analysis of the leachates reveals that low pH (2.5-3.1) samples containing mM levels of soluble Fe, Al and Ca are antibacterial. The acidic pH is not the only factor contributing to the antibacterial effect. STXM iron maps of single cells reveals that soluble Fe²⁺ and Fe³⁺ are adsorbing to the bacterial cell walls. The adsorption of reduced iron to the cell walls of bacteria can result in lipid peroxidation and the concurrent release of toxic aldehydes. Results from the TBARS assay reveal that the antibacterial leachates are resulting in lipid peroxidation and the release of mono-aldehydes at micro molar levels from bacterial cell walls. The hydrogen peroxide and ferrous/ferric iron assay of the mineral leachates indicates that H₂O₂ is being generated in the presence of Fe²⁺, ultimately generating hydroxyl radicals which can be highly toxic to bacterial lipids, proteins and DNA. Results from this research may lead to the realization of new antibacterial mechanisms involving minerals and transition metals.