EVIDENCE FOR CLAY MINERAL SORPTION OF BACTERIAL NUTRIENTS CAUSING GROWTH INHIBITION

The overuse and misuse of antibiotics threatens both environmental and human health. Antibacterial clays are an alternative to antibiotics that can reduce even antibiotic resistant bacteria and can be safely disposed of in the environment after use. However, to use antibacterial clays properly we need to understand their various antibacterial modes of actions. To inhibit or halt bacterial growth, antibacterial clays buffer the fluid pH outside of the near-neutral environment that suits human pathogens, and mineral-microbe interactions are affected by ion exchange.

Here we investigate a natural clay from the Colombian Amazon (AMZ) known as a healing clay to the Uitoto natives of the Colombian Amazon. With tribal permission to collect clay samples, and investigate the antibacterial action of the AMZ clay we found that the ingested clays are primarily composed of kaolinite and smectite, containing a range of transition metals (e.g., Fe, Cu, Ni, Mn). A single dose of AMZ clay (250mg/mL) induced a 4-6 order of magnitude in vitro reduction of Escherichia coli (ATCC 25922) when incubated 24hr at 37˚C, unlike control clays (kaolinite, smectite) that did not impact bacterial survival. In aqueous suspensions the AMZ minerals buffer fluid pH between 4.4-4.6 and soluble metals are released. However, their individual aqueous concentrations are too low to be toxic.

Chemical analyses of E. coli populations were compared before and after reaction with AMZ and showed that essential bacterial nutrients (Mg and P) were depleted, relative to control bacterial populations. By adding back the P and Mg to the growth media, the E. coli population resumed growth. We conclude that the AMZ inhibits bacterial growth primarily by absorbing Mg and P. PO₄^3– adsorption is enhanced by defect sites (+ charges) associated with surface dissolution. The Hinkley Index for the AMZ clay is 0.8 indicating moderate surface defects. Withholding essential nutrients imperative for bacterial metabolism is effective but less aggressive than metal-induced toxicity, which could make this type of antibacterial action more suitable for certain applications such as treatment of gastro-intestinal infections or decontamination of water.