EXPLORATION OF THE CYTOTOXICITY OF ANTIBACTERIAL NATURAL CLAY OF LACUSTRINE ORIGIN

The emergence of antibiotic resistant bacteria drive the quest for new natural antimicrobials. We study natural clays that can destroy or significantly inhibit bacterial growth. Toxicity may come from soluble metal species, i.e. reduced Fe, capable of generating deleterious oxygen radicals. Yet, other antibacterial clays seem to differ in their mode of action.

This study addresses antibacterial clay (AMZ clay) formed in an ancient lake in the Colombian Amazon. The clay mineral fraction is composed of kaolinite (28.9%), halloysite (17.8%), illite (12%), and smectite (16.7%). Fe content is below 3%, and it host traces of other transition metals. The pH of a suspension (200mg/ml) is 4.1 and its Eh is 0.36V. These conditions determine surface charge, element solubility and speciation, and cell responses to environmental stresses.

Standard microbiology methods were used to assess the viability of two model bacteria (Escherichia coli and Bacillus subtilis) after incubation with clay for 24 hrs. Bacteria treated with clay suspensions had reduced viability by 3-5 orders of magnitude. The chemical composition of bacteria was analyzed before and after clay treatment, using ICP- MS. Results showed elevated concentrations of Al, Fe and Cu on clay treated cells relative to a control (bacteria without clay). However, unlike other antibacterial clays, the aqueous leachate derived from the clay does not affect the cells.

The zeta potentials (ζ) of clay and bacteria were measured to determine the force of attraction. At pH 4, ζ were -15, -32 and -39mV for AMZ, E.coli and B.subtilis respectively. Based on charge, bacteria and clay are not strongly attracted. In summary, a reduction of cell viability is observed when bacteria are exposed to AMZ clay, presumably due to toxic metal species, but the cells remain viable when exposed to the aqueous leachate, in absence of clay. Therefore the interaction of mineral and bacterial surfaces, is implicated in the antibacterial process.