Evaluation of the medicinal use of clay minerals_ as antibacterial agents

摘要

Natural clays have been used to heal skin infections since the earliest recorded history.Recently, our attention was drawn to a clinical use of French green clay (rich inFe-smectite) for healing Buruli ulcer, a necrotizing fasciitis (`flesh-eating' infection)caused by Mycobacterium ulcerans. These clays and others like them are interesting asthey may reveal an antibacterial mechanism that could provide an inexpensive treat-ment for this and other skin infections, especially in global areas with limited hospitalsand medical resources. Microbiological testing of two French green clays and other clays used traditionallyfor healing identified three samples that were effective at killing a broad spectrum ofhuman pathogens. A clear distinction must be made between 'healing clays' and thosewe have identified as antibacterial clays. The highly adsorptive properties of manyclays may contribute to healing a variety of ailments, although they are not antibacterial.The antibacterial process displayed by the three identified clays is unknown. Therefore,we have investigated the mineralogical and chemical compositions of the antibacterialclays for comparison with non-antibacterial clays in an attempt to elucidate differencesthat may lead to identification of the antibacterial mechanism(s). The two French green clays used to treat Buruli ulcer, while similar in mineralogy,crystal size, and major element chemistry, have opposite effects on the bacterial popu-lations tested. One clay deposit promoted bacterial growth whereas another killed thebacteria. The reasons for the difference in antibacterial properties thus far show that thebactericidal mechanism is not physical (e.g. an attraction between clay and bacteria),but by a chemical transfer or reaction. The chemical variables are still under investigation. Cation exchange experiments showed that the antibacterial component of the claycan be removed, implicating exchangeable cations in the antibacterial process. Further-more, aqueous leachates of the antibacterial clays effectively kill the bacteria. Progres-sively heating the clay leads first to dehydration (200°C), then dehydroxylation(550°C or more), and finally to destruction of the clay mineral structure (-900°C). By identifying the elements lost after each heating step, and testing the bactericidal effectof the heated product, we eliminated many toxins from consideration (e.g. microbes,organic compounds, volatile elements) and identified several redox-sensitive refractorymetals that are common among antibacterial clays. We conclude that the, pH and oxida-tion state buffered by the clay mineral surfaces is key to controlling the solution chem-istry and redox-related reactions occurring at the bacterial cell wall.