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Mineralogical variables that control the antibacterial effectiveness of a natural clay deposit

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Abstract

As antibiotic-resistant bacterial strains emerge and pose increased global health risks, new antibacterial agents are needed as alternatives to conventional antimicrobials. Naturally occurring antibacterial clays have been identified which are effective in killing antibiotic-resistant bacteria. This study examines a hydrothermally formed antibacterial clay deposit near Crater Lake, OR (USA). Our hypothesis is that antibacterial clays buffer pH and Eh conditions to dissolve unstable mineral phases containing transition metals (primarily Fe2+), while smectite interlayers serve as reservoirs for time release of bactericidal components. Model pathogens (Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990) were incubated with clays from different alteration zones of the hydrothermal deposit. In vitro antibacterial susceptibility testing showed that reduced mineral zones were bactericidal, while more oxidized zones had variable antibacterial effect. TEM images showed no indication of cell lysis. Cytoplasmic condensation and cell wall accumulations of <100 nm particles were seen within both bacterial populations. Electron energy loss analysis indicates precipitation of intracellular Fe3+-oxide nanoparticles (<10 nm) in E. coli after 24 h. Clay minerals and pyrite buffer aqueous solutions to pH 2.5–3.1, Eh > 630 mV and contain elevated level (mM) of soluble Fe (Fe2+ and Fe3+) and Al3+. Our interpretation is that rapid uptake of Fe2+ impairs bacterial metabolism by flooding the cell with excess Fe2+ and overwhelming iron storage proteins. As the intracellular Fe2+ oxidizes, it produces reactive oxygen species that damage biomolecules and precipitates Fe-oxides. The ability of antibacterial clays to buffer pH and Eh in chronic non-healing wounds to conditions of healthy skin appears key to their healing potential and viability as an alternative to conventional antibiotics.

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Acknowledgments

The authors are grateful for the use of the United States Geological Survey (Boulder CO), X-ray Diffraction Lab. Various laboratories at Arizona State University supported this work, and we thank David Lowry and Robert Roberson in the School of Life Sciences (SOLS) Bioimaging Facility—Electron microscopy division; Stanley Williams for observations in the field; Sandra Londoño Arias for assistance with the TEM; Thomas Groy for XRD facilities at ASU; Everett Shock for use of his biogeochemistry lab; Steve Romaneillo for ICP-MS analyses in the W.M. Keck Foundation Laboratory for Environmental Biogeochemistry; Rajeev Misra (SOLS) and Amisha Poret-Peterson in the Astrobiology Inst. for help with microbiology techniques; Karl Weiss and Jiangtao Zhu in the LeRoy Eyring Center for Solid State Science for help with the STEM–EELS imaging. This research was funded by Grant (EAR-1123931) from the National Science foundation, the Clay Minerals Society, and the Geological Society of America.

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Morrison, K.D., Underwood, J.C., Metge, D.W. et al. Mineralogical variables that control the antibacterial effectiveness of a natural clay deposit. Environ Geochem Health 36, 613–631 (2014). https://doi.org/10.1007/s10653-013-9585-0

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