Abstract
We have identified a natural clay mixture that exhibits in vitro antibacterial activity against a broad spectrum of bacterial pathogens. We collected four samples from the same source and demonstrated through antibacterial susceptibility testing that these clay mixtures have markedly different antibacterial activity against Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA). Here, we used X-ray diffraction (XRD) and inductively coupled plasma – optical emission spectroscopy (ICP-OES) and – mass spectrometry (ICP-MS) to characterize the mineralogical and chemical features of the four clay mixture samples. XRD analyses of the clay mixtures revealed minor mineralogical differences between the four samples. However, ICP analyses demonstrated that the concentrations of many elements, Fe, Co, Cu, Ni, and Zn, in particular, vary greatly across the four clay mixture leachates. Supplementation of a non-antibacterial leachate containing lower concentrations of Fe, Co, Ni, Cu, and Zn to final ion concentrations and a pH equivalent to that of the antibacterial leachate generated antibacterial activity against E. coli and MRSA, confirming the role of these ions in the antibacterial clay mixture leachates. Speciation modeling revealed increased concentrations of soluble Cu2+ and Fe2+ in the antibacterial leachates, compared to the non-antibacterial leachates, suggesting these ionic species specifically are modulating the antibacterial activity of the leachates. Finally, linear regression analyses comparing the log10 reduction in bacterial viability to the concentration of individual ion species revealed positive correlations with Zn2+ and Cu2+ and antibacterial activity, a negative correlation with Fe3+, and no correlation with pH. Together, these analyses further indicate that the ion concentration of specific species (Fe2+, Cu2+, and Zn2+) are responsible for antibacterial activity and that killing activity is not solely attributed to pH.
Highlights
Recent epidemiological studies have demonstrated a steady increase in infections due to antibiotic-resistant bacteria [1,2]
E. coli was grown on Luria-Bertani (LB) agar or in LB broth, and methicillin-resistant Staphylococcus aureus (MRSA) was grown on trypticase soy agar (TSA) or in trypticase soy broth (TSB)
Univariate regression analyses revealed that Zn2+, Ni2+, Co2+, and Cu2+ concentrations were positively correlated with antibacterial activity in E. coli and that Zn2+, Co2+, and Cu2+ concentrations were positively correlated with antibacterial activity in MRSA
Summary
Recent epidemiological studies have demonstrated a steady increase in infections due to antibiotic-resistant bacteria [1,2]. Reinbacher [5] describes German physician Dr Julius Stumpf’s treatment strategy in 1898 of a patient who had long been suffering from a deep and suppurating ulcer of the tibia. This patient refused amputation, so the physician began treatment with a thick layer of fine clay powder. Clays have been applied in a similar manner for the treatment of bacterial infections caused by Mycobacterium ulcerans, the causative agent of Buruli ulcer, which is a difficult-to-treat necrotic skin disease. After several months of treatment, the infections often healed with some scarring and a resumption of normal motor function [6,7,8]
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