To test the effect of the mixed-metal solution, it was incubated (37 ☌) overnight with E. The stock was sterile filtered (<0.2 μm filter Genemate vacuum filtration system) and stored at 4 ☌. ![]() The mixed-metal stock was prepared with the chloride salts of the metals mixed in DIW. Therefore, the toxicity of the most abundant metals combined ( Table 2 and Figure 3) was tested by making a mixed-metal solution. The toxicity of individual transition metals in the LB-incubated-with-AMZ solution was below their MIC under the mineral-buffered pH–Eh conditions. Therefore, AMZ is an atypical antibacterial clay that appears to work by a mechanism different from that previously reported. This differs from previous research (5) showing a more important role of redox sensitive metals in the antibacterial process. (15-17) This paper shows (via nanoscale secondary ion mass spectrometry measurements) that an elevated Al level is associated with bacterial membranes and that Al is the only element that exceeds the minimum inhibitory concentration (MIC) in solution. Acid rain studies have shown an effect of Al on living organisms. (11) Furthermore, the aqueous leachate of AMZ is not antibacterial, (11) indicating that exchangeable elements are not responsible for the antibacterial action, yet the AMZ clay releases aluminum. (4, 13, 14) However, AMZ contains primarily kaolin group minerals (kaolins) and lacks reduced Fe-bearing minerals. (6) Aqueous leachates of many antibacterial clays are also antibacterial. Most of the known antibacterial clays are of hydrothermal origin and contain illite-smectite (expandable clay) and reduced Fe phases. These results highlight the importance of dissolved Al for amplifying the toxicity of transition metals to human pathogens. coli after reaction with AMZ, together with evidence of membrane permeabilization, suggests that Al reacts with membrane phospholipids, enhancing intracellular transport of metals. Ion imaging showed elevated Al levels in the bacterial membrane, and high intracellular Fe levels, relative to those of untreated controls. Only Al derived from the clay exceeded the minimum inhibitory concentrations for E. Chemical analysis of AMZ and bacteria showed that Al, P, and transition metals (Fe, Cu, Mn, and Zn) were exchanged during incubation at 37 ☌. The clay buffered the media pH to ∼4.6 and Eh values to +360 mV. Antibacterial susceptibility testing shows minimum inhibitory concentrations of 80 mg/mL against a model Escherichia coli (ATCC 25922). Here we show that Al toxicity plays a central role in the antibacterial action of a kaolin-rich clay from the Colombian Amazon (AMZ). Some clays have been shown to kill antibiotic-resistant human pathogens and may provide an alternative to known antibiotics. ![]() The problems of antibiotic overuse compel us to seek alternative antibacterial agents.
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