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(353e) Natural Organic Matter and Bacterial Inoculum Concentration Affect Copper Toxicity to Escherichia coli

Authors: 
Bertuccio, A. J., Carnegie Mellon University
Moore, J. D., Carnegie Mellon University
Tilton, R. D., Carnegie Mellon University
Copper-based engineered nanomaterials (Cu ENM), in the forms of copper hydroxide and copper oxide nanoparticles, are used in a variety of commercial products for agriculture. Copper has known antimicrobial effects, and usage of Cu ENMs can impact naturally occurring microbial communities. The primary source of Cu ENM toxicity is believed to be the copper ion. Here we consider its toxicity to Escherichia coli using growth inhibition experiments at various bacterial inoculation concentrations and with additions of humic acid (HA). We observe an inoculum effect for copper toxicity to bacteria where increases in initial bacterial concentration are associated with higher copper ion minimal inhibitory concentrations and minimal bactericidal concentrations. Furthermore, we find that HA at environmentally relevant concentrations can mitigate copper toxicity, requiring up to ten times higher concentrations of copper ion to have the same antimicrobial effect as when HA is absent. Bacterial growth rate constants generally decrease as ionic copper concentration increases. Copper ion binding isotherms are used to interpret results and estimate copper partitioning to E. coli and to HA. Transcriptional responses of three copper-responsive genes, copA, cueO, and cusC, were measured to identify changes in gene expression for different initial bacterial concentrations and the effect of HA when the same copper concentration is used for each system. Reactive oxygen species (ROS) were also measured to identify differences in the amount of ROS produced with different inoculum and HA concentrations.
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