Abstract
Although the bactericidal effect of copper has been known for centuries, there is a current resurgence of interest in the use of this element as an antimicrobial agent. During this study the use of dendritic copper microparticles embedded in an alginate matrix as a rapid method for the deactivation of Escherichia coli ATCC 11775 was investigated. The copper/alginate produced a decrease in the minimum inhibitory concentration from free copper powder dispersed in the media from 0.25 to 0.065 mg/ml. Beads loaded with 4% Cu deactivated 99.97% of bacteria after 90 minutes, compared to a 44.2% reduction in viability in the equivalent free copper powder treatment. There was no observed loss in the efficacy of this method with increasing bacterial loading up to 106 cells/ml, however only 88.2% of E. coli were deactivated after 90 minutes at a loading of 108 cells/ml. The efficacy of this method was highly dependent on the oxygen content of the media, with a 4.01% increase in viable bacteria observed under anoxic conditions compared to a >99% reduction in bacterial viability in oxygen tensions above 50% of saturation. Scanning electron micrographs (SEM) of the beads indicated that the dendritic copper particles sit as discrete clusters within a layered alginate matrix, and that the external surface of the beads has a scale-like appearance with dendritic copper particles extruding. E. coli cells visualised using SEM indicated a loss of cellular integrity upon Cu bead treatment with obvious visible blebbing. This study indicates the use of microscale dendritic particles of Cu embedded in an alginate matrix to effectively deactivate E. coli cells and opens the possibility of their application within effective water treatment processes, especially in high particulate waste streams where conventional methods, such as UV treatment or chlorination, are ineffective or inappropriate.
Highlights
Elemental copper has been used as an anti-spoiling agent and preservative for over two millennia, predating the discovery of the role of microbes in disease during the late nineteenth century [1]
Salmonella enterica and Campylobacter jejuni are destroyed within two hours on a dry copper surface [2] and Santo et al showed that Staphlococcus haemolyticus was deactivated within minutes, accompanied by extensive membrane damage [3]
The other treatments resulted in a dose related reduction of E. coli viability, with between 79 and 97% of bacteria non-viable after 90 minutes (Figure 2)
Summary
Elemental copper has been used as an anti-spoiling agent and preservative for over two millennia, predating the discovery of the role of microbes in disease during the late nineteenth century [1]. The anti-microbial mechanism of copper in such applications is poorly characterised, recent work suggests that toxicity is due to the oxidation of the surface layer and consequent donation of electrons to cell wall components, such as lipids and proteins, resulting in a rapid reduction of cell integrity [3]. Other mechanisms such as internal protein binding and DNA mutation have been suggested [4], [5] and in vitro experimentation has showed that Cu can bind to DNA. Macomber et al indicated that in vivo, Cu2+ ions were sequestered by the glutathione naturally produced by E. coli preventing oxidative damage to DNA through Fenton-like reactions [6]
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