Evaluation of biocidal efficacy of copper alloy coatings in comparison with solid metal surfaces: generation of organic copper phosphate nanoflowers

Abstract

Aims: To analyse the biocidal efficacy of thermal sprayed copper surfaces. Methods and Results: Copper alloy sheet metals containing >60% copper have been shown to exhibit potent biocidal activity. Surface biocidal activity was assessed by epifluorescence microscopy. After 2-h exposure at 20°C in phosphate-buffered saline (PBS), contact killing of Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis by brass sheet metal and phosphor bronze was 3-4-times higher than that by stainless steel. SEM observations revealed that the surface membranes of both bacterial strains were slightly more irregular when exposed to brass sheet metal than stainless steel. However, when exposed to phosphor bronze coating, E. coli were 3-4 times larger with irregular membrane morphology. In addition, the majority of the cells were associated with spherical carbon-copper-phosphate crystalline nanostructures characteristic of nanoflowers. The membranes of many of the S. epidermidis exhibited blebbing, and a small subset was also associated with nanoflowers. Conclusions: Our data indicate that increasing the surface roughness of copper alloys had a pronounced impact on the membrane integrity of Grampositive and, to a lesser degree, Gram-negative bacteria. In the presence of PBS, carbon-copper-phosphate-containing nanoflowers were formed, likely nucleated by components derived from killed bacteria. The intimate association of the bacteria with the nanoflowers and phosphor bronze coating likely contributed to their nonreversible adhesion. Significance and Impact of the Study: Thermal spraying of copper alloys provides a strategy for the rapid coating of three-dimensional organic and inorganic surfaces with biocidal copper alloys. Our study demonstrates that the macroscale surface roughness generated by the thermal spray process enhances the biocidal activity of copper alloys compared with the nanoscale surface roughness of copper sheet metals. Moreover, the coating surface topography provides conditions for the rapid formation of organic copper phosphate nanocrystals/nanoflowers.

Contact Killing of Bacteria on Copper Is Suppressed if Bacterial-Metal Contact Is Prevented and Is Induced on Iron by Copper Ions

“…It has been shown in previous studies with Gram-positive and Gram-negative bacteria and yeast that membrane damage is a key event in contact killing (16,17,(27)(28)(29)(30). These findings combined with our observations support a hypothesis whereby bacterialmetal contact permeabilizes the cells, thus facilitating access of copper ions to cellular components.…”

Copper as an antimicrobial agent: recent advances

“…One method is the physical interaction of the CuNPs with the cell, or virus, plasma membrane leading to its destruction, making the microbe susceptible to damage from copper ions. [9][10][11][12][13][14][15] The smaller nanoparticles (NPs), between 1-10 nm, favour this mechanism as they can attach to the membrane and inltrate the cell. 16,17 Another method of copper's action is the generation of reactive oxygen species (ROS) by reduction of copper through a Fenton-like reaction, leading to enzyme and non-enzyme mediated oxidative damage involving lipid peroxidation, protein oxidation and DNA damage.…”

Biocidal Mechanisms of Metallic Copper Surfaces

“…The degree of surface homogeneity was also shown to modulate the killing efficiency [37]. A rougher surface is characterized by larger area of contact with bacteria cells; hence cells are exposed to more corrosion products, thus more toxicity.…”