Abstract
Release of metal ions from metal-based surfaces has been considered one of the main drivers of their antimicrobial activity. Here we describe a method that enables parallel assessment of metal ion release from solid metallic surfaces and antimicrobial efficacy of these surfaces in a short time period. The protocol involves placement of a small volume of bioluminescent bacteria onto the tested surface and direct measurement of bioluminescence at various time points. In this study, two recombinant Escherichia coli strains, one expressing bioluminescence constitutively and applicable for general antimicrobial testing, and the other induced by Cu ions, were selected. Decrease in bioluminescence of constitutive E. coli on the surfaces showed a good correlation with the decrease in bacterial viability. Response of Cu-inducible E. coli showed a correlation with Cu content in the tested surfaces but not with Cu dissolution suggesting the role of direct bacteria-surface contact in Cu ion-driven antibacterial effects. In summary, the presented protocol enables the analysis of microbial toxicity and bioavailability of surface-released metal ions directly on solid surfaces within 30–60 min. Although optimized for copper and copper alloy surfaces and E. coli, the method can be extended to other types of metallic surfaces and bacterial strains.
Highlights
Metal, metal alloy or metal nanoparticle-based surfaces have been used extensively as antimicrobial surfaces in water treatment, in ventilation and air-conditioning systems, general high touch surfaces such as door handles and knobs, and more importantly in healthcare settings[1,2,3,4]
We present a novel, robust and rapid test method that can be applied for the analysis of solid surfaces which in this study was optimized for copper and copper alloy-based surfaces
A variety of natural as well as recombinant bioluminescent bacteria can be obtained through commercial suppliers or from research laboratories are available, we here used two E. coli strains because this bacterium is a medically relevant test organism and because we have constructed a series of E. coli-based bioluminescent sensor strains that enable detection of metal ions[25] as well as other intracellular stress conditions[26]
Summary
Metal-, metal alloy or metal nanoparticle-based surfaces have been used extensively as antimicrobial surfaces in water treatment (filters, water pipes), in ventilation and air-conditioning systems, general high touch surfaces such as door handles and knobs, and more importantly in healthcare settings[1,2,3,4]. Copper-based surfaces were the first registered solid antimicrobial surfaces by US EPA in 2008, after being supported by extensive antimicrobial efficacy testing According to those studies, copper and copper alloys such as brass and bronze were able to decrease the counts of bacteria, including pathogens such as methicillin resistant Staphylococcus aureus, by 7 to 8 logs within hours[5]. Due to the fact that release of copper ions is generally considered as the key mechanism of antimicrobial activity of copper surfaces[4,17] a recombinant bioluminescent E. coli strain that enables the quantitative assessment of bioavailable Cu ions release is used in the same test format in parallel to the constitutively bioluminescent E. coli. A constitutively bioluminescent E. coli strain can be individually used as a rapid first tool to evaluate the antibacterial effect of other (non-metallic) solid surfaces
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