Copper nanoparticles have negligible direct antibacterial impact

Introduction. Soluble copper that can be acquired by bacteria is toxic and therefore antimicrobial. Whether nanostructured copper materials, in either disperse or agglomerated form, have antimicrobial impact, aside from that of their dissolution products, is not clear and was herein addressed. Methods. We took five nanostructured copper materials, two metallic, and three oxo-hydroxides with one of these being silicate-substituted. Four agglomerated in the bacterial growth media whilst the silicate-substituted material remained disperse and small (6.5 nm diameter). Antibacterial activity against E. coli was assessed with copper phase distribution measured over time. Using the dose of soluble copper, and benchmark dose non-linear regression modelling, we determined how well this phase predicted antimicrobial activity. Finally, we used Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis to investigate whether membrane adhesion effects by copper were plausible or if intracellular uptake most likely explained the bacterial impact of copper. Results. Comparison over time of antimicrobial activity against particulate or soluble phases of the aquated materials clearly demonstrated that soluble copper but not particulate forms were associated with inhibition of bacterial growth. Indeed, the benchmark dose modelling showed the soluble dose required to cause a 50% reduction in E. coli growth was strongly clustered – for all particle formulations – at 14.5 mg/L (10 – 19 mg/L 90% confidence interval). By comparison, total copper levels associated with the same reduction in viability varied widely (45-549 mg/L). Finally, in favour of this soluble product dominance in terms of antimicrobial activity, copper had low association with bacterial membrane (something both soluble and particulate materials could do) but showed high intra-bacterial levels (something only soluble copper could do). Conclusion. Taken together our data show that it is the uptake of soluble but not particulate copper, and the intracellular loading not just contact and membrane association, that drives copper toxicity to bacteria. Therapeutic strategies for novel antimicrobial copper compounds should consider these findings.