Nanoscale imaging and hydrophobicity mapping of the antimicrobial effect of copper on bacterial surfaces

• We show a technique for real time spatial and biochemical monitoring of single cells using AFM.
• The same cell can be imaged at high resolution and over time for spatio-temporal study of effect of antimicrobial agents.
• Mechanism of antimicrobial action of copper particles shown to be due to membrane destruction.
• Loss of membrane integrity is accompanied by a simultaneous loss in hydrophobicity.

Copper has a long historical role in the arena of materials with antimicrobial properties. Various forms of copper ranging from surfaces to impregnation in textiles and particles, have attracted considerable interest owing to their versatility, potency, chemical stability, and low cost. However, the effects and mechanisms of their antimicrobial action is still unclear. In this study, the effect of copper particles on Escherichia coli was studied at the nanoscale using atomic force microscopy (AFM). Time-lapse AFM images at the single cell level show the morphological changes on live E. coli during antimicrobial treatment, in which for the first time, this process was followed in situ on the same cell over time. AFM-based hydrophobicity mapping further showed that incubating cells with Cu decreased the surface hydrophobicity with an increase of incubation time. Specifically, we are able to visualize both morphology and physico-chemical nature of the bacterial cell surface change in response to copper treatment, leading to the membrane damage and cytoplasm leakage. Overall, the time-lapse AFM imaging combined with hydrophobicity mapping approach presented here provides spatio-temporal insight into the antimicrobial mechanisms of copper at the single cell level, and can be applied to design of better metallic antimicrobial materials as well as investigate different microorganisms.