Particle-Driven Effects at the Bacteria Interface: A Nanosilver Investigation of Particle Shape and Dose Metric

Design criteria for controlling engineered nanomaterial (ENM) antimicrobial performance will enable advances in medical, food production, processing and preservation, and water treatment applications. In pursuit of this goal, better resolution of how specific ENM properties, such as nanoparticle shape, influence antimicrobial activity is needed. This study probes the antimicrobial activity toward a model Gram-negative bacterium, Escherichia coli (E. coli), that results from interfacial interactions with differently shaped silver nanoparticles (AgNPs): cube-, disc-, and pseudospherical-AgNPs. The EC₅₀ value (i.e., the concentration of AgNPs that inactivates 50% of the microbial population) for each shape is identified and presented as a function of mass, surface area, and particle number. Further, shifts in relative potency are identified from the associated dose-response curves (e.g., shifts left, to lower concentrations, indicate greater potency). When using a mass-based dose metric, the disc-AgNPs present the highest antimicrobial activity of the three shapes (EC₅₀: 2.39 ± 0.26 μg/mL for discs, 2.99 ± 0.96 μg/mL for cubes, 116.33 ± 6.43 μg/mL for pseudospheres). When surface area and particle number are used as dose metrics, the cube-AgNPs possess the highest antimicrobial activity (EC₅₀-surface area: 4.70 × 10^-5 ± 1.51 × 10^-5 m²/mL, EC₅₀-particle: 5.97 × 10⁹ ± 1.92 × 10⁹ particles/mL), such that the relative trend in potency becomes cubes > discs > pseudospheres and cubes ≫ discs ⩾ pseudospheres, respectively. The results reveal that the antimicrobial potency of disc-AgNPs is sensitive to the dose metric, significantly decreasing in potency (∼5-30×) upon conversion from a mass-based concentration to surface area and particle number and influencing the conclusions drawn. The shift in relative particle potency highlights the importance of investigating various dose metrics within the experimental design and signals different particle parameters influencing shape-based antimicrobial activity. To probe shape-dependent behavior, we use a unique empirical approach where the physical and chemical properties (ligand chemistry, surface charge) of the AgNP shapes are carefully controlled, and total available surface area is equivalent across shapes as made through modifications to particle size and concentration. The results herein suggest that surface area alone does not drive antimicrobial activity as the different AgNP shapes at equivalent particle surface area yield significantly different magnitudes of antimicrobial activity (i.e., 100% inactivation for cube-AgNPs, <25% inactivation for disc- and pseudospherical-AgNPs). Further, the particle shapes studied possess different crystal facets, illuminating their potential influence on differentiating interactions between the particle surface and the microbe. Whereas surface area may partly contribute to antimicrobial activity in certain ENM shapes (i.e., disc-AgNPs in relation to the pseudospherical-AgNPs), the different magnitudes of antimicrobial activity across shape provide insight into the likely role of other particle-specific factors, such as crystal facets, driving the antimicrobial activity of other shapes (i.e., cube-AgNPs).