Enhanced dark field microscopy for rapid artifact-free detection of nanoparticle binding to Candida albicans cells and hyphae.

We surveyed a panel of 13 metal nanoparticle (NP) catalysts for their antifungal activities against Candida albicans ATCC 90028. Initial characterization using scanning electron microscopy (SEM) suggested that our ability to detect NP binding to Candida surfaces with this method was impeded by preparation artifacts. As an alternative method for visualizing NP binding, we used an enhanced dark field illumination system (CytoViva) attached to a standard light microscope. When viewed using this system, all NP produced intense optical signals due to resonant light scattering. To assay binding, NP were allowed to interact with C. albicans hyphae and cells in spent RPMI broth for 15 min with gentle inversion, followed by viewing with the CytoViva system. The antifungal efficacy of NP preparations was determined separately using a 24-h broth microdilution test. For single-metal NP, observations of binding at 15 min made via CytoViva corresponded to antifungal efficacy at 24 h, with the most antifungal NP yielding complete coverage of hyphal surfaces. Our work suggests the utility of visual screening using the CytoViva system for rapid, simple and artifact-free viewing of NP-cell interactions in support of antimicrobial screening efforts. This approach provides a quick and accessible alternative to SEM for imaging of NP-cell interactions.