Single-cell force spectroscopy of Als-mediated fungal adhesion

Macroscopic assays that are traditionally used to investigate the adhesion behaviour of microbial cells provide averaged information obtained on large populations of cells and do not measure the fundamental forces driving single-cell adhesion. Here, we use single-cell force spectroscopy (SCFS) to quantify the specific and non-specific forces engaged in the adhesion of the human fungal pathogen Candida albicans. Saccharomyces cerevisiae cells expressing the C. albicans adhesion protein Als5p were attached on atomic force microscope tipless cantilevers using a bioinspired polydopamine wet polymer, and force-distance curves were recorded between the obtained cell probes and various solid surfaces. Force signatures obtained on hydrophobic substrates exhibited large adhesion forces (1.25 +/- 0.2 nN) with extended rupture lengths (up to 400 nm), attributed to the binding and stretching of the hydrophobic tandem repeats of Als5p. Data collected on fibronectin (Fn)-coated substrates featured strong adhesion forces (2.8 +/- 0.6 nN), reflecting specific binding between Fn and the N-terminal immunoglobulin-like region of Als5p, followed by weakly adhesive macromolecular bonds. Both hydrophobic and Fn adhesion forces increased with contact time, emphasizing the important role that time plays in strengthening adhesion.