Fungi such as Candida albicans exist in biofilm phenotypes, which present as viscoelastic materials; however, a method to measure linear viscoelastic moduli, yield stress, and yield strain is lacking. Characterization methods for fungal materials have been limited to techniques specific to particular industries. Here, we present a method to measure the shear stress, strain amplitude, and creep of C. albicans BWP17 biofilms. Our method includes features tailored to the analysis of fungi including an in vitro growth protocol attuned to the slow growth rates of C. albicans biofilms and a resultant cultured biofilm that has sufficient integrity to be transferred to the rheometer tooling without disrupting its structure. The method's performance is demonstrated by showing that results are insensitive to gap, evaporative sealant, length of experiment, and specimen radius. Multiscale imaging of the fungal biofilm showed complex entanglement networks at the hundred-micrometer scale. For a wild-type strain cultivated for 14 days, using small-amplitude oscillatory rheology, we found that the elastic (G′) and viscous (G″) moduli were nearly independent of frequency over the range 0.1–10 s−1, with magnitudes of and , respectively. The yield stress was approximately . We modeled the linear creep response of the fungal biofilm and found that C. albicans has a characteristic relaxation time of and a viscosity of . We applied this method to probe the effects of altered chitin deposition in the C. albicans cell wall. Differences between the biofilm's phenotypic cell shape and rheological properties in mutants with altered chitin synthase activity were resolved. Discovering how genotypic, phenotypic, and environmental factors impact the material properties of these microbial communities can have implications for understanding fungal biofilm growth and aid in the development of remediation strategies.
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See supplementary material at https://www.scitation.org/doi/suppl/10.1122/8.0000427 for information pertaining to the effects of sandpaper grit size on rheological measurement, images of the biofilm samples pre- and post-rheological testing, the impact of silicon oil on biofilm samples over time, and the characteristic rheological values for multiple C. albicans mutant strains.
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