Surface Structures and Mechanical Properties of Model Fungus Aspergillus nidulans

Although a better understanding of the mechanical properties of common fungi has the potential to yield improvements in dealing with both beneficial and pathogenic fungi, few previous studies have focused on assessing these mechanical properties because of relatively small size of fungal cells. To address this challenge we have used the atomic force microscope (AFM) to study a model filamentous fungus, Aspergillus nidulans. The wild type spores were compared with spores from two isogenic rodA+ and rodA- strains. AFM images of wild type and rodA+ spores showed characteristic ""rodlet"" protein structures covering spore surfaces. In comparison, rodA- spores were rodlet free. Nanoindentation measurements showed that rodA- spores have larger values of stiffness and elastic modulus than do the wild type and rodA+ strains. We developed an AFM approach which for the first time allowed us to measure cell-wall mechanical properties of A. nidulans hyphae. Using finite element analysis to simulate AFM indentation, the elastic modulus of wild type hyphae was determined to be 110+/-10 MPa. This decreased to 64+/-4 MPa when grown under osmotic stress, implying growth medium osmotic conditions have significant effects on cell wall elasticity. We studied the effect of cell-wall degrading, enzymes on fungal hyphae. Fungal elements in the stationary phase is found to be smaller than those in the exponential phase. This likely results from increased cell-wall hydrolase activity present in the stationary phase. Real-time lyticase digestion of the cell wall gradually removes rod-shaped structures, and tends to reduce the wall rigidity. We added various amounts of rapamycin (a gratuitous inducer of autophagy) to cultures of A. nidulans. While no statistically significant difference in elastic modulus was found for the hyphae grown in low rapamycin (4 mg/L) against the control, a statistically significant difference was found for hyphae grown in high rapamycin (20 mg/L). These results imply that rapamycin leads to stiffer cell walls. We have developed a novel AFM approach to address important biological problems. The results will likely lead to insights which could improve bioprocess performance or provide novel targets or strategies for new antifungal therapeutics.