Surface properties of Aspergillus oryzae spores investigated by atomic force microscopy

Abstract

Spores of the filamentous fungus Aspergillus oryzae have a great biotechnological potential for the production of highly active proteins. To date, little is known about the molecular mechanisms of spore aggregation, a phenomenon observed during germination in liquid medium. Here, atomic force microscopy (AFM) imaging and force measurements were used to characterize, under aqueous conditions, the surface morphology and macromolecular interactions of A. oryzae spores in relation to their aggregation behavior. Dormant spores were covered with a discontinuous layer of about 35 nm thickness, as revealed by height images. High-resolution deflection images showed that this layer consisted of rodlets, 10±1 nm in diameter, that were assembled in parallel to form fascicles interlaced with different orientations. The germinating spore surface was much rougher and showed streaks oriented in the scanning direction, indicating that the probe was interacting with soft material. Retraction force curves were strikingly different depending on the spore physiological state: while dormant spores exhibited non-adhesive properties, germinating spores showed single or multiple attractive forces of 400±100 pN magnitude, along with characteristic elongation forces and rupture lengths ranging from 20 to 500 nm. These elongation forces are attributed to the stretching of long, flexible cell surface macromolecules and suggested to play a role in the aggregation process by promoting bridging interactions.