Colloids and Surfaces B: Biointerfaces 24 (2002) 277 – 284 Surface properties of Aspergillus oryzae spores investigated by atomic force microscopy Bruno C. van der Aa a , Marcel Asther b , Yves F. Dufreˆne a, * a Unite´ de chimie des interfaces, Uniersite´ catholique de Louain, Croix du Sud 2 /18, 1348 Louain -la -Neue, Belgium b Institut National de la Recherche Agronomique and Unite´ de Biotechnologie des Champignons Filamenteux, Faculte´ des Sciences de Luminy, 163, aenue de Luminy —C.P. 925, F -13288 Marseille, France Received 14 May 2001; received in revised form 25 July 2001; accepted 13 August 2001 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 measure- ments 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. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aggregation; Atomic force microscopy; Cells; Macromolecular interactions; Ultrastructure www.elsevier.com/locate/colsurfb 1. Introduction The behavior of fungal cells in the natural environment as well as their performance in in- dustrial processes directly depend on the cell sur- face properties. Ascospores from marine fungi were shown to attach to wood samples through sticky appendages and through extracellular mate- rial released upon spore germination [1]. Hamer et al. [2] provided direct evidence for rapid attach- ment of conidia of a plant pathogenic fungus to polymer substrata by secretion of mucilage. Ag- gregation of the spores of Phanerochaete chrysosporium during germination was shown to * Corresponding author. Tel.: +32-10-473589; fax: +32- 10-472005. E-mail address: dufrene@cifa.ucl.ac.be (Y.F. Dufreˆne). 0927-7765/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7765(01)00277-6