In 2017, the Dutch Central Bureau of Fungal Cultures was renamed the Westerdijk Fungal Biodiversity Institute, after Johanna Westerdijk. Westerdijk was the first female professor of The Netherlands and director of the centre from 1907 to 1952. She had a great interest in fungi and, under her leadership, the institute’s collection grew to be the largest in the world. A century on from her appointment as professor, Westerdijk’s accomplishments were celebrated with the centre’s renaming and the unveiling of several extraordinary images of Aspergillus restrictus.
A. restrictus is a mould that can grow in environments with very limited water. The species is often found in indoor air and house dust, and is considered a potential cause of respiratory issues; A. restrictus can also contribute towards cereal and cotton rot. In this project, high resolution images of the species were taken at various magnifications, using both light and electron microscopy. These images, copied below, allow the viewer to zoom in on the structure of the mould at differing levels of detail. This means that we can explore the various stages of fungal growth, from different angles and magnifications. For reference, simplified diagrams of the Aspergillus life cycle and structure are included first.
The life cycle of Aspergillus. [Image copied from: https://aspergillusproject11.wordpress.com/2013/04/18/life-cycle/]
Structure of the conidial head. [Image adapted from: https://mycology.adelaide.edu.au/descriptions/hyphomycetes/aspergillus/]
Images of Aspergillus restrictus:
An Aspergillus restrictus colony, measuring approximately 1 cm (bar = 1 mm). White aerial hyphae can be seen throughout the colony.
An image taken closer to the centre of the colony (bar = 0.5 mm). White aerial hyphae and green columnar conidiophores can be seen throughout.
The rim of the colony (bar = 0.5 mm). White aerial hyphae, with the early stages of conidiophore formation, can be seen at the edge of the colony (bottom of the image). Green conidia, organised into rows and columns, are seen more centrally (top of the image).
The rim of the colony (bar = 0.1 mm). Conidial head formation is visible throughout the image. Hyphae can be seen growing into the agar plate and upwards; aerial hyphae also cross above these structures.
Image showing various stages of conidial head formation. Aerial hyphae can be seen in the bottom right corner. Vesicles, found at the end of stipes, are transparent. A row of phialides grow from several vesicles, and conidia organise into rows and columns after these. Rows of conidia often twist clockwise (bar = 0.1 mm).
Image false-coloured to clearly show the various stages of conidial head formation and growth (bar = 0.05mm).
False-coloured image showing ornamentation on the surface of mature conidia (bar = 0.01 mm). These details are not visible on the newly formed conidia.
Conidial heads seen at 3 different developmental stages (bar = 0.01 mm). In the centre, a vesicle has formed at the end of the stipe. On the bottom right, a layer of phialides has grown on the vesicle. In the top of the image, the development of mature, ornamented conidia can be seen.
False-coloured image showing material on the surface of the phialides (brown) and the developing ornamentation of the early conidia (green) (bar = 0.001 mm)
Cell surface and ornamentation of the conidia from various magnifications and angles. Image D shows the formation of a crown at each end of the conidia (bars = 0.001 mm in images A, B and D, and 0.0001 mm in images C and E)
A) Image showing the appearance of material on the surface of the developing phialide tips (bar = 0.001 mm). B) The area where the phialide meets the developing conidia. The structure of the material on the surface of the phialide can be seen; each unit consists of a head and a tail (bar = 0.0001 mm).
These images, produced by the Westerdijk Fungal Biodiversity Institute, show the structure and development of Aspergillus restrictus in incredible detail. There are several surprising questions and discoveries that arise from this level of detail. For example, the clockwise twisting of conidial rows has not previously been described, and the chemical composition of the material found on the phialide surfaces is unknown. Therefore, this technology not only provides us with these impressive images, but may also lead to further research and greater understanding of the structure and development of fungi. Greater knowledge of Aspergillus growth and function can assist in the development of drugs which impede its growth.
Read the full paper: Jan Dijksterhuis, Wim van Egmond and Andrew Yarwood (2020), From colony to rodlet: “A six meter long portrait of the xerophilic fungus Aspergillus restrictus decorates the hall of the Westerdijk institute.”