Fun With Microbiology (What's Buggin' You?): Aspergillus fumigatus

Aspergillus fumigatus (Fungus)

Aspergillus species (in general);

There are about 180 species of Aspergillus, 20 of which are recognized as being human pathogens. Aspergillus fumigatus is the species most commonly implicated, followed by Aspergillus flavus and Aspergillus niger.

Pathogenicity;

Most commonly, Aspergillus infections in humans are pulmonary (ie. involve the respiratory system) however they can disseminate to produce deep seated infections particularly in immunocompromised individuals such as transplant and neutropenic patients. Aspergillus may also cause allergic aspergillosis and fungal sinusitis. Aspergillus is also capable of causing illness in other species such as birds and cattle.

Ecology;

Aspergillus fumigatus is cosmopolitan & ubiquitous. Primarily a saprobic fungus of soils which can be found living on substrates such as plants, compost, seeds and wood chips, etc. Hospital renovations have been implicated in the dissemination of Aspergillus fumigatus which can be detrimental to the patient and a nuisance contaminant for the laboratory. Careful consideration must be given before dismissing Aspergillus fumigatus as a laboratory culture contaminant.

Physiology;

A.fumigatus is rapidly growing fungus which matures in about 3 days. Aspergillus fumigatus is thermophillic, with good growth to 45^oC and often up to 50^oC. This ability can be used to differentiate it from other species.

Colony Morphology;

The surface growth is velvety, downy or powdery, showing various shades of green, most commonly a blue-green to a grey-green with a narrow white border. The colour typically darkens with age. The reverse is white to tan to pale yellowish. Colouration or shade can be dependent on the media on which the fungus is cultured upon.

Aspergillus fumigatus on SAB Agar after 5 days incubation at 30^oC (Surface)

(Click on any photo to enlarge for better viewing)

Microscopic Morphology;

Hyphae are septate with smooth walled conidiophores (usually less than 300 µm in length and 5-10 µm wide). Vesicles are subclavate in shape, roughly 20-30 µm in width. Conidiogenous cells (phialides) are flask shaped, uniseriate, compact (closely spaced), usually forming on the upper two-thirds of the vesicle and mature parallel to the axis of the conidiophore (columnar formation). Young conidial heads may radiate. Conidia are verrucose (smooth or slightly rough), (sub)spherical and about 2-3.5 µm in diameter and develop in chains.

Photos below were taken with the DMD-108 digital microscope. Magnifications are as noted. In my opinion, the uniseriate phialides (or biseriate in other species) are best visualized and determined under oil. The DMD-108 microscope has an addition optional digital magnification factor of X10 which can be utilized for even greater enlargement of features. Both adhesive tape and slide culture preparations were made and stained with Lactophenol Cotton Blue (LPCB). The formed chains of conidia were easily disrupted even with the slide culture technique. A little disappointing however the photos below were the result.

(Most photos taken with the DMD-108 digital scope, Nikon where noted.)

Structures described in test above shown for clarity (X1000 oil immersion)
(Click on photo to enlarge for better viewing)

Aspergillus fumigatus (LPCB - Adhesive tape preparation) (X250)

Aspergillus fumigatus (LPCB - Adhesive tape preparation) (X400)
(Phialides generally on upper 2/3 of vesicle and more or less parallel to the conidiophore axis)