Saturday, 4 February 2012

Aspergillus flavus

Aspergillus flavus (fungus)

Ecology & Pathogenicity;

Found worldwide. Ubiquitous in nature and is the second most common cause of invasive aspergillosis next to Aspergillus fumigatus. Has been implicated in pulmonary, systemic, sinus, ear and other infections. Most widely reported food-borne fungus and can be found colonizing decaying vegetation, crops and seeds. Has also been implicated as both an insect and animal pathogen. (For a slightly more expanded description of pathogenicity, see the intro to the Aspergillus fumigatus post.)

Macroscopic Colony Morphology;

Very rapid rate of growth, maturing in about three days. Surface is greenish-yellow to olive and may have a white border. Texture is often floccose, especially near the center and overall can be velvety to woolly. Unremarkable cream to tan to yellowish reverse on Sabouraud Dextrose media.

Aspergillus flavus on SAB media at 72 hour at 30oC

(note greenish-yellow colour with white edge)

(click on any photo to enlarge for better viewing)

Microscopic Morphology;

Septate hyphae with rather long conidiophores (~400-800 X 8-17 µm) which have a rather rough texture or even spiny, especially just below the vesicle. Vesicles are spherical to elongate and about 20 – 45 µm wide. Aspergillus flavus can be variable in seriation with most strains being about 20% biseriate however some strains can be almost entirely uniseriate. Metulae (8-10 X 5-7 µm) cover three quarters to the entire surface of the vesicle from which the phialides (7-12 X 3-4 µm) form. The metulae support the phialides and together form the biseriate structure. Conidia are globose to ellipsoidal (3-6 µm) with smooth to finely roughened walls.

(Photos below are of a ~30hr old slide culture preparation taken with the DMD-108 digital microscope - except where noted)

Aspergillus flavus mycelium &, conidiophores (LPCB) (X250)

Tangled web of the Aspergillus flavus mycelium as above (X250)

Aspergillus flavus conidiophore bearing vesicle (X1000)

look for the biseriate structure (metulae & phailide) on this and the following photos

Aspergillus flavus vesicle bearing metulae & phialides from which forms the conidia

Aspergillus flavus (LPCB X1000)

Look for the Biseriate structure - Rough surface of conidiophore at its apex (where it meets the vesicle ) is evident in this photo. This roughness is a diagnostic feature of Aspergillus flavus.

Aspergillus flavus (LPCB X1000) Biseriate structure may be easier seen in this photo.

Aspergillus flavus (LPCB X1000)

(Too many photos, but I just like them!)

Aspergillus flavus (LPCB X1000 + 10X digital magnification from DMD-108 Scope)

Though difficult to see, the slighly rough wall can be visualized on several of the conidia, particularly in the lower left of the photo. The conidiophore also has this rough or gently spiked texture particularly at the apex (where it meets the vesicle)

Also note that phialides radiate from vesicle in all directions as opposed to A.fumigatus where they tend found on the upper 2/3rds of the vesicle and extend parallel to the conidiophore.

Aspergillus flavus (LPCB X1000)

(again, enlarge photo and look for the rough texture of the conidia's surface)

Aspergillus flavus - more typical appearance with phialides radiating from vesicle in all directions. This photo taken from adhesive tape preparation of 72 hr fully mature culture. This technique was quite disruptive, dispersing conidia throughout the preparation.

(LPCB X400 -Nikon)


Many Aspergillus flavus isolates are capable of producing aflatoxins, very potent carcinogens. (Aflatoxin B1, cyclopiazonic acid, 3-nitroproprionic acid.)

Intended as Aspergillus flavus computer screen 'Wallpaper' (1024X768 when posted)

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