Aspergillus flavus complex
Aspergillus section Flavi historically includes species with conidial heads in shades of yellow-green to brown and dark sclerotia. Hedayati et al. (2007) reviewed the A. flavus complex and included 23 species or varieties, including two sexual species, Petromyces alliaceus and P. albertensis. Several species of section Flavi produce aflatoxins, among which aflatoxin B1 is the most toxic of the many naturally occurring secondary metabolites produced by fungi. Aflatoxins are mainly produced by A. flavus and A. parasiticus, which coexist and grow on almost any crop or food (Varga et al. 2011). Within the complex, A. flavus is the principle medically important pathogen of both humans and animals. However, some other species in the A. flavus complex, notably A. oryzae, A. avenaceus, A. tamari, A. alliaceus and A. nomius, may cause rare mostly superficial infections (Hedayati et al. 2007, de Hoog et al. 2015).
Note: Accurate species identification within A. flavus complex remains difficult due to overlapping morphological and biochemical characteristics. For morphological identifications, it is recommended to report as Aspergillus flavus complex.
Molecular Identification: ITS sequence analysis is sufficient to identify to species complex level only. Definitive identification requires analysis of β-tubulin, calmodulin and actin genes (Samson et al. 2007, Balajee et al. 2005a).
Aspergillus flavus Link ex Grey
Aspergillus flavus has a worldwide distribution and normally occurs as a saprophyte in soil and on many kinds of decaying organic matter, however, it is also a recognised pathogen of humans and animals. It is a causative agent of otitis, keratitis, acute and chronic invasive sinusitis, and pulmonary and systemic infections in immunocompromised patients. A. flavus is second only to A. fumigatus as the cause of human invasive aspergillosis (Hedayati et al. 2007).
Morphological Description: On Czapek Dox agar, colonies are granular, flat, often with radial grooves, yellow at first but quickly becoming bright to dark yellow-green with age. Conidial heads are typically radiate, later splitting to form loose columns (mostly 300-400 µm in diameter), biseriate but having some heads with phialides borne directly on the vesicle (uniseriate). Conidiophore stipes are hyaline and coarsely roughened, often more noticeable near the vesicle. Conidia are globose to subglobose (3-6 µm in diameter), pale green and conspicuously echinulate. Some strains produce brownish sclerotia.
Key Features: Spreading yellow-green colonies, rough-walled stipes, mature vesicles bearing phialides over their entire surface and conspicuously echinulate conidia.
Culture of Aspergillus flavus.
Conidial head of A. flavus.
Note: conidial heads with both uniseriate and biseriate
arrangement of phialides may be present.