Description
Aspergillus Flavus Parasiticus Agar (AFPA) was developed by modification of Bothast and Fennell’s Aspergillus Differential Medium, which was used for the rapid detection of aflatoxigenic species and was incubated at 28°C for three days.
Ferric citrate in the formulation induced production of the intense orange yellow reverse colour in colonies of these species described as Cadmium yellow.
Salkin & Gordon reviewed the medium with a view to using it in medical mycology. From the 63 isolates that they tested only A. flavus, A. parasiticus & A. oryzae produced the pigment.
A. flavus & A. parasiticus are capable of producing potent aflatoxins. These isolates are most commonly associated with nuts & oilseeds (peanuts, corn & cottonseed). A. flavus may also be detected in spices, but because of the small quantity consumed, aflatoxin in spices is not considered a real hazard.
A. oryzae & A. sojae are closely related to A. flavus & A. parasiticus. They are important in the production of fermented foods in Asia, especially soy sauce, but they do not produce aflatoxins.
Hamsa & Ayres reviewed the steps in production & incubation of Aspergillus Differential Medium. They incorporated streptomycin to suppress bacterial growth & dichloran to reduce the spread of fungi. Incubation time was increased to five days at 28°C, but the original formula remained untouched.
Assante reported that the orange yellow reverse colouration is connected to production of aspergillic acid, produced by A. flavus & other species in the A. flavus group, & neoaspergillic acid produced by A. ochraceus & some related species.
Extensive work was performed by Pitt, Hocking & Glenn, who reviewed each component in the ADM formulation & it’s concentration to maximise rapid growth in conjunction with maximum production of distinctive, non-diffusible orange yellow pigment.
They experimented with a range of antibiotics to exclude growth of competing bacterial flora, because they found that low pH compromised the production of reverse colour, & incorporated dichloran to reduce growth of spreading moulds.
An incubation temperature of 30°C was chosen because A. flavus & A. parasiticus grow better & produce pigment more rapidly at this temperature.
AFPA described here was recommended to produce quality, speedy results on samples which may be contaminated with A. flavus or A. parasiticus after incubation of the medium for 42-48 hours at 30°C.
It should be noted that A. oryzae is rarely isolated in products other than those associated with Asian fermented foods & that A. ochraceus grows very slowly on AFPA at 30°C, & does not produce pigment in less than three days. A. niger may produce colonies similar in texture & size to A. flavus & associated members of the group, but exhibits a pale yellow reverse colony colouration & black conidial heads occur after 48 hours incubation at 30°C.
