Comparative study of toxigenic potential of Aspergillus flavus and Aspergillus niger isolated from Barley as affected by temperature, water activity and carbon source

The objective of the present work was to evaluate the ability of two mycotoxigenic species Aspergillus flavus and Aspergillus niger, isolated from barley, to produce aflatoxin B1 (AFB1) and ochratoxin A (OTA), respectively, as affected by nutritional and environmental factors. Six carbon sources (D-fructose, D-glucose, D-galactose, lactose, sucrose and starch) and different water activities (0.90, 0.95 and 0.98), temperature (20 and 28 °C) and incubation time (5 and 10 days) were tested. The results showed that optimal conditions for growth on Barley Meal Extract Agar (BMEA) medium were 28 °C and 0.95 aw for A. niger strain and 28 °C and 0.98 aw for A. flavus strain. Optimal conditions for OTA and AFB1 production were largely different for the two tested strains. A. niger had an optimal OTA production at 0.98 aw and 20 °C after 10 days of incubation while A. flavus had an optimal AFB1 production at 0.95-0.98 aw and 28 °C after 5 days of incubation. These results indicates that A. flavus has a higher optimum temperature for mycotoxin synthesis than A. niger and takes greater advantage of drier conditions for maximum AFB1 production. In the current study, both OTA and AFB1 production of A. niger and A. flavus were highly influenced by carbon sources. The sugar that provided the highest toxin levels in the cultures of the two species was sucrose with the lowest levels given by starch. OTA production by A. niger was also highly induced by fructose as carbon sources, while, AFB1 production by A. flavus was favored by glucose. Globally, our results showed, significantly different optimal conditions for production of AFB1 and OTA, respectively, by A. flavus and A. niger.