Effect of water activity on ochratoxin A production by Aspergillus niger aggregate species

The effect of water activity ( a w) (0.82–0.99) on growth and ochratoxin A (OTA) production by twelve Aspergillus niger aggregate strains, cultured in Czapek Yeast Autolysate agar (CYA) and Yeast Extract Sucrose agar (YES), was studied for an incubation period of 30 days. The strains were selected to include diverse sources, different reported abilities to produce OTA and different ITS-5.8 S rDNA Restriction Fragment Length Polymorphism (RFLP) pattern. They were characterized by Random Amplification of Polymorphic DNA (RAPD) and ITS-5.8 S rDNA and 28 S rDNA (D1/D2) sequencing. Regardless of the a w value tested, YES was a better culture medium than CYA for OTA production. The a w range for OTA production was narrower than that for growth. OTA was produced from 0.90, 0.92, 0.94 or 0.96 to 0.99 a w depending on the strain and the culture medium. The molecular study differentiated strains into two groups which corresponded to the RFLP types N and T although it did not distinguish them by their source of isolation or OTA producing abilities. Our results show that A. niger aggregate strains are able to grow and produce OTA over a wide a w range. These results will lead to a better understanding of the contribution of A. niger aggregate in OTA contamination of food and feed.