Influence of physiological factors on growth, sporulation and ochratoxin A/B production of the new Aspergillus ochraceus grouping

Recently, new species within the Aspergillus section Circumdati group responsible for ochratoxin production were reported. This study has examined the impact of interactions between water activity (a(w), 0.99-0.90), temperature (20-35 degrees C) on growth, asexual spore production and ochratoxin A and B (OTA and OTB) on strains of each of the three species from this new grouping (A. ochraceus, A. steynii, and A. westerdijkiae) for the first time. The maximum growth occurred at 0.95 a(w) and 30 degrees C for both A. ochraceus and A. westerdijkiae, while it was at 0.99 a(w) and 30 degrees C for A. steynii. No conidial spore production occurred at 0.99 a(w) in cultures of A. ochraceus and A. steynii but large numbers of spores (2.3x10(7)/cm(2)) were produced by A. westerdijkiae. Optimum temperature for spore production was 0.95 a(w) and 30 C for A. westerdijkiae and A. ochraceus, and 0.95 a(w) and 35 degrees C for A. steynii. Quantification of OTA showed that optimum was produced at 0.99 a(w), by A. steynii at 30 degrees C, for A. westerdijkiae at 25 degrees C and for A. ochraceus at 20 degrees C. As water stress was imposed (0.95 a(w)), the temperature for maximum OTA production changed. For example, for A. steynii and A. westerdijkiae this was at 35 degrees C, for A. ochraceus, 25 degrees C. Much less OTB was produced relative to OTA, but the production followed the same pattern at all a(w) levels and temperatures. This is the first detailed study to examine the similarities and differences in ecology of these related species in this important mycotoxigenic group.