The production of aflatoxin B1 or G1 by Aspergillus parasiticus at various combinations of temperature and water activity is related to the ratio of aflS to aflR expression

Abstract

The influence of varying combinations of wateractivity (aw) and temperature on growth, aflatoxin biosynthesisand aflR/aflS expression of Aspergillus parasiticuswas analysed in the ranges 17-42°C and 0.90-0.99 aw.Optimum growth was at 35°C. At each temperature studied,growth increased from 0.90 to 0.99 aw. Temperatures of 17and 42°C only supported marginal growth. The externalconditions had a differential effect on aflatoxin B1 or G1biosynthesis. The temperature optima of aflatoxin B1 andG1 were not at the temperature which supported optimalgrowth (35°C) but either below (aflatoxin G1, 20-30°C) orabove (aflatoxin B1, 37°C). Interestingly, the expression ofthe two regulatory genes aflR and aflS showed anexpression profile which corresponded to the biosynthesisprofile of either B1 (aflR) or G1 (aflS). The ratios of theexpression data between aflS:aflR were calculated. Highratios at a range between 17 and 30°C corresponded withthe production profile of aflatoxin G1 biosynthesis. A lowratio was observed at >30°C, which was related to aflatoxinB1 biosynthesis. The results revealed that the temperaturewas the key parameter for aflatoxin B1, whereas it waswater activity for G1 biosynthesis. These differences inregulation may be attributed to variable conditions of theecological niche in which these speci