Glass-fiber disks provide suitable medium to study polyol production and gene expression in Eurotium rubrum

Eurotium species often dominate the fungal population in stored grain and are responsible for spoilage. In this study we tested the usefulness of glass fiber disks to aid the analysis of growth, polyol content and gene expression in E. rubrum in response to various water activities. Growth measurements based on ergosterol content and conidial production indicated that E. rubrum grew as well at 0.86 aw as 0.98 aw. The rate of growth was considerably reduced at 0.83 aw and 0.78 aw. In contrast, under our conditions, Aspergillus flavus and A. nidulans were able to grow only in the highest water activity (0.98 aw). Mannitol was the predominant polyol in all three fungal species grown at 0.98 aw. When E. rubrum was grown at 0.86 aw or lower, glycerol comprised greater than 90% of the total polyols. After a shift from 0.86 aw to 0.98 aw, mannitol levels in E. rubrum increased to 89% of the total polyols within 24 h. Of six genes whose expression was measured by quantitative real-time PCR, three were affected by water activity. Expression of putative hydrophobin and mannitol dehydrogenase genes was higher at 0.98 aw than at 0.86 aw. A putative triacylglycerol lipase gene was expressed at higher levels in 0.86 aw.. The results of this study indicate that the disk method is suitable to study the effects of water activity on growth, polyol biosynthesis and gene expression in E. rubrum. The results also indicate the potential competitiveness of E. rubrum over A. flavus and A. nidulans in low water environments associated with stored grain.