Mixed Substrate Utilization

In composting plant material, the hydrolysis of polysaccharide constituents by the secreted enzymes is expected to produce a mixture of sugars in the growth environment of thermo-philic fungi. One of the adaptive strategies for their growth could be the simultaneous utilization of a mixture of sugars. To test this, the thermophilic fungi Thermomyces lanug-inosus and Penicillium duponti were grown in a mixture of glucose and sucrose in liquid media (Maheshwari and Balasubramanyam, 1988). Both fungi concurrently utilized glucose and sucrose at 50°C, with sucrose utilized faster than glucose (Figure 10.6). This is quite the opposite of the phenomenon of diauxy observed in bacteria that utilize one carbon source at a time, e.g., glucose is utilized before lactose in a mixture of the two sugars. The simultaneous utilization of sucrose in the presence of glucose occurred because invertase is insensitive to catabolite repression by glucose and because the activity of the glucose uptake system is repressed by sucrose.

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