Why Solid-State Fermentation Seems to be Resistant to Catabolite Repression?

A critical review of the phenomenon of resistance to catabolite repression of enzyme synthesis by solid-state fermentation (SSF) has been made. The practical and theoretical importance of such phenomenon is commented, together with the current ideas to explain it. Namely, the possible existence of microscopic gradients within the mass of cell aggregates, or the changes in cell permeability to sugars, which would explain the fact that no catabolite repression is observed in SSF when sugar concentration is as high as 100 g/L, despite the fact that in conventional submerged fermentation (SmF) strong inhibition of inducible enzymes is observed whenever sugar concentration is higher than 10 g/L. Two alternative mathematical models are presented in order to explore the feasibility of those hypotheses, but also to help the planning of future experiments in order to understand the microscopic physiology of SSF. A priori, both hypotheses will explain the phenomenon, but only if the local diffusivity or permeability of sugars in SSF have changed in various orders of magnitude as compared to the observed magnitudes in SmF systems.