Morphological and enzymatic responses of a recombinant Aspergillus niger to oxidative stressors in chemostat cultures.

Continuous chemostat cultures of a recombinant strain of Aspergillus niger (B1-D), engineered to produce the marker protein hen egg white lysozyme, were investigated with regard to their susceptibility to oxidative stress. The culture response to oxidative stress, produced either by addition of exogenous hydrogen peroxide (H(2)O(2)) or by high dissolved oxygen tension (DOT), was characterised in terms of the activities of two key defensive enzymes: catalase (CAT) and superoxide dismutase (SOD). Since the morphology is so critical in submerged fungal bioprocesses, the key morphological indices were analysed using a semi-automated image analysis system. Both oxidant stressors, H(2)O(2) and elevated DOT, increased both enzyme activities, however, the extent was different: exogenous H(2)O(2) led mainly to increased CAT activity, whereas gassing with O(2) enriched air, which resulted in a DOT of 165% of air saturation, increased both enzyme activities more than 2-fold compared with the control steady state culture. Addition of exogenous H(2)O(2) resulted in shorter hyphae compared with control steady state cultures. These findings indicate that it is unsound to use exogenous H(2)O(2) to simulate oxidative stress induced by elevated dissolved oxygen levels since the response to each might be quite different, both in terms of enzymatic (defensive) responses and in terms of culture morphology.