Fungal morphology in submerged cultures and its relation to glucose oxidase excretion by recombinant Aspergillus niger

The effect of culture conditions such as medium composition and shear stres s on the fungal pellet morphology in shake-flask cultures and its relation to glucose oxidase (GOD) excretion by recombinant Aspergillus niger NRRL 3 (GOD 3-18) was investigated. It was shown that culture conditions resulting in the formation of smaller fungal pellets with an increased mycelial dens ity result in higher yields of exocellular GOD. The pellets obtained in sha ke-flask cultures showed distinct layers of mycelial density with only the thin outer layer consisting of a dense mycelial network. The performance of the recombinant strain and the process of pellet formation was also analyz ed during batch cultivation in a stirred-tank bioreactor. It was shown that the process of pellet formation occurred in two steps. (1) aggregation of free spores to spore clusters with subsequent germination and formation of small aggregates surrounded by a loose hyphal network, and (2) aggregation of the primary aggregates to the final full-size pellets. The fungal pellet s formed during bioreactor cultivation were smaller, did not show large dif ferences in mycelial density, and were more efficient with respect to the p roduction of exocellular GOD. The decreasing pellet size also correlated wi th an increased mycelial density, indicating an improvement of the transpor t of nutrients to the inner parts of the pellet.