Isolation of a fluffy mutant of Aspergillus niger from chemostat culture and its potential use as a morphologically stable host for protein production.

Chemostat cultivation of Aspergillus niger and other filamentous fungi is often hindered by the spontaneous appearance of morphologic mutants. Using the Variomixing bioreactor and applying different chemostat conditions we tried to optimize morphologic stability in both ammonium- and glucose-limited cultures. In most cultivations mutants with fluffy (aconidial) morphology became dominant. From an ammonium-limited culture, a fluffy mutant was isolated and genetically characterized using the parasexual cycle. The mutant contained a single morphological mutation, causing an increased colony radial growth rate. The fluffy mutant was subjected to transformation and finally conidiospores from a forced heterokaryon were shown to be a proper inoculum for fluffy strain cultivation.