Agitation effects on morphology and protein productive fractions of filamentous and pelleted growth forms of recombinant Aspergillus niger

Recombinant Aspergillus niger genetically engineered to produce glucose oxidase using the constitutive gpdA promoter and the glucoamylase signal sequence for secretion was grown in batch cultures at agitation speeds of 200-800 rpm covering the industrial relevant power input range of 0.1-5 W kg(-1). The growth morphology ranged from large pellets with an average diameter of 1500 mu m at low power input up to micropeilets embedded in a filamentous network at high power input. A correlation of agitation intensity with growth morphology and glucose oxidase production revealed an increase of the protein production capability with the change of the growth morphology from pelleted to filamentous growth forms. However, the exposure to higher shear stress with increasing power input also resulted in lower biomass yields as well as increased transient formation of polyol (xylitol) and higher final concentrations of oxalic acid. The highest specific production rates were found in young filamentous growth forms at high power input. Although intermediate agitation intensity leading to small pellets became more favorable during prolonged cultivation. An acridine orange staining procedure discriminating between RNA rich (red) and RNA poor regions (green) of the fungal biomass proved that active protein production is restricted to filamentous growth forms and the outer layer of fungal pellets. A correlation between the RNA rich fraction of the biomass determined by image analysis and the productivity is shown. (c) 2006 Elsevier Ltd. All rights reserved.