Impact of out-of-phase conditions on screening results in shaking flask experiments

Screening projects dealing with filamentous microorganisms in shaking flaks may generate strains showing a less filamentous morphology with a decreased apparent viscosity of the fermentation broth. The apparent viscosity of the fermentation broths showing pseudo-plastic flow behavior can be calculated by known relations, if the average shear rate is known. A method is presented allowing the determination of the relevant average shear rate, and thus, apparent viscosity of the fermentation broth at given operating conditions of the shaking flask experiment. At elevated apparent viscosity, shaking flask fermentations are subject to the recently discovered out-of-phase conditions. Measurements of the oxygen transfer capacity (OTRmax) in a highly viscous fluid have clearly shown reduced mass transfer, and therefore a reduced productivity of the investigated strains, when out-of-phase conditions are present. This leads to a selection pressure preferring a less filamentous morphology accompanied by lower apparent viscosity in screening projects in shaking flasks. In two completely different cases, the apparent broth viscosity of several consecutive strain generations was investigated. The later strain generation showed a lower apparent broth viscosity compared to the predecessor strain. In a third case, it was shown that out-of-phase conditions prevent the development of an improved culture medium. (C) 2003 Elsevier B.V. All rights reserved.