Morphological characterization of Aspergillus nidulans: growth, septation and fragmentation

The influence of the sepA gene on the growth of Aspergillus nidulans has be en investigated by characterizing and comparing the parental strain A28 (pa baA6 biA1) with the sepA null mutant (sepA4 Delta Bm). The sepA gene is kno wn to affect the septation process in A. nidulans, therefore the sepA4 Delt a Bm strain does not produce any septa during the first hours of growth. Du ring batch cultivations sepA4 Delta Bm shows an abrupt decrease in specific growth rate and more pronounced fragmentation (in response to elevated sti rrer speed) than the parental strain. Higher specific fragmentation rates ( q(frag)) were obtained for the sepA4 Delta Bm strain. The physiological rea sons for the differences have been investigated by employing fluorescent st ains. Computerized image analysis revealed that the more pronounced fragmen tation in the mutant was due to the lower number and irregular spacing of s epta (visualized by calcofluor white staining), which resulted in a weaker hyphal structure that is more vulnerable to shear stress and fragmentation than the parental strain. This led to a loss of active biomass (determined by Mag fura staining) from the hyphae of the mutant, which had failed to co mpartmentalize by formation of septa, in turn resulting in decreased specif ic growth rates for the culture.