New morphological aspects of Penicillium chrysogenum were found during physiological characterisation of two NADPH-dependent glutamate dehydrogenase mutant strains. A morphological characterisation of the previously constructed strains, together with the two beta-lactam producing industrial recipient strains, was conducted. The reference strains showed a compact structure with highly branched hyphal elements whereas the morphology of the Delta gdhA strains consisting of long elongated hyphal elements with few branches. On solid medium, the hyphal growth unit (length) increased from an average of 47 mu m tip(-1) in the reference strains to 117 mu m tip(-1) in the Delta gdhA strains and in submerged cultures a decrease of 18% in branching frequency was measured due to the gdhA deletion. P. chrysogenum Wis 54-1255, the ancestor of most production strains was also characterised and this strain showed morphology similar to the industrial strains. Interestingly, the constructed strains showed morphology similar to wild type Aspergillus nidulans another species carrying the penicillin biosynthetic cluster. Thus, the results showed that elimination of glutamate dehydrogenase activity in high producing strains of P chrysogenum has a radical impact on morphology. (c) 2009 Elsevier B.V. All rights reserved.