The Influence of Maintenance Energy and Growth Rate on the Metabolic Activity, Morphology and Conidiation of Penicillium chrysogenum

Summary The rates of utilization of energy-yielding substrates (glucose and oxygen) by Penicillium chrysogenum in glucose-limited chemostat cultures were resolved into requirements or ‘rations’ for growth and maintenance. The maintenance ration of glucose was almost all oxidized to carbon dioxide. Over the growth rate range 0·023–0·075 hr‒1 only vegetative growth occurred; although the filamentous growth form predominated, the occurrence of pellets and swollen organisms increased with growth rate. At growth rates of 0·014 hr‒1 and below, conidiation occurred and was maximal at a specific growth rate of 0·009 hr‒1 (average doubling time 78 hr). After growth in chemostat culture the organism could be maintained in a non-growing state by supplying only the maintenance ration of glucose (0·022 g. glucose g. mycelial dry wt/hr). When growth in the chemostat was suddenly stopped by stopping the glucose feed, the mould autolyzed; autolysis was prevented by supplying the maintenance ration of glucose. When the glucose feed rate in chemostat cultures was decreased to the maintenance ration, mycelial differentiation occurred. Differentiation involved increased hyphal vacuolation, a decreased degree of oxidation of glucose, breakdown and resynthesis of nucleic acids and conidiation. The rates at which these changes occurred were inversely related to the growth rate prior to stopping growth. For maximum conidia formation there was an optimal glucose feed rate about 0·038 g. glucose/g. mycelial dry wt/hr, that is 1·7 × maintenance ration. The maintenance ration of glucose was shown to play a role in preventing autolysis and facilitating conidiation. Electron microscope studies showed that there was no change in the observed ultrastructure of cells (except degree of vacuolation) with change in specific growth rate from near the maximum to zero. The septa of the hyphae were found to be plugged.