Dissolved oxygen levels affect dimorphic growth by the entomopathogenic fungus Isaria fumosorosea

The entomopathogenic fungus Isaria fumosorosea is capable of dimorphic growth (hyphal or yeast-like) in submerged culture. Using 250-mL baffled flasks, culture volumes of 50, 100, 150, and 200 mL were grown in a shaker incubator at 350 rpm and 28°C. Dissolved oxygen (DO) was continuously monitored using a non-invasive oxygen monitoring system. Culture volumes of 50 mL maintained DO concentrations above 10% throughout the 3-day growth period and accumulated biomass and produced blastospores more rapidly (1.2×10⁹ blastospores mL^?1 in 2 days) than the other culture volumes tested. Dissolved oxygen was depleted in culture volumes of 100, 150, and 200 mL after 20.5, 16.8, and 13.5 h, respectively. The DO in 150 and 200 mL cultures remained exhausted (<3%) throughout the growth period resulting in significantly lower blastospore yields and increased hyphal growth. These results were used to establish oxygen levels (>20% DO) for I. fumosorosea growth in 100-L bioreactors resulting in blastospore production (1.1×10⁹ blastospores mL^?1 in 2 days) comparable to highly aerated, low volume shake flask cultures. In addition, maintaining higher DO levels resulted in increased blastospore production by cultures of I. fumosorosea grown on low-cost nitrogen sources (cottonseed meal and soy flour) that previously elicited excessive hyphal growth. These studies showed that oxygen availability is essential for significant yeast-like growth by I. fumosorosea cultures and that continuous monitoring of oxygen concentrations in shake flask cultures can be used to establish aeration conditions for bioreactors.