Bioreactor and substrate selection for solid-state cultivation of the malaria mosquito control agent Metarhizium anisopliae

Metarhizium anisopliae spores are a promising alternative to chemical insecticides against malaria mosquitoes. In-house application resulting in infection of mosquitoes with the fungus can strongly contribute to reducing malaria transmission. For such application, fungal spores need to be produced in large quantities. Cultivation of the fungus on several solid substrates and in two bioreactor types for solid state fermentation was studied. Our experiments showed that M. anisopliae cannot withstand mixing; therefore aerated static packed beds are the most suitable reactor type. Based on spore yields (0.18 kg spores kg−1 substrate, 5.5 × 1014 spores m−3 reactor volume) and its favorable physical properties, hemp impregnated with a nutrient medium was selected for further research. Total nutrient concentrations between 250 and 313 kg m−3 medium gave the highest productivities; the maximum concentration imposed by water losses for evaporative cooling was estimated at 240 kg m−3. Combined autoclaving of nitrogen and carbon sources consistently gave lower average productivities. Pilot-scale fermentation demonstrated the feasibility of scale-up of the aerated packed bed with impregnated hemp, showing its potential for large scale production of M. anisopliae spores.