Changing the Inoculum Type From Preculture to Spore Suspension Markedly Alters the Production of Secondary Metabolites in Filamentous Microbial Coculture

The shake flask cocultures of Aspergillus terreus and Streptomyces rimosus were investigated with regard to the production of mevinolinic acid (lovastatin), oxytetracycline, and other secondary metabolites (SMs). The aim of the study was to determine the effect of inoculum type (spore suspension or preculture) on the levels of SMs in the fermentation broth. Altogether, 17 SMs were detected, including 4 products with confirmed identities, 10 putatively annotated metabolites, and 3 unknown molecules. As observed over the course of qualitative and quantitative analyses, the selection of inoculum type markedly influenced the SM-related outcomes of cocultures. Depending on the coculture initiation procedure, replacing the preculture with spore inoculum positively affected the biosynthesis of oxytetracycline, butyrolactone I, (+)-geodin, as well as the molecules putatively identified as rimocidin, CE-108, and (+)-erdin. It was concluded that the comparative analyses of SM production in filamentous microbial cocultures and monocultures are dependent on the type of inoculum and thus the diversification of inocula is highly recommended in such studies. Furthermore, it was demonstrated that designing a coculture experiment that involves only a single type of inoculum may lead to the underestimation of biosynthetic repertoires of filamentous microorganisms.