Mycelial differentiation linked avermectin production in Streptomyces avermitilis studied with Raman imaging

Streptomyces avermitilis is a gram-positive bacterium that undergoes complex physiological and morphological differentiation during its life cycle, which has implications in secondary metabolites production. Avermectin, produced by S. avermitilis , is widely used as an anthelmintic and insecticidal agent. In this study, we have applied Raman microspectroscopic imaging to elucidate the correlation between production of avermectin and the morphological differentiation in S. avermitilis . We demonstrate distinctive variations in the localization of avermectin at various morphological stages, such as, substrate mycelium, spore-bearing mycelium, spiral spore chains under solid culture conditions. Under liquid culture condition, however, avermectin is detected only in mycelia after early MII stage of differentiation. Morphological differentiation was observed in liquid and solid cultures, but the chemical profiles of the mycelia were substantially different. Spherical bodies containing avermectin with characteristically different chemical composition to that of spores were also observed under solid culture, which suggests possible release of extracellular vesicles (EVs). Key points Avermectin production is regulated during mycelial differentiation Liquid and solid culture conditions affects mycelial differentiation Raman microspectroscopic analysis reveals localization profiles of avermectin