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 metabolite 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 secondary metabolites at various stages of morphological differentiation. Under solid culture, avermectin was detected in the mycelia formed at the later stages of morphological differentiation (e.g., spore-bearing mycelium and spiral spore chains), but not in the early-stage substrate mycelium. On the contrary, under liquid culture condition, avermectin was found concentrated in the mycelial pellet formed at the early MII stage of differentiation. Furthermore, the chemical profiles of the mycelia were substantially different depending on the culture condition. Raman spectra corresponding to proteins, lipids, and cytochrome were observed in the mycelia irrespective of the stage of morphological differentiation, however, carotenoid was observed under solid culture condition particularly in spore-bearing mycelium and spiral spore chains.

Automated summary

This study used Raman microspectroscopic imaging to investigate the correlation between avermectin production and morphological differentiation in Streptomyces avermitilis. The results showed distinct variations in the localization of secondary metabolites at different stages of morphological differentiation. Avermectin was detected in the later stages of differentiation under solid culture, but concentrated in the early stages under liquid culture. The chemical profiles of the mycelia also differed significantly depending on the culture condition.