Effects of bioreactor inoculation method on secondary metabolite production and morphological characteristics of Streptomyces rimosus in microparticle-enhanced cultivation (MPEC).

Streptomyces rimosus is renowned for complex metabolic pathways and the ability to produce diverse bioactive secondary metabolites, including antibiotics, antifungals, and other biologically active compounds. Despite their industrial significance, the cultivation of filamentous microorganisms still remains a complex challenge. In this work, the production of secondary metabolites and morphological characteristics of Streptomyces rimosus were examined in microparticle-enhanced bioreactor cultivations (MPEC) initiated with either spore or preculture inocula. The addition of 10 μm talc microparticles in the concentrations equal to 2.5, 5, 7 and 10 g L−1 to S. rimosus cultivations conducted in the 5-litre stirred tank bioreactors markedly influenced the biosynthesis of secondary metabolites: oxytetracycline, decarboxamidooxytetracycline (ADOTC), rimocidin, rimocidin CE-108, rimocidin with 27 ethyl group, an unnamed angucycline, rimosamide A and rimosamide B. The example of rimocidin showed that the inoculation procedure was among the factors determining the efficiency of metabolite bioproduction in MPEC. In the spore-inoculated variant of the experiment, the addition of 7 g TALC L−1 enhanced rimocidin production 17-fold, while in the corresponding run inoculated with the preculture, microparticles had a negative effect or did not substantially enhance the biosynthesis of the metabolite. The presence of microparticles in S. rimosus cultivations reduced the projected area of agglomerates regardless of whether the bioreactor was inoculated with spores or preculture. In the spore-inoculated control run, the size of the largest agglomerates was equal to 2.1·104 µm2 (96 h, 0 g TALC L−1) while for the corresponding MPEC, it was equal to 1.1·104 µm2 (96 h, 10 g TALC L−1). At the end of the preculture-inoculated experiment, in the control run the agglomerates reached 1.0·105 µm2 (96 h, 0 g TALC L−1) and in the MPEC their sizes decreased below 3.5·104 µm2 (96 h, 5 g TALC L−1). The addition of microparticles induced various effects on the morphology number Mo of S. rimosus, dependent on the bioreactor inoculation procedure. In the preculture-inoculated runs with 7 and 10 g TALC L−1 the morphology number exceeded 0.50, which indicated the development of regular pellets. In the other cases, the agglomerates were irregular, with the range of morphology number from 0.22 to 0.48.