"Microbial Wars" in a Stirred Tank Bioreactor: Investigating the Co-Cultures of Streptomyces rimosus and Aspergillus terreus, Filamentous Microorganisms Equipped With a Rich Arsenal of Secondary Metabolites.

Tomasz Boruta,Marcin Bizukojć,Anna Ścigaczewska

doi:10.3389/fbioe.2021.713639

Tomasz Boruta, Marcin Bizukojć + Show 1 more

Open Access

https://doi.org/10.3389/fbioe.2021.713639

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Abstract

Microbial co-cultivation is an approach frequently used for the induction of secondary metabolic pathways and the discovery of novel molecules. The studies of this kind are typically focused on the chemical and ecological aspects of inter-species interactions rather than on the bioprocess characterization. In the present work, the co-cultivation of two textbook producers of secondary metabolites, namely Aspergillus terreus (a filamentous fungus used for the manufacturing of lovastatin, a cholesterol-lowering drug) and Streptomyces rimosus (an actinobacterial producer of an antibiotic oxytetracycline) in a 5.5-L stirred tank bioreactor was investigated in the context of metabolic production, utilization of carbon substrates and dissolved oxygen levels. The cultivation runs differed in terms of the applied co-culture initiation strategy and the composition of growth medium. All the experiments were performed in three bioreactors running in parallel (corresponding to a co-culture and two respective monoculture controls). The analysis based upon mass spectrometry and liquid chromatography revealed a broad spectrum of more than 40 secondary metabolites, including the molecules identified as the oxidized derivatives of rimocidin and milbemycin that were observed solely under the conditions of co-cultivation. S. rimosus showed a tendency to dominate over A. terreus, except for the runs where S. rimosus was inoculated into the already developed bioreactor cultures of A. terreus. Despite being dominated, the less aggressive strain still had an observable influence on the production of secondary metabolites and the utilization of substrates in co-culture. The monitoring of dissolved oxygen levels was evaluated as a fast approach of identifying the dominant microorganism during the co-cultivation process.

Highlights

Microbial secondary metabolism is a rich source of bioactive molecules with pharmaceutical and industrial relevance, e.g., antibiotics and pigments (Ruiz et al, 2010; O’Brien and Wright, 2011; Pham et al, 2019)
Instead of focusing solely on oxytetracycline and lovastatin, being the extensively studied secondary metabolic products of S. rimosus and A. terreus, respectively, the intention was to identify as many molecules as possible to have a wide perspective on the stimulatory or inhibitory effect exerted on the given strain by the accompanying microorganism and to appreciate the entire “metabolic arsenal” unlocked during the microbial confrontation
The bioreactor co-cultivation of S. rimosus and A. terreus unlocks the formation of several molecules, including the ones identified as the oxidized derivatives of rimocidins and milbemycins, which may be of entirely biosynthetic origin or based on microbial biotransformation

Summary

Introduction

Microbial secondary metabolism is a rich source of bioactive molecules with pharmaceutical and industrial relevance, e.g., antibiotics and pigments (Ruiz et al, 2010; O’Brien and Wright, 2011; Pham et al, 2019). In terms of elucidating their ecological roles, biosynthetic origins and physiological impacts, these molecules continue to serve as challenging and exciting research subjects Eliciting their production under laboratory conditions is typically far from trivial, as it requires a specific set of environmental cues leading to the awakening of the underlying biosynthetic routes. In the cases when the conflict of interest arises, the competition is unfolded, the fight for nutrients and space begins and two species establish an antagonistic relationship In this kind of “microbial war” (Bauer et al, 2018), the competing microbes take advantage of their biochemical “armor and weaponry” (Keller 2015) to eliminate the opponent. On the other hand, investing scarce resources and energy in biosynthesizing and releasing such molecular bioweapons can be regarded as truly justified only in the face of external microbial threats Such reasoning stands behind the co-cultivation experiments focused on the awakening of secondary metabolite production. The “winner” of the competition can be considered as the strain that maintains observable metabolic productivity throughout the co-cultivation run and manages to inhibit the proliferation of its rival

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