Abstract

The bacterial genus Streptomyces is an important source of antibiotics, and genome mining is a valuable tool to explore the potential of microbial biosynthesis in members of this genus. This study reports an actinomycete strain 135, which was isolated from Qinghai-Tibet Plateau in China and displayed broad antimicrobial activity. The fermentation broth of strain 135 displayed strong antifungal activity (>70%) against Sclerotinia sclerotiorum, Botrytis cinerea, Valsa mali, Phytophthora capsici, Glomerella cingulata, Magnaporthe grisea, Bipolaris maydis, Exserohilum turcicum in vitro, meanwhile possessed significant preventive and curative efficacy against S. sclerotiorum, Gaeumannomyces graminis, and P. capsici on rape leaves (54.04 and 74.18%), wheat (90.66 and 67.99%), and pepper plants (79.33 and 66.67%). X-14952B showed the greatest antifungal activity against S. sclerotiorum and Fusarium graminearum which the 50% inhibition concentration (EC50) were up to 0.049 and 0.04 μg/mL, respectively. Characterization of strain 135 using a polyphasic approach revealed that the strain displayed typical features of the genus Streptomyces. 16S rRNA gene sequencing and phylogenetic analysis demonstrated that the isolate was most closely related to and formed a clade with Streptomyces huasconensis HST28T (98.96% 16S rRNA gene sequence similarity). Average nucleotide identity (ANI) and DNA-DNA hybridization (DDH) values in strain 135 and related type strains were both below the threshold of species determination (91.39 and 56.5%, respectively). OrthoANI values between strain 135 and related type strains are under the cutoff of determining species (<95%). The biosynthetic gene cluster (BGC) designated to X-14952B biosynthesis was identified through genome mining and the possible biosynthesis process was deduced.

Highlights

  • Actinomycetes have excellent development potential as a microbial resource

  • The inhibition rates of 12 actinomycetes were greater than 70% on S. sclerotiorum, and 12 isolates had an inhibition rate greater than 60% on B. cinerea (Table 2), with some isolates featuring on both lists

  • The inhibition rates of fermentation broth of the different isolates appeared disparate on all tested pathogenic fungi, meaning that fermentation broth with a strong inhibitory effect on one type of pathogenic fungi may not have the same strength on another type of pathogenic fungi

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Summary

Introduction

Numerous structurally diverse natural products have been isolated from various actinomycetes, and almost two-thirds of natural antibiotics and anticancer agents, pesticides, and antimicrobial agents used in medicine are produced by members of this bacterial group. Streptomyces, as the most prominent genus and the most resourceful producer of antibiotics among the actinomycetes, has attracted considerable interest from the beginning of the golden age of natural product-based drug discovery. With the continuous search in discovering new species of Streptomyces and the ever-increasing studies of natural product separation, known bioactive compounds have been rediscovered even from phylogeny distinct Streptomyces. The latest research trends have focused on developing new screening technologies for efficient dereplication and investigating rare samples from extreme and unexplored environments where microorganisms may have unique characteristics compared with those of known species and might produce novel bioactive secondary metabolites (Suzuki, 2001). Extreme and unexplored environments include marine environments (Yang et al, 2020), mountains (Wei et al, 2020), deserts (Sivakala et al, 2021), and rainforests (Schneider et al, 2018)

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