Resource exploration and antibacterial activity screening of actinomycetes from Chahan Nur

Abstract:

[Background] Due to the wide spread of drug-resistant bacteria, new antibiotics are urgently needed. Actinomycetes are major antibiotic-producing pharmaceutical resources. Saline lakes are special water bodies with high concentrations of dissolved salts. Studies have demonstrated that actinomycetes with high diversity, rich novel metabolites, and a wide range of biological activities from saline lakes are potential resource for drug discovery. [Objective] This study aims to reveal the composition of actinomycete species in the soil samples collected from Chahan Nur and identify the strains with antibacterial activities, which will accumulate strains for the discovery of novel antibacterial compounds. [Methods] The actinomycetes were isolated by dilution coating method in 19 selective culture media. The diversity and novelty of the isolates were analyzed by comparison of 16S rRNA genes. According to the taxa and novelty, representative isolates were selected for detection of the genes involved in the biosynthesis of type Ⅰ, type Ⅱ polyketide synthases (PKS) and non-ribosomal polypeptide synthases (NRPS) by PCR. The fermented broths and mycelia of representative isolates were extracted with ethyl acetate and acetone, respectively, and then antibacterial tests were carried out with the disk diffusion method. [Results] A total of 250 actinomycete strains were isolated from nine soil samples, belonging to 28 genera, 16 families of 9 orders, and Streptomyces (88 isolates, 35.2%) and Nocardiopsis (68 isolates, 27.2%) were the dominant genera. Fifteen Streptomyces isolates showed the highest 16S rRNA gene similarities below 98.65% to relative strains, which implied they might belong to four novel Streptomyces species. Meanwhile, novel species of Nocardiopsis and Nocardioides were discovered. Among the 75 tested strains, 66 strains were detected with at least one gene involved in antibiotic biosynthesis, and 20 strains carried all the three functional genes. Seven actinomycete strains with strong antibacterial activities were screened out, including five isolates of Streptomyces, one isolate of Nocardiopsis, and one isolate of Promicromonospora. [Conclusion] Soil samples from Chahan Nur have abundant culturable actinomycetes, and the active substances from some strains have strong antibacterial activities. Therefore, the secondary metabolites of these strains are worthy of further study.