The isolation of novel terrestrial Streptomyces strains with antimicrobial and cytotoxic properties

Streptomyces are very important Gram-positive filamentous bacteria because they can produce many beneficial secondary metabolites with various biological activities. The study aims to isolate and identify novel Streptomyces strains from terrestrial habitats in the United Arab Emirates (UAE) with antimicrobial and cytotoxic properties. The antimicrobial efficacy was assessed using the disk and well susceptibility methods. Crude ethyl acetate extracts of active-producing strains were used to determine the antimicrobial activity by minimum inhibitory concentration (MIC) against E. coli, S. aureus, C. albicans, S. cerevisiae, B. subtilis, and E. coli-ESBL (extended-spectrum Beta-Lactamase), and cytotoxic properties against HeLa cells by MTT assay and DAPI staining. The novelty of the strains was determined by sequencing the 16S rRNA fragment. Results showed that antimicrobial activity was observed for CSK1, CSK3, CSW2, CSU1, CSU2, and CSG1 strains, with zones of inhibition ranging between 16 and 35 mm. Minimum inhibitory concentrations of ethyl acetate extracts ranged from 0.21 to 12.17 µg/mL, with the highest inhibitory effect against S. aureus ranging between 0.21 and 0.29 µg/mL. Some strains (CSK3, CSW2, CSU1 and CSG1) also displayed cytotoxic activity against the HeLa cancer cell line with IC50 values ranging between 3.46 and 9.74 µg/µL, and apparent DNA fragmentation and chromatin condensation. This study indicates that Streptomyces sp. strains isolated from different soil habitats in the UAE can produce antimicrobial compounds that can treat microbial infections. In addition, some strains’ cytotoxicity suggests that they are producing bioactive compounds that can lead to drug discovery.