Antioxidant and Antibacterial Activities of Secondary Metabolites Produced by <i>Streptomyces</i> Isolates Against Extended‐Spectrum β‐Lactamase‐Producing Bacteria

Abstract

Extended-spectrum β-lactamase (ESBL)-producing Gram-negative bacteria pose a severe therapeutic challenge globally. Streptomyces remain one of the most prolific natural sources of antibacterial and antioxidant secondary metabolites, yet their activity against ESBL-producing pathogens remains under-explored. Soil-derived Streptomyces isolates were screened for bioactivity, and the most potent strain (SM7) was identified by 16S rRNA sequencing. Secondary metabolites were extracted using ethyl-acetate and evaluated for antibacterial activity against ESBL-producing Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae using agar diffusion, MIC, and MBC assays. Antioxidant activity was assessed using DPPH and ABTS assays, while GC-MS and molecular docking were employed to identify and characterize bioactive compounds. Streptomyces sp. SM7 exhibited strong antibacterial activity, producing inhibition zones of 21.4 ± 0.6 mm, 19.2 ± 0.4 mm, and 17.6 ± 0.5 mm against ESBL-producing E. coli, K. pneumoniae, and E. cloacae, respectively. MIC values ranged from 62.5 to 250 µg/mL, with bactericidal MBC/MIC ratios of 2. The extract showed potent antioxidant activity with DPPH and ABTS IC₅₀ values of 48.9 µg/mL and 61.4 µg/mL, respectively. GC-MS identified 18 bioactive compounds, with 2,4-di-tert-butylphenol (18.6%) as the major constituent, which exhibited a docking affinity of -7.1 kcal/mol against bacterial DHFR. Streptomyces sp. SM7 produces phenolic- and fatty-acid-rich metabolites with potent bactericidal and antioxidant activities against ESBL-producing pathogens, highlighting its promise as a natural source of next-generation antimicrobial agents. These findings support Streptomyces sp. SM7 as a promising lead for downstream purification, mechanism-guided optimization, and future drug-development efforts targeting difficult-to-treat ESBL-producing Enterobacterales.