Phenotypic Characterization and Broad-Spectrum Antibacterial Evaluation of Streptomyces Strains from Rhizospheric Soil of Vegetable Crops

The emerging prevalence of multidrug-resistant human pathogens has intensified the global demand for novel antimicrobial agents from environmental reserves. This study aims to isolate, characterise and evaluate the therapeutic potential of actinomycetes derived from agricultural farmlands. Soil samples were serially diluted and selectively plated on nutrient agar supplemented with nalidixic acid, yielding four pure actinomycetes strains designated as ACT-A1, ACT-A3, ACT-A5 and A3. Comprehensive phenotypic characterization following International Streptomyces Project guidelines revealed dominant white and greyish-white aerial mycelia, pale yellow diffusible pigments and diverse spore chain morphologies (flexible, microverticillus and open loop). Biochemical and physiological profiling demonstrated robust extracellular enzymatic capabilities, including active casein, gelatin and starch hydrolysis, along with positive Methyl Red and Indole metabolic reactions. Growth tolerance assays confirmed that all isolates favour neutral-to-alkaline, successfully growing between pH 8.0 and 9.0 while exhibiting no growth under acidic conditions. Primary and secondary antimicrobial screening against five clinical test pathogens (Escherichia coli, Pseudomonas sp., Bacillus sp., Staphylococcus aureus and Klebsiella sp.) established significant broad-spectrum antagonistic activity across all strains, with strain ACT-A5 exhibiting maximal inhibitory potency. These findings highlight that agricultural soil isolates represent valuable pharmacological candidates for extracting bioactive secondary metabolites to combat emerging antibiotic resistance.