Aquatic-derived Streptomyces cavourensis BA1 as a promising source of antibacterial metabolites against...

Multidrug-resistant Gram-negative bacteria remain a major therapeutic challenge, highlighting the need for new antibacterial metabolites from underexplored microbial sources. This study investigated Streptomyces cavourensis strain BA1, isolated from the Zubaida fish farm southwest of Lake Burullus, Egypt, as a potential producer of antibacterial agents against multidrug-resistant Escherichia coli and Klebsiella pneumoniae. The isolate was characterized morphologically, microscopically, and molecularly, and its antibacterial activity was evaluated using culture filtrate and solvent fractions. Bioactive metabolites in the most active extract were analyzed by GC-MS and HPLC, while cytotoxicity against VERO cells and ultrastructural effects on bacterial cells were also assessed. Morphological and SEM observations showed branched mycelia and flexuous spore chains, and molecular analysis confirmed 100% similarity to S. cavourensis strain PES4. The culture filtrate exhibited marked bactericidal activity, whereas the ethyl acetate organic phase showed the strongest activity among solvent extracts, with MIC values of 0.125-0.5 µg/mL, exceeding the activity of ampicillin under the tested conditions. The extract showed a favorable in vitro safety profile, with a CC50 of 296.1 µg/mL against VERO cells. GC-MS revealed fatty acid derivatives, including 9,12-octadecadienoic acid and palmitic acid derivatives, while HPLC identified trans-ferulic acid, salicylic acid, and caffeic acid as antibacterial-associated phenolic/flavonoid compounds. SEM of treated bacteria demonstrated wrinkling, surface collapse, membrane damage, and cellular debris, suggesting disruption of cell envelope integrity. Overall, S. cavourensis BA1 represents a promising source of antibacterial metabolites, although genome sequencing, bioassay-guided purification, and mechanistic studies are required to confirm the active principles and therapeutic safety in vivo.