Inhibition of Bacterial Quorum Sensing Systems Using Streptomyces Extracts: A Study on Pseudomonas aeruginosa and Chromobacterium violaceum.

Background: The effectiveness of ethyl acetate Streptomyces extracts as quorum-sensing (QS) inhibitors in Pseudomonas aeruginosa PAO1 was evaluated as an antivirulence strategy to counteract antibiotic resistance. Materials and Methods: Streptomyces isolates were obtained from local soil samples (Tikrit, Iraq), and bioactive metabolites were extracted using ethyl acetate. Anti-QS activity was assessed using two bacterial models: Chromobacterium violaceum CV026 to quantify inhibition of violacein production, and P. aeruginosa PAO1 to evaluate elastase, pyocyanin, protease, and biofilm formation. Gene expression analysis of the las, rhl, and pqs QS pathways (lasI/lasR, rhlI/rhlR, pqsA/pqsR) was conducted using SYBR Green–based qPCR, with rpoD as the reference housekeeping gene. Results: The extract significantly reduced the production of key virulence factors (elastase: p = 0.002, pyocyanin: p = 0.003, protease: p = 0.001) without affecting bacterial growth (p = 0.120). In parallel, a significant downregulation was observed in the expression of all investigated QS-related genes (lasI: p < 0.001, rhlI: p = 0.001, pqsA: p = 0.002), indicating effective disruption of QS signaling at the transcriptional level. Conclusion: These findings demonstrate that ethyl acetate Streptomyces extracts exhibit inhibitory activity against QS-regulated virulence in P. aeruginosa PAO1, supporting their potential as natural antivirulence agents that attenuate pathogenicity without exerting strong selective pressure for resistance. Further work is warranted to purify and characterize the active compounds, investigate their synergy with conventional antibiotics, and evaluate their efficacy against multidrug-resistant clinical isolates.