Abstract

By playing a major role in the virulence of several pathogenic bacteria, N-Acyl homoserine-lactone (AHL) -dependent quorum sensing, is now considered a useful target for antimicrobial therapy. In the present study a total number of 63 Streptomyces isolates recovered from different soil samples collected from different localities in Egypt were screened for quorum quenching activity using a fast, reliable, simple screening method and Chromobacterium violaceum mutant strain (CV026) as a biosensor. Primary screening against synthetic hexanoyl homoserine lactone (HHL) revealed that 8 isolates showed quorum quenching activity while secondary screening against the extracted naturally produced acyl homoserine lactone (AHL) signals of seven Pseudomonas aeruginosa clinical isolates showed a variable profile of activity. Characterization of activity showed that isolate that coded St62, showing highest activity among other Streptomyces isolates, could degrade 2 µM HHL in 4 h. The crude enzyme of this isolate was found to have high thermal stability retaining >83% of its activity when pre-incubated at 80°C for 60 min. It also retained >90% of the activity when pre-incubated at pH 8, pH 9 and pH 10 and >80% when pre-incubated at pH 6. All the tested metal ions and EDTA had no effect on activity except for Cu⁺⁺ which caused partial reduction in activity. Enzyme extract was found to have acylase activity equal to 5.2 U/mg total protein.  Vmax and Km were found to be equal to 19.92 nM.min^-1 per mg protein and 23.05 nM, respectively. Maximum catalytic activity was observed at pH 8 over a temperature range of 20-50°C. The enzyme also showed preference in hydrolyzing AHLs with long acyl chains than short acyl chains but yet digested all the tested AHL standards. Finally, identification of the selected isolate using 16S ribosomal RNA gene analysis and phylogenetic analysis showed it to be a Streptomyces minutiscleroticus which is, to the best of our knowledge, the first identified quorum quenching bacterial species of its type.

Key words: Quorum sensing, Streptomyces minutiscleroticus, Antipathogenic drugs, Acyl homoserine lactones.