Screening and preliminary characterization of quenching activities of soil Bacillus isolates against acyl homoserine lactones of clinically isolated Pseudomonas aeruginosa

Abstract

Aim: Quorum quenching of Pseudomonas aeruginosa homoserine lactone signal molecules represents a new approach for control of infection of this pathogenic highly resistant microorganism. The present study aimed at screening of soil for quorum sensing inhibitory bacteria active against clinically isolated P. aeruginosa together with the characterization of their activity and finally identification of promising isolates. Methodology and results: Using a fast, reliable and simple screening method, 161 bacterial isolates collected from soil from different places in Egypt were screened for their quorum quenching activity against synthetic hexanoyl homoserine lactone using Chromobacterium violaceum mutant strain (CV026) as a biosensor. All 32 positive isolates were found to belong to Bacillus species. Secondary screening against the signals extracted from seven Pseudomonas isolates, analyzed by thin layer chromatography, was done. The activity of all the positive isolates was found to be intracellular. Activity against different concentrations of synthetic hexanoyl homoserine lactone showed that some isolates could degrade more than 20 µM even when diluted 100 fold. Selected isolates were found to have broad spectrum activity against other synthetic homoserine lactone standards. Maximum activity for most of the selected isolates was found to occur between 25-60 °C. Crude enzyme extracts of the promising isolates were collected by sonication, protein concentrations of the obtained extracts were measured and their activities were compared by well diffusion method. Finally, the isolates with promising quorum quenching activities were identified using 16S ribosomal RNA sequencing. Conclusion, significance and impact of study: Having high activity against homoserine lactone autoinducers, the enzyme produced by Bacillus isolates represents a new promising antipathogenic drug suppressing Pseudomonas virulence.