Determination of an Expedient Method for Screening Quorum Quenching (QQ) Rhizobacteria with Choromobacterium violaceum (CV) RU9 as Bioindicator

Quorum quenching (QQ) is the process of hindering the auto-inducer molecules that acts as quorum sensing (QS) signals. Quorum sensing signals, the Acyl homoserine lactones play a major role in induction of virulence in Pectobacterium carotovorum subsp. carotovorum (Pcc), the most acclaimed causative agent of soft rot disease in several vegetable crops. The disease sounds so alarming because it is also an infamous post-harvest affliction. Disruption of the QS signals will halt the production of QS induced secretion of the plant cell wall degrading virulence enzymes. Thus, in the present study it was attempted to screen rhizobacterial isolates for their ability to produce degradative enzymes for the disruption of the QS signal molecules. Three methods, soft agar diffusion assay, soft agar overlay assay and disc diffusion assay were trailed out with a basic modification to choose the most befitting method. For the assay we used Choromobacterium violaceum (CV) strain RU9 as a bioindicator rather than the mutant strain CV026. Of the three methods of screening, the soft agar overlay assay was chosen as the pre-eminent method for screening which further required standardization of cell load of CV cells for the overlay. 1% to 5% CV RU9 cells were overlaid and their violacein units were determined simultaneously. A concentration of 3% CV RU9 cells had 812 violacein units for overlay was determined to be the appropriate concentration. Soft agar overlay assay with 3% CV RU9 cells was used to determine the quorum quenching ability of the rhizobacterial strains isolated from various crops.