Anti-quorum sensing activity in plant extracts and its effect on the virulence of Pectobacterium carotovorum

Bacteria use a unique sophisticated mechanism named as Quorum Sensing (QS) for regulation of diverse physiological processes in consent with their population activities and gene expression. This phenomenon depends on the synthesis and perception of small signal molecules such as acyl-homoserine lactones (AHLs) which regulates the important functions including motility, antibiotic and pigment production, biofilm formation, and production of virulence determinants in many bacteria. The presence of Anti-QS activity in methanolic extract of 25 plants was assessed through inhibition of violacein pigmentation in Chromobacterium violaceum CV026 growing on LB agar plates at the presence of 5 mg/l C6-homoserin lactone. The results revealed that the QS inhibition activity was observed in the leaves and stem extracts of coriander (Coriandrum sativum), licorice (Glycyrrhiza glabra), rocket (Eruca sativa), hollyhock (Althea officinalis) and mountainous celery (Kelussia odoratissma) which repress violacein production in C. violaceum CV026 via formation of clear hallo with colorless bacterial colonies around the plant extracts. In addition, the methanolic extracts of Glycyrrhiza glabra and Eruca sativa were also able to inhibit QS-regulated virulence in Pectobacterium carotovorum and severelydecreased soft rot symptoms on potato tubers. Both LuxI and LuxR activity were affected by crude extracts of rocket and mountainous celery suggesting that the phytochemicals target both QS signal and receptor. The overall results suggest that plants have mimic quorum-sensing signals, which could be serving as potential sources to disrupt quorum sensing in associated bacteria and inhibited secretion of pectolytic enzymes in Pectobacterium.