Food Microbiology

Plants produce a diverse array of low molecular-mass compounds, with more than 8,000 phenolics. Phenolic compounds play a pivotal role in plant defense and fitness. Here, we provide an example of the interference of plant small molecules (phenolics) with bacterial virulence, via inhibition of the quorum sensing (QS) machinery of the soft rot bacterium P. carotovorum. Interestingly, biofilm formation and exoenzymes activity were significantly impaired, at compounds concentrations that did not affect bacterial cell growth or membrane integrity. Since biofilm production and exoenzymes are virulence determinants known to be under the strict control of QS, the effect of specific molecules on QS was studied. Common volatile and soluble phenolic compounds were tested for their influence on the expression of central QS system and QS-controlled genes. The N-acyl-homoserine lactone (AHL) reporter strains (CV026 and pSB401) demonstrated a prominent reduction in the level of QS signal molecules accumulation, following exposure to the compounds. Moreover, infection capabilities were strongly impaired on potato, cabbage, and calla-lily; but fully recovered upon external application of AHL. To further confirm the interaction of the plant compounds with bacterial QS targets, drug discovery tools were occupied using the schrodinger® molecular docking suit. Pectobacterium central QS proteins ExpI/ExpR were used as targets for salicylic acid and carvacrol (plant phenolics). Finally using isothermal calorimetry (ITC), SA and CAR were directly bound to cloned and purified ExpI, by this, experimentally supporting the computational docking results. True binding of plant-derived phenolics to bacterial QS synthase protein as a target was demonstrated. The importance of plant-bacteria chemical signaling shown here supports a rising field of research endeavoring interkingdom communication between plants and bacterial pathogens.