Environmental factors affecting twitching motility, biofilm development, and aggregation by Xylella fastidiosa

The bacterial pathogen Xylella fastidiosa causes many important plant diseases in different crops such as citrus, grapes, almond and coffee. While disease symptoms expressed by this pathogen are not completely understood, it is widely accepted that blockage of xylem vessels by aggregates of the bacteria and extracellular polysaccharides is a major factor leading to disease. Work from our laboratory has provided evidence that this bacterium migrates within xylem elements by twitching motility facilitated by extension and retraction of type IV pili. This movement helps the bacterium colonize new areas of the xylem, upstream from the point of inoculation. The use of 'artificial' xylem vessels (microfluidic flow chambers) has provided information on temporal and spatial activities of the bacterium in vitro. This tool has been very helpful for learning new aspects of the biology of X. fastidiosa. Using these microfluidic chambers we have examined the effect of various environmental factors on bacterial movement and biofilm formation. Preliminary results indicate that the addition of chelating agents to the medium reduced movement of the bacteria. When a dilution of sap from a susceptible grape variety (cv. Chardonnay) was added to the microfluidic chambers, cells formed thicker and structurally different biofilms than when they were grown in PD2 media alone. Other factors such as surfactants and DNase are being studied on their influence in both biofilm formation and movement The information obtained from these studies provides invaluable insight into the biology of the pathogen and disease development caused by X. fastidiosa.