No abstract

The availability of the genome sequence of the bacterial plant pathogen Xylella fastidiosa, the causal agent of citrus variegated chlorosis, is accelerating important investigations concerning its pathogenicity. Plant vessel occlusion is critical for symptom development. The objective of the present study was to search for information that would help to explain the adhesion of X. fastidiosa cells to the xylem. Scanning electron microscopy revealed that adhesion may occur without the fastidium gum, an exopolysaccharide produced by X. fastidiosa, and X-ray microanalysis demonstrated the presence of elemental sulfur both in cells grown in vitro and in cells found inside plant vessels, indicating that the sulfur signal is generated by the pathogen surface. Calcium and magnesium peaks were detected in association with sulfur in occluded vessels. We propose an explanation for the adhesion and aggregation process. Thiol groups, maintained by the enzyme peptide methionine sulfoxide reductase, could be active on the surface of the bacteria and appear to promote cell-cell aggregation by forming disulfide bonds with thiol groups on the surface of adjacent cells. The enzyme methionine sulfoxide reductase has been shown to be an auxiliary component in the adhesiveness of some human pathogens. The negative charge conferred by the ionized thiol group could of itself constitute a mechanism of adhesion by allowing the formation of divalent cation bridges between the negatively charged bacteria and predominantly negatively charged xylem walls.

No abstract

Two chemically defined media based on xylem fluid chemistry were developed for Xylella fastidiosa. These media were tested and compared to chemically defined media XDM 2 , XDM 4 and XF-26. New media were evaluated for the Pierce's disease (PD) strain UCLA-PD. Our media either was similar to the concentration of some amino acids found in the xylem fluid of the PD-susceptible Vitis vinifera cv. Chardonnay (medium CHARD2) or incorporated the tripeptide glutathione found in xylem fluid composition (medium 3G10-R). CHARD2 and 3G10-R are among the simplest chemically defined media available. Xylem fluid chemistry-based media supported X. fastidiosa growth and especially stimulated aggregation and biofilm formation.

No abstract