Genomics and X-ray microanalysis indicate that Ca2+ and thiols mediate the aggregation and adhesion of Xylella fastidiosa

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.

Nutritional deficiency in citrus with symptoms of citrus variegated chlorosis disease

“…Despite that X. fastidiosa is normally found in plant vessels (Leite et al, 2002), it was observed by using specific primers that X. fastidiosa could be found in leave tissues.…”

Calcium Increases Xylella fastidiosa Surface Attachment, Biofilm Formation, and Twitching Motility

“…Experiments from the present study indicate that the addition of Ca significantly increases the strength of the attachment of the cells to the surface. Previous investigations into the mechanism of X. fastidiosa cell adhesion using X-ray microanalyses of occluded citrus xylem vessels led Leite et al to propose a model where Ca and Mg ions act as cross-binding molecules that create a bridge between the negatively charged xylem vessels and the negatively charged bacterial EPS (30). According to this model, extracellular calcium is responsible for cell attachment, acting as an external physical force.…”

“…However, it has been suggested to be nonspecific in some cases (e.g., Ca and Mg), with these elements playing a role as bridges for the adhesion between negatively charged bacterial cells and xylem vessels (30). While previous studies using gene expression showed that Fe regulates virulence factors such as the type IV pilus and bacteriocins (55), other studies showed that cells in biofilms were more resistant to Cu than planktonic cells (44) and that the Cu and Zn present in xylem fluid are correlated with the in vitro growth of X. fastidiosa (1).…”

The Iron Stimulon of Xylella fastidiosa Includes Genes for Type IV Pilus and Colicin V-Like Bacteriocins

“…This suggests that iron sensing might be important in the early stages of plant colonization, to activate systems that allow efficient translocation throughout xylem vessels and competition against endophytes, and not for biofilm formation. According to current models of biofilm formation by Xylella, cells initially multiply and adhere to each other and host surfaces; later, they synthesize fastidian gum and hemagglutinin-like adhesins for biofilm maturation, when vessel clogging occurs and symptoms become evident (43,60,64,77). These findings warrant further studies in order to better understand the regulation of X. fastidiosa pathogenicity determinants as well as to evaluate their importance for its survival within both host and vector.…”