Fastidian gum: theXylella fastidiosaexopolysaccharide possibly involved in bacterial pathogenicity

Abstract

The Gram-negative bacterium Xylella fastidiosa was the first plant pathogen to be completely sequenced. This species causes several economically important plant diseases, including citrus variegated chlorosis (CVC). Analysis of the genomic sequence of X. fastidiosa revealed a 12 kb DNA fragment containing an operon closely related to the gum operon of Xanthomonas campestris. The presence of all genes involved in the synthesis of sugar precursors, existence of exopolysaccharide (EPS) production regulators in the genome, and the absence of three of the X. campestris gum genes suggested that X. fastidiosa is able to synthesize an EPS different from that of xanthan gum. This novel EPS probably consists of polymerized tetrasaccharide repeating units assembled by the sequential addition of glucose-1-phosphate, glucose, mannose and glucuronic acid on a polyprenol phosphate carrier.

Response of Xylella fastidiosa to Zinc: Decreased Culturability, Increased Exopolysaccharide Production, and Formation of Resilient Biofilms under Flow Conditions

“…The same is true for EPSs, which binds cation to their negatively charged groups (71,72). The negatively charged residues in the EPS from X. fastidiosa (21,41) could chelate Zn, which was shown to accumulate Zn in biofilms (13), thus preventing accumulation of toxic levels inside the bacterial cell. A similar well-characterized case involves the response of E. amylovora to copper, where the bacterium increases EPS production upon contact with high levels of Cu (73).…”

“…Here, the expression of genes involved in EPS synthesis in X. fastidiosa (gtaB, xanB, gumD, and gumH) was analyzed to identify changes in expression induced by the presence of Zn. GtaB (UTP-glucose-1-phosphate uridylyltransferase) transfers a UDP unit to glucose-1-P. XanB (phosphomannose isomerase-GDP-mannose pyrophosphorylase) transfers a GDP unit to mannose-1-P. GumD (UMP-gluco- syltransferase) and GumH (GDP-mannosyltransferase) add the first glucose and mannose units, respectively, to the EPS repeating unit (21,41). As shown by RT-qPCR, Zn affects the initial steps of EPS synthesis in X. fastidiosa, increasing the availability of nucleotidyl precursors.…”

“…Bacterial EPS synthesis can be divided into three stages: (i) conversion of monosaccharides to nucleotidyl derivative precursors, (ii) assembly of the oligosaccharide repetitive unit attached to a polyprenol phosphate carrier, and (iii) polymerization of the repetitive unit and secretion of the polymer (41). Here, the expression of genes involved in EPS synthesis in X. fastidiosa (gtaB, xanB, gumD, and gumH) was analyzed to identify changes in expression induced by the presence of Zn.…”

“…Biofilm formed on the glass slides was stained with calcofluor white (MP Biomedicals, Illkirch, France) to stain polysaccharides, as previously described (38). Calcofluor white stains beta-linked polysaccharides (38)(39)(40), and X. fastidiosa EPS has been shown to have both beta-and alpha-linked polysaccharides (41). The biofilms were observed with a Nikon Eclipse A1 confocal laser scanning inverted microscope (Nikon, Melville, NY) using a 60ϫ oil immersion objective and the appropriate filters.…”

Xylella fastidiosa: Host Range and Advance in Molecular Identification Techniques

“…Deficiencies of P and K have been reported in leaves of CVC affected trees, together with high concentrations of Fe, Mn, and Zn (Silva-Stenico et al, 2009). On the leaves, chlorotic lesions appear on the upper side, while on the lower leaf side gummy lesions may appear, due to the production by X. fastidiosa of fastidian gum, an exopolysaccharide that was proposed to be involved in the formation of biofilms that allow the attachment and survival of bacteria inside the xylem vessels (da Silva et al, 2001). While the leaf matures, the gummy lesions can enlarge and become necrotic.…”