Influence of xylem fluid chemistry on planktonic growth, biofilm formation and aggregation ofXylella fastidiosa

Xylella fastidiosa is the causal agent of Pierce's disease in grapevines. The mechanisms of pathogenicity are largely due to occlusion of xylem vessels by aggregation of X. fastidiosa and biofilm formation. Xylella fastidiosa was subjected to xylem fluids with varying chemistries to examine the effects of nutritional components on bacterial growth in vitro. The exposure of X. fastidiosa to xylem fluids collected from different Vitis genotypes resulted in highly significant differences in both planktonic growth and biofilm formation. Planktonic growth of X. fastidiosa in Vitis xylem fluid was correlated to the concentration of citric acid, amino acids (glutamic acid, glutamine, histidine, valine, methionine, isoleucine and phenylalanine) and inorganic ions (copper, magnesium, phosphorus and zinc). Biofilm formation was correlated to many amino acids at 1 h of incubation. Xylem fluid from Vitis rotundifolia cv. Noble (fluid that supported low planktonic growth) was supplemented with the compounds that were correlated above to levels found in Vitis champinii cv. Ramsey (fluid that supported high planktonic growth) to determine the direct impact of xylem constituents on the growth characteristics of X. fastidiosa. Augmentation of fluid from Noble with the amino acids listed above, citric acid, calcium and magnesium resulted in increased planktonic growth and aggregation.

Quorum sensing-dependent biofilms enhance colonization in Vibrio cholerae

Vibrio cholerae is the causative agent of the diarrheal disease cholera. By an incompletely understood developmental process, V. cholerae forms complex surface-associated communities called biofilms. Here we show that quorum sensing-deficient mutants of V. cholerae produce thicker biofilms than those formed by wild-type bacteria. Microarray analysis of biofilm-associated bacteria shows that expression of the Vibrio polysaccharide synthesis (vps) operons is enhanced in hapR mutants. CqsA, one of two known autoinducer synthases in V. cholerae, acts through HapR to repress vps gene expression. Vibrio biofilms are more acid resistant than planktonic cells. However, quorum sensing-deficient biofilms have lower colonization capacities than those of wild-type biofilms, suggesting that quorum sensing may promote cellular exit from the biofilm once the organisms have traversed the gastric acid barrier of the stomach. These results shed light on the relationships among biofilm development, quorum sensing, infectivity, and pathogenesis in V. cholerae.

Differential expression of the Escherichia coli autoaggregation factor antigen 43

Antigen 43 (Ag43) is a self-recognizing surface adhesin found in most Escherichia coli strains. Due to its excellent cell-to-cell aggregation characteristics, Ag43 expression confers clumping and fluffing of cells and promotes biofilm formation. Ag43 expression is repressed by the cellular redox sensor OxyR. Here we used mutant versions of OxyR that are locked in either the reduced or the oxidized form as well as the addition of a simple redox-changing chemical to show that the redox state of OxyR influences Ag43 expression. Furthermore, the redox state of OxyR influences the biofilm-forming potential of E. coli. Finally, we demonstrated that Ag43-mediated cell aggregation confers significant protection against hydrogen peroxide killing.