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Pathogen Biology

Pathogen morphology

Xylella fastidiosa is a Gram-negative, rod-shaped bacterium that lacks flagella for motility and requires oxygen for respiration. The bacterial cells often possess a rippled (undulating) cell wall and terminal fimbriae (surface structures, shorter than flagella, that help to anchor the cells together in the xylem stream) (Figures 15 and 16). X. fastidiosa, as the name suggests, has fastidious nutrient requirements and can be difficult to grow in culture. The bacterium grows slowly on selective medium to form small colonies that appear white to yellow (Figure 17). The entire genome of a strain of X. fastidiosa isolated from citrus has been recently sequenced (the first bacterial plant pathogen for which this has been performed). Based on what is known for other microbes, the X. fastidiosa genome encodes for proteins involved in cell-cell interaction, degradation of plant cell walls, synthesis of toxins, and ability to cause disease (pathogenicity).


Figure 15	Figure 16	Figure 17

Host specificity

When the bacterium was first described in 1987, 25 bacterial strains isolated from 10 different hosts were included as a single species. Strains differ, however, in characteristics such as host range, pathogenicity, nutritional requirements, and genetic homology. In 2004, several new subspecies of X. fastidiosa were proposed based on pathogenicity, phylogenetic characteristics, and DNA relatedness. For example, the bacterium that causes Pierce’s disease of grapevine is X. fastidiosa subsp. piercei. Placed in the X. fastidiosa subsp. multiplex taxon are strains of X. fastidiosa that affect elm, sycamore, oak, and maple. Even within the same subspecies, however, differences in host range exist. For example, strains isolated from elm are not pathogenic to sycamore, and vice versa.

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