Two Xylella fastidiosa subsp. multiplex strains isolated from almond in Spain differ in plasmid content and virulence traits

The plant pathogenic bacterium Xylella fastidiosa (Xf) is a major threat to agriculture and the environment worldwide. Recent devastating outbreaks in Europe highlight the potential of this pathogen to cause emergent diseases. Xf subsp. multiplex ESVL and IVIA5901 strains that belong to sequence type (ST) 6 were isolated from almond orchards within the outbreak area in Alicante province (Spain). Both strains share more than 99% of the chromosomal sequences), but the ESVL strain harbours two plasmids (pXF64-Hb_ESVL and pUCLA-ESVL). Here, virulence phenotypes and genome content were compared between both strains, using three strains from USA as a reference for the phenotypic analyses. Experiments in microfluidic chambers, used as simulation of xylem vessels, showed that twitching motility was absent in the IVIA5901 strain, while the ESVL strain had reduced twitching motility. Both Spanish strains had less biofilm formation, cell aggregation and lower virulence in tobacco compared with USA reference strains. Genome analysis of the two plasmids from ESVL revealed 51 unique coding sequences (CDS) that were absent in the chromosome of IVIA5901. Comparison of the chromosomes of both strains showed some unique CDS and SNPs in each strain with potential deleterious mutations. Genomic differences found in genes previously associated with adhesion and motility might explain the differences in the phenotypic traits studied. Although additional studies are necessary to infer the potential role of Xf plasmids, our results indicate that the presence of plasmids should be considered in the study of the mechanisms of pathogenicity and adaptation in Xf to new environments.