Adaptation of genetically monomorphic bacteria: evolution of copper resistance through multiple horizontal gene transfers of complex and versatile mobile genetic elements.

Copper-based antimicrobial compounds are widely used to control plant bacterial pathogens. Pathogens have adapted in response to this selective pressure. Xanthomonas citri pv. citri, a major citrus pathogen causing Asiatic citrus canker, was first reported to carry plasmid-encoded copper resistance in Argentina. This phenotype was conferred by the copLAB gene system. The emergence of resistant strains has since been reported in Réunion and Martinique. Using microsatellite-based genotyping and copLAB PCR, we demonstrated that the genetic structure of the copper-resistant strains from these three regions was made up of two distant clusters and varied for the detection of copLAB amplicons. In order to investigate this pattern more closely, we sequenced six copper-resistant X. citri pv. citri strains from Argentina, Martinique and Réunion, together with reference copper-resistant Xanthomonas and Stenotrophomonas strains using long-read sequencing technology. Genes involved in copper resistance were found to be strain dependent with the novel identification in X. citri pv. citri of copABCD and a cus heavy metal efflux resistance-nodulation-division system. The genes providing the adaptive trait were part of a mobile genetic element similar to Tn3-like transposons and included in a conjugative plasmid. This indicates the system's great versatility. The mining of all available bacterial genomes suggested that, within the bacterial community, the spread of copper resistance associated with mobile elements and their plasmid environments was primarily restricted to the Xanthomonadaceae family.

Natural Infection of Cashew (Anacardium occidentale) by Xanthomonas citri pv. mangiferaeindicae in Burkina Faso.

Xanthomonas citri pv. mangiferaeindicae is the causal agent of bacterial canker of mango (Mangifera indica, Anacardiaceae), a disease of international importance. Since the original description of the bacterium in the 1940s, the status of cashew (Anacardium occidentale, Anacardiaceae) as a host species has been unclear. Here, we report the first outbreak of a cashew bacterial disease in Burkina Faso (Western Africa) where X. citri pv. mangiferaeindicae recently emerged on mango. A comprehensive molecular characterization, based on multilocus sequence analysis, supplemented with pathogenicity assays of isolates obtained during the outbreak, indicated that the causal agent on cashew in Burkina Faso is X. citri pv. mangiferaeindicae and not X. citri pv. anacardii, which was previously reported as the causal agent of a cashew bacterial leaf spot in Brazil. Pathogenicity data supported by population biology in Burkina Faso suggest a lack of host specialization. Therefore, the inoculum from each crop is potentially harmful to both host species. Symptoms induced on cashew leaves and fruit by X. citri pv. mangiferaeindicae and nonpigmented strains of X. citri pv. anacardii are similar, although the causative bacteria are genetically different. Thus, xanthomonads pathogenic on cashew may represent a new example of pathological convergence in this bacterial genus.