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International audience; The essential genome of a bacterium encompasses core genes associated with basic cellular processes and conditionally essential genes dependent upon environmental conditions or the genetic context. Comprehensive knowledge of those gene sets allows for a better understanding of fundamental bacterial biology and offers new perspectives for antimicrobial drug research against detrimental bacteria such as pathogens. We investigated the essential genome of Xanthomonas hortorum pv. vitians, a gammaproteobacterial plant pathogen of lettuce (Lactuca sativa L.) which belongs to the plant-pathogen reservoir genus Xanthomonas and is affiliated to the family Xanthomonadaceae. No practical means of disease control or preventionagainst this pathogen is currently available, and its molecular biology is virtually unknown. To reach a comprehensive overview of the essential genome of X. hortorum pv. vitians LM16734, we developed a mixed approach combining high-quality full genome sequencing, saturated transposon insertion sequencing (Tn-Seq) in optimal growth conditions, and coupled computational analyses such as comparative genomics, synteny assessment and phylogenomics. Among the 370 essential loci identified by Tn-Seq, a majority was bound to critical cell processes conserved across bacteria. The remaining genes were either related to specific ecological features of Xanthomonas or Xanthomonadaceae species, or acquired through horizontal genetransfer of mobile genetic elements and associated with ancestral parasitic gene behaviour and bacterial defence systems. Our study sheds new light on our usual concepts about gene essentiality and is pioneering in the molecular and genomic study of X. hortorum pv. vitians.

International audience; Assessment of the taxonomy and diversity of the Xanthomonas strains causing bacterial leaf spot of lettuce (BLSL), commonly referred to as X. campestris pv. vitians, has been a long-lasting issue which held back the global efforts made to understand this pathogen. In order to provide a sound basis essential to its study, we conducted a polyphasic approach on a panel of strains obtained through sampling campaigns or acquired from collections. Results of a multilocus sequence analysis combined with phenotypic assays revealed that the pathotype strain of X. campestris pv. vitians does not match the description of the nomenspecies provided by Brown in 1918 [1]. However, LMG 938 = CFBP 8686 does fit this description. Therefore, we propose that LMG 938 = CFBP 8686 replaces LMG 937 = CFBP 2538 as pathotype strain of X. campestris pv. vitians. Then, overall genome relatedness indices (OGRIs) calculations and crossed-pathogenicity assays on lettuce allowed to validate the nomenclature X. hortorum pv. vitians comb. nov. originally proposed by Vauterin et al. in 1995 [2]. Interestingly, genome-based phylogenies exhibited an evolutionary continuum between closely related species X. hortorum and X. cynarae, confirmed by multiple OGRIs calculations between all the taxonomically relevant members of these two species. Phenotypic profiles characterized with Biolog GEN III microplates did not reveal any stable diagnostic traits legitimizing the distinction into two different species but a strong stable core phenotype. Thus, to be in accordance with nowadays standards in bacterial taxonomy and with respect to the global taxonomical framework of the genus Xanthomonas [3, 4], we propose the new combination of X. hortorum Vauterin et al. 1995 and X. cynarae (Trébaol 2000) Timilsina et al. 2018 into a new X. hortorum comb. nov. This new stable species would allow to integrate new uncharacterized members of X. hortorum isolated over the years on a wide range of host plants, making it a promising model for the study of genomic and ecologic determinants underlying adaptation to host plants. 1. Brown NA. Some bacterial diseases of lettuce. J Agric Res 1918;13:367. 2. Vauterin L, Hoste B, Kersters K, Swings J. Reclassification of Xanthomonas. Int J Syst Evol Microbiol 1995;45:472-489. 3. Chun J, Oren A, Ventosa A, Christensen H, Arahal DR, et al. Proposed minimal standards for the use of genome data for the taxonomy of prokaryotes. Int J Syst Evol Microbiol 2018;68:461-466. 4. Rosselló-Móra R, Amann R. Past and future species definitions for Bacteria and Archaea. Syst Appl Microbiol 2015;38:209-216.

International audience; Assessment of the taxonomy and diversity of the Xanthomonas strains causing bacterial leaf spot of lettuce (BLSL), commonly referred as Xanthomonas campestris pathovar vitians, has been a long-lasting issue which held back the global efforts made to understand this disease and struggle against the damages it causes in the field. In order to restore sane bases essential to the study of this pathogen, we conducted an extensive polyphasic approach on a panel of strains obtained through sampling campaigns or collection strains acquisitions. Results of a multilocus sequence analysis crossed with pathogenicity assays revealed that BLSL-causing strains formed a monophyletic pathovar sparsely diversified in three clonal groups. However, genome-based phylogenies yielded only two major groups of strains. From the bigger picture perspective, phylogenies exhibited the intermediate position of pathovar vitians between close species X. hortorum and X. cynarae, resulting in a phylogenetic continuum regarding the known diversity of the genus. Finally, whole-genome comparisons of all type, pathotype or representative strains of these two species by multiple overall genome relatedness indices (OGRIs) calculation endorsed the hypothesis of one single specie. Carbon and nitrogen sources utilization tested using Biolog microplates did not revealed any particular phenotypic differences which would have legitimate the distinction into two species. Therefore, we propose the combination of X. hortorum, X. cynarae and X. campestris pv. vitians into a new X. hortorum comb. nov. with CFBP 4925 T = LMG 937 T remaining the type strain, and LMG 938 neoPT = NCPB 2248 neoPT becoming the pathotype strain of X. hortorum pv. vitians.