Towards an Improved Taxonomy of Xanthomonas

Abstract

Improvement of the taxonomy of the genus Xanthomonas and especially of Xanthomonas campestris, which is subdivided into more than 125 pathovars, is discussed. Recent contributions to the taxonomy of Xanthomonas are reviewed, and on the basis of these data and unpublished data from several laboratories, the usefulness of different phenotypic, chemotaxonomic, and genotypic techniques is discussed. The heterogeneity of several X. campestris pathovars has been demonstrated by sodium dodecyl sulfate electrophoresis of whole-cell proteins and fatty acid fingerprinting. The host selectivity of the pathovars is not correlated with their relationships as revealed by DNA-DNA hybridization experiments. In order to reveal the phylogenetic relationships among X. campestris pathovars and their relationships to other Xanthomonas species, it will be necessary to perform extensive DNA-DNA homology studies as an essential part of a polyphasic approach. At present, six DNA homology groups within X. campestris have been delineated. A systematic approach to improve the taxonomy of the genus Xanthomonas is proposed.

Reclassification of Xanthomonas

“…vesicatoria. Vauterin et al (49) reported that this pathovar is composed of two unrelated genotypes, which were designated types A and B. These types were later confirmed by protein electrophoresis data (48).…”

“…All of the strains used in this study were selected on the basis of the results of a fingerprint analysis of more than 1,000 Xanthomonas strains in which sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of proteins was used and on the basis of the results of a FAME analysis, as described previously (48,49,51,52,60). We verified that these organisms were authentic xanthomonads by determining their Xanthomonas-specific protein patterns and fatty acid contents (48,60).…”

“…Schroth and Hildebrand (39) have stressed the need for genomic homology matrices to clarify the taxonomic relationships within the genera Pseudomonas and Xanthomonas. Extensive studies based on electrophoresis of proteins (48), gas chromatographic analysis of fatty acid methyl esters (FAME) (60), restriction fragment length polymorphisms of genomic DNA (32), restriction patterns of rRNAs (4), and DNA hybridization (28,37,49,50) have shown that a number of more or less well-defined groups exist in X campestris and that a number of pathovars are heterogeneous. Most of the DNA hybridization results that have been published are fragmentary (49,50) or deal with specific groups, such as pathovars isolated from members of the Poaceae (51) or from citrus (52), and are part of a polyphasic study on the taxonomy of the genus Xanthomonas conducted in our laboratory.…”

“…Most of the DNA hybridization results that have been published are fragmentary (49,50) or deal with specific groups, such as pathovars isolated from members of the Poaceae (51) or from citrus (52), and are part of a polyphasic study on the taxonomy of the genus Xanthomonas conducted in our laboratory. A global DNA homology matrix for the genus Xanthornonas should serve as a basis for reclassification of the species and pathovars of the genus (39,49).…”

Comment on the Reinstatement of Xanthomonas citri (ex Hasse 1915) Gabriel et al. 1989 and X. phaseoli (ex Smith 1897) Gabriel et al. 1989: Indication of the Need for Minimal Standards for the Genus Xanthomonas

“…Vauterin et al (30) observed that the creation of species solely on the basis of RFLP analysis raises the same objections and poses the same problems for classification and nomenclature as does creating species solely on the basis of pathogenicity for particular plant hosts. It is generally recognized that an orderly nomenclature of plant-pathogenic bacteria will be best served if an accurate overview of the relationships of organisms at the molecular level is obtained and more phenetic data are gathered before individual plantpathogenic species, presently named as pathovars, are reinstated.…”