No abstract

A total of 61 strains, including members of all five currently described pathogenicity groups of Xanthomonas campestris pv. citri (groups A, B, C, D, and E) and representing a broad geographical diversity, were compared by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis of whole-cell proteins, gas chromatographic analysis of fatty acid methyl esters, and DNA-DNA hybridization. We found that all of the pathogenicity groups were related to each other at levels of DNA binding of more than 60%, indicating that they all belong to one species. Our results do not confirm a previous reclassification of X. campestris pathogens isolated from citrus in two separate species (Gabriel et al., Int. J. Syst. Bacteriol. 39:14-22, 1989). Pathogenicity groups A and E could be clearly delineated by the three methods used, and group A was the most homogeneous group. The delineation of pathogenicity groups B, C, and D was not clear on the basis of the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins and gas chromatography of fatty acid methyl esters, although these groups constituted a third subgroup on the basis of the DNA homology results.

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 Recent contributions to the taxonomy of Xanthomonas are reviewed, and on the basis of these data and unpublished data from several laboratories, the usefulness of different phenotypic, chemotaxonomic, and genotypic techniques is discussed.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.