A numerical approach to the taxonomy of xanthomonas and some phytopathogenic pseudomonas spp. as determined by phenotypic properties and protein gel electrophoresis

Abstract

The applicability of polyacrylamide gel electrophoresis (PAGE) of whole cell protein extracts to the classification and rapid identification of phytopathogenic Pseudomonas and Xanthomonas species and pathovars was investigated. Results of numerical analysis of protein electrophoregrams were compared with those obtained from numerical analysis of classical phenotypic features. Shortcomings of and objections to certain taxonomic criteria for these genera are discussed. The PAGE technique was also tested in actual practice. It is concluded that electrophoresis of the total soluble proteins of the bacterial cell is a powerful, rapid and relatively simple method, applicable to everyday use, for the differentiation and identification of phytopathogenic pseudomonads and xanthomonads up to an infrasubspecific level. Results correlate to a high degree with the clustering obtained from classical phenotypic features, as well as DNA homology and, to a certain extent, rRNA homology groupings. Subgeneric relationships were determined by numerical taxonomy, as well as protein electrophoresis, within rRNA homology groups I, II, III and V, resulting in several proposals concerning the taxonomic positions of members of these genera. The reliability of PAGE as a taxonomic marker to supplement and evaluate phylogenetic relationships, together with its universal applicability to phytobacteriology, is illustrated, emphasising its potential role in the proposed polyphasic approach to Proteobacteria taxonomy. As a result of this study, the present emphasis on phylogenetic taxonomy and natural relationships between bacterial groups is questioned in view of the implications of this approach for practical, everyday phytobacteriology. The applicability of PAGE to applied phytobacteriology is illustrated by the rapid identification of a field isolate, while other advantages of PAGE for practical microbiology are also emphasised by this study. The value and power of computer-assisted numerical analysis is demonstrated.