SUMMARY: Using lysates and purified DNA, base ratios were determined by the buoyant density ultracentrifugation and the absorbance-temperature transition techniques; the correspondence was very high between the two methods. The DNAs from a large number of Erwinia species, mostly phytopathogenic and nonphytopathogenic, but including some ‘Erwinia-like’ bacteria from animal sources and from other genera of the family Enterobacteriaceae were studied. The DNA base ratios of these bacteria fell mainly in the range 50–58 % GC, consistent with their taxonomic placement in the family Enterobacteriaceae. Certain assemblages of erwinias could be grouped on the basis of GC content into clearly differentiated clusters, which, accidentally or otherwise, correlated neatly with certain existing nomenclatural groupings. It was not possible, from the GC values alone, to decide whether the present multiplicity of Erwinia species can be justified as separate species, or whether and how they might be combined into fewer species, or whether and how they might be demoted to some infraspecific level. Indeed, it is not clear whether the genus , defined as it is solely on the basis of plant habitat, should be maintained or whether its species might not, just as rationally, be distributed throughout other genera of the family Enterobacteriaceae. Classifications of the genus on over-all phenotypic similarity result in a small number of specific taxa which, on the basis of the present study, are seen to contain organisms of different GC contents. In the present state of the art, one can only be agnostic about the amount of such differences in GC content which corresponds to separate taxa or to a particular categoreal level (generic, specific, infraspecific) in the present hierarchical taxonomic scheme.


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