Bacterial strains isolated from wide ranges of nematode hosts and geographic sources and strains isolated from human clinical specimens were used to assess the taxonomic structure of the genus . The following two methods were used: DNA relatedness and phenotypic characterization. Analysis of the DNA relatedness data revealed that all of the strains studied were congeneric and that the genus is, on the basis of DNA relatedness data, more homogeneous than the other genus of nematode-symbiotic bacteria, the genus . In contrast to previous reports, only two DNA relatedness groups were identified in the genus . These groups corresponded to the symbiotic strains and the clinical strains. There appeared to be some subgroups within the symbiotic strain group on the basis of the interactions of the strains with nematodes, which corresponded to some extent with the DNA relatedness data. However, there were significant ambiguities in the DNA relatedness data, and this group could not be subdivided on the basis of DNA relatedness data or phenotypic data. The distinct functional differences within and between the DNA relatedness groups of symbiotic strains indicated that there are biologically significant subgroups within the genus that cannot be defined at this time. Further investigation of the taxonomy of by using different approaches and a suitably wide range of strains is recommended. However, it is clear that the clinical strains form a recognizable subgroup within the genus even though no formal subtaxon can be defined at this time.


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