PHENOTYPIC AND DNA RELATEDNESS BETWEEN NEMATODE SYMBIONTS AND CLINICAL STRAINS OF THE GENUS PHOTORHABDUS (ENTEROBACTERIACEAE)

Bacterial strains isolated from wide ranges of nematode hosts and geog raphic sources and strains isolated from human clinical specimens were used to assess the taxonomic structure of the genus Photorhabdus. The following two methods were used: DNA relatedness and phenotypic chara cterization. Analysis of the DNA relatedness data revealed that all of the strains studied were congeneric and that the genus Photorhabdus i s, on the basis of DNA relatedness data, more homogeneous than the oth er genus of nematode-symbiotic bacteria, the genus Xenorhabdus. In con trast to previous reports, only two DNA relatedness groups were identi fied in the genus Photorhabdas. These groups corresponded to the symbi otic strains and the clinical strains, There appeared to be some subgr oups within the symbiotic strain group on the basis of the interaction s of the strains with nematodes, which corresponded to some extent wit h the DNA relatedness data, However, there were significant ambiguitie s 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 o f symbiotic Photorhabdus strains indicated that there are biologically significant subgroups within the genus Photorhabdus that cannot be de fined at this time, Further investigation of the taxonomy of Photorhab dus 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.