Phylogenetic relationships of Xenorhabdus spp. and Phororhabdus sp. were investigated on the basis of 16s rRNA gene sequences. Xenorhabdus spp. and Photorhabdus sp. were grouped together with Proteus vulgaris and Arsenophonus nasoniae. This group was distant from other members of the family Enterobacteriaceae. Xenorhabdus japonicus, previously proposed as a new species, was nearly located to Xenorhabdus nematophilus. Signature nucleotides of X. japonicus were identified that distinguish it other members of the family Enterobacteriaceae.
The members of the genera Xenorhabdus and Photorhabdus are bacteria symbiotically associated with the genera of entomopathogenic nematodes Steinemema and Heterorhabditis, respectively (AKHURST 1983, BOEMARE et al. 1993, THOMAS and POINAR 1979). THOMAS and POINTER (1979) proposed the new genus and the type species Xenorhabdus nematophilus for entomopathogenic nematophilic bacteria. AKHURST (1983) classified the species X. nematophilus into four subspecies based on the biochemical and physiological features. In 1988, based on the numerical analysis of the conventional taxonomic features, AKHURST and BOEMARE (AKHURST and BOEMARE 1988, BOEMARE and AKHURST 1988) proposed elevating each subspecies to the status of species, X . nematophilus, Xenorbabdus beddingii, Xenorhabdus bovienii and Xenorhabdus poinarii, and emended the description of the species Xenorhabdus luminescens associated with nematode Heterorhabditis. In 1993, X . luminescens was transferred to new genus, Photorhabdus; the type species is Photorhabdus luminescens (BOEMARE et al. 1993). The exclusion of l? luminescens from the genus Xenorhabdus was based on low interspecies DNA reassociation values (