Phylogenetic evidence for the taxonomic heterogeneity of Photorhabdus luminescens.

The sequences of the 16S rRNA gene of 40 strains of bacterial symbionts isolated from the nematodes Heterorhabditis spp. and seven bacterial symbionts of the nematodes Steinernema spp. which were isolated from different geographical areas, as well as the type strain of Xenorhabdus japonicus, were determined and compared to each other and to the sequences of several reference strains of members of the Enterobacteriaceae. The data confirmed the separate status of the two genera of symbionts of entomopathogenic rhabditid nematodes. The symbionts of Heterorhabditis spp. clustered with the type strain of Photorhabdus luminescens, while the symbionts of Steinernema spp. grouped with Xenorhabdus species. X. japonicus clustered with the other Xenorhabdus species. Phylogenetic analysis of 15 almost complete 16S ribosomal DNA (rDNA) sequences of the Heterorhabditis symbionts indicated that there were several subclusters. The properties correlated with these subclusters are not yet apparent, although there may be some geographical and ecological correlations. For example, among the nematode-symbiotic bacteria, the members of subclusters I and III are from southeastern and midwestern North America, respectively, while the members of subclusters II and IV are primarily from Europe and Australia, respectively. The nonsymbiotic strains of P. luminescens form a highly homologous subcluster by themselves. The results of DNA-DNA hybridization studies performed with a few selected strains of five of the 16S rDNA subclusters support the existence of several genospecies within P. luminescens.