F-54501 Vandewre-les-Nancy, and URA CNRS 1977, Ecologie Microbienne, USBSE,The taxonomic position of nitrogen-fixing strains that were isolated from rhizosphere macerates of rice cultivated in the Binh Thanh region of Vietnam was determined by using polyphasic taxonomy. We determined the phylogenetic relationships of these organisms by performing DNA-rRNA hybridization experiments with a labeled rRNA probe from the type strain of Burkholderia cepacia, and we found that they belong to a single rRNA complex. Other members of this rRNA complex were also studied, and the N,-fixing strains were found to be closely related to B. cepacia. In addition, all members of the rRNA complex containing B. cepacia were studied by performing auxanographic and DNA-DNA hybridization experiments. Phenotypically and genotypically, the N,-fixing isolates constitute a single cluster together with two strains of clinical origin. These organisms constitute a new Burkholderia species, for which the name Burkholderia vietnamiensis is proposed; the type strain of this species is TVV75 (= LMG 10929). All members of this species can fix nitrogen. On the basis of our polyphasic taxonomy results and previously published data we concluded that the genus Burkholderia should be restricted to the following species: B. cepacia (the type species), Burkholderia mallei, Burkholderia pseudomallei, B. vietnamiensis, Burkholderia gladioli, Burkholderia caryophylli, Burkholderia plantarii, Burkholderia glumue, Burkholderia vandii, BurkhoMeria cocovenenans comb. nov., and Burkholderia andropogonis comb. nov. On the basis of genotypic and phenotypic results [Alcaligenes] eutrophus, [BurkhoZderia] solanacearum, and[Burkholderia] pickettii belong to two other clusters whose internal structures must be studied further.On the basis of the results of extensive phenotypic studies and DNA-rRNA and DNA-DNA hybridization experiments performed by Palleroni, Stanier, and their collaborators, the genus Pseudomonas was divided into five groups (24, 43, 46, 56). Additional phylogenetic data have shown that these groups are only very remotely related and that each of them contains species belonging to other genera (21-23, 70, 71). In addition to the former pseudomonads that have been shown to belong to the genus Xanthomonas and to a smaller group related to Pseudomonas diminuta and Pseudomonas vesicularis (21-23,43) classified in the recently described genus Brevundirnonas (52), three large groups of pseudomonads can be considered. Pseudomonas rRNA group I (43) is part of rRNA superfamily I1 (18) or the gamma subclass of the Proteobacteria (S), where it constitutes a separate rRNA complex (18,21,22,68); this group represents the authentic pseudomonads which are grouped with Pseudomonas aeruginosa, the type species (43). Although the internal relationships within this group have not been determined yet, it is evident that the genus Pseudomonas must be limited to this group and that all other Pseudomonas species have been generically misnamed as determined by phylogenetic da...

The diversity among 43 isolates of the genus Frankia was studied by determining levels of deoxyribonucleic acid relatedness (S1 nuclease method) and DNA base compositions. The guanine + cytosine contents ranged from 66 to 75 mol% . At least nine genomic species were differentiated, including three genomic species among strains compatible with members of the genus Alnus, five genomic species among strains compatible with members of the family Elaeagnaceae, and one genomic species among strains compatible with members of the genus Casuan'na. Genomic species 1, which contained proposed type strain CpIl and nine strains that were compatible with members of the genus Ahus and were 64 to 97% related to strain ACoN24d, is Frankia alni. Four genomic species contained single strains that were 0 to 30% related to the other genomic species. Typical strains isolated from members of the genus Casuarina were found to be very homogeneous (69 to 100% related to strain ORS020606) and clearly separated from atypical strains. The nine genomic species delineated in this study cannot be named since no phenotypic tests are available for identification.

Symbioses between the root nodule-forming, nitrogen-fixing actinomycete Frankia and its angiospermous host plants are important in the nitrogen economies of numerous terrestrial ecosystems. Molecular characterization of Frankia strains using polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) analyses of the 16S rRNA-ITS gene and of the nifD-nifK spacer was conducted directly on root nodules collected worldwide from Casuarina and Allocasuarina trees. In their native habitats in Australia, host species contained seven distinctive sets of Frankia in seven different molecular phylogenetic groups. Where Casuarina and Allocasuarina trees are newly planted outside Australia, they do not normally nodulate unless Frankia is introduced with the host seedling. Nodules from Casuarina trees introduced outside Australia over the last two centuries were found to contain Frankia from only one of the seven phylogenetic groups associated with the host genus Casuarina in Australia. The phylogenetic group of Frankia found in Casuarina and Allocasuarina trees introduced outside Australia is the only group that has yielded isolates in pure culture, suggesting a greater ability to survive independently of a host. Furthermore, the Frankia species in this group are able to nodulate a wider range of host species than those in the other six groups. In baiting studies, Casuarina spp. are compatible with more Frankia microsymbiont groups than Allocasuarina host spp. adapted to drier soil conditions, and C. equisetifolia has broader microsymbiont compatibility than other Casuarina spp. Some Frankia associated with the nodular rhizosphere and rhizoplan, but not with the nodular tissue, of Australian hosts were able to nodulate cosmopolitan Myrica plants that have broad microsymbiont compatibility and, hence, are a potential host of Casuarinaceae-infective Frankia outside the hosts' native range. The results are consistent with the idea that Frankia symbiotic promiscuity and ease of isolation on organic substrates, suggesting saprophytic potential, are associated with increased microsymbiont ability to disperse and adapt to diverse new environments, and that both genetics and environment determine a host's nodular microsymbiont.