Rhizobia are bacteria that form nitrogen-fixing nodules in legume plants. The sets of genes responsible for both nodulation and nitrogen fixation are carried in plasmids or genomic islands that are often mobile. Different strains within a species sometimes have different host specificities, while very similar symbiosis genes may be found in strains of different species. These specificity variants are known as symbiovars, and many of them have been given names, but there are no established guidelines for defining or naming them. Here, we discuss the requirements for guidelines to describe symbiovars, propose a set of guidelines, provide a list of all symbiovars for which descriptions have been published so far, and offer a mechanism to maintain a list in the future.

The distribution of dispersed repetitive DNA (repetitive extragenic palindromic [REP] and enterobacterial repetitive intergenic consensus [ERIC]) sequences in the genomes of a number of gram-negative soil bacteria was examined by using conserved primers corresponding to REP and ERIC sequences and the polymerase chain reaction (PCR). The patterns of the resulting PCR products were analyzed on agarose gels and found to be highly specific for each strain. The REP and ERIC PCR patterns of a series of Rhizobium meliloti isolates, previously ordered in a phylogenetic tree based on allelic variations at 14 enzyme loci (B. D. Eardly, L. A. Materon, N. H. Smith, D. A. Johnson, M. D. Rumbaugh, and R. K. Selander, Appl. Environ. Microbiol. 56:187-194), were determined. Isolates which had been postulated to be closely related by multilocus enzyme electrophoresis also revealed similar REP and ERIC PCR patterns, suggesting that the REP and ERIC PCR method is useful for the identification and classification of bacterial strains.

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