INSERTION-SEQUENCE (IS) HYBRIDIZATION SUPPORTS CLASSIFICATION OF RHIZOBIUM-MELILOTI BY PHAGE TYPING

Sixty-one isolates of Rhizobium meliloti from two field sites which ha d been previously classified into 15 phage types on the basis of sensi tivity to 16 typing phages, were subjected to insertion sequence (IS) hybridization using DNA probes for ISRm3 and ISRm5. Isolates from all but one phage type contained ISRm3 (apparent copy no. 1-11) and all is olates contained ISRm5 (apparent copy no. 3-11). The isolates were pla ced into 24 IS classes based on differences in their respective ISRm3 and ISRm5 hybridization profiles. At either field site, isolates repre senting different phage types possessed IS hybridization profiles that differed from each other, while those comprising a specific type had identical or closely related profiles.-Isolates from one phage type we re unusual since they did not react with any of the typing phages and were shown by IS hybridization to constitute a heterogeneous group. Ev idence for spatial effects were provided by isolates from two of six t ypes present at both sites which fell into separate IS classes on the basis of their site of origin. These data have ecological implications and suggest that for a particular site, phage typing may be employed for the rapid assessment of the genetic diversity among field isolates .