Composition and field distribution of the population of Rhizobium meliloti in root nodules of uninoculated field-grown alfalfa

Only four antibiotics (kanamycin, spectinomycin, ampicillin and novobiocin) of ten tested were capable of discriminating between root nodule isolates of Rhizobium meliloti obtained from uninoculated field-grown alfalfa (Medicago sativa L.). The 300 isolates in the collection were subdivided into seven groups based on their intrinsic antibiotic resistance characteristics with 204 and 55 isolates placed into two groups, C and F, respectively. Isolates from group C dominated the root nodule population on plants in eight of the nine quadrats analyzed. Furthermore they were one of the two dominant groups found in nodules formed on plants grown in a plant infection-soil dilution experiment and challenged with a composite soil sample from the field site. Antiserum raised to a group C isolate (No. 31) cross-agglutinated with 46 of 55 group C isolates to a titer identical with that of the parent antigen. There were no cross-reactions between isolates from any of the other six groups. Only 9 of 34 isolates from group F cross-agglutinated with antiserum raised to a group F isolate (No. 17). Thirty-three of 35 cross-agglutinating field isolates from group C had the same sodium dodecyl sulphate-polyacrylamide-gel electrophoretic protein-profile pattern as isolate No. 31 whereas non-agglutinating isolates from the same group had distinctly different protein profile patterns. The data suggest that intrinsic antibiotic resistance characteristics can be a useful complementary tool to be used in conjunction with other methods to identify and discriminate isolates of R. meliloti. It should not be used on its own as a strain identification method.