SEMISELECTIVE MEDIUM FOR ISOLATION OF RHIZOBIUM-LEGUMINOSARUM FROM SOILS

A medium which controlled the growth of soil bacteria without inhibiti ng Rhizobium leguminosarum strains and other fast- and slow- growing r hizobia was developed. This medium, MNBP, included antibiotics (penici llin G and bacitracin) which inhibit the growth of Gram-positive bacte ria, fungicides (cycloheximide and benomyl) and also pentachloronitrob enzene in order to control actinomycetes. Colony counts of total bacte ria from different soils were reduced by 88-95% on this medium, compar ed to those obtained on a nutrient-rich agar medium, MGYF, commonly us ed for culture of rhizobia. The numbers of both Gram-positive and Gram -negative bacteria were lowered on medium MNBP. When used in conjuncti on with a complementary selection step (inability to grow on Luria-Ber toni agar medium) and a specific identification method (colony blot hy bridization with a biovar-specific DNA probe), this medium allowed the isolation of a population of 66 R. leguminosarum by. viciae from a so il sample. The effectiveness of the isolation procedure was found to b e satisfactory because the number of R. leguminosarum by. viciae isola ted agreed with that estimated by indirect plant infection counts.