MEDIUM FOR DIRECT ISOLATION OF RHIZOBIUM-MELILOTI FROM SOILS

A nutrient poor medium containing 100 mg l-1 yeast-extract, 400 mg 1-1 tryptone, 100 mg l-1 calcium chloride, 150 mg l-1 cycloheximide, 25 mg l-1 Congo-red and 10 g l-1 agar (medium A) greatly reduced polysaccharide production and permitted development of discrete bacterial colonies from soil dilutions of 10(-2) and greater; pentachloronitrobenzene (5 mg l-1) suppressed the growth of actinomycetes. Colony counts of diverse strains of Rhizobium meliloti and other fast growing rhizobia on this medium were either significantly greater or did not differ from those on traditionally-used nutrient-rich agar media. Medium A was made partially selective for R. meliloti by the addition of 5 g l-1 sodium chloride (medium AS). The utility of medium AS for detection and direct isolation of R. meliloti from soil was demonstrated by using colony blot hybridization in conjunction with a species-specific DNA probe derived from nodH: strains SU47 and 1521 were readily recovered from a field soil (deficient in native rhizobia of the same species) in numbers which did not differ significantly from those used at inoculation.