A modified sucrose fractionation procedure for the isolation of frankiae from actinorhizal root nodules and soil samples

Summary

The isolation and pure culture of the symbiotic nitrogen-fixing frankiae has always been difficult. In the past the isolation of these actinomycetes directly from soil samples has proven impossible and isolations from root nodules of many genera has been only poorly successful. We report here a modified sucrose fractionation procedure which increased the success of isolations from root nodules and which permitted the isolation of Frankia directly from soil samples. Crushed nodule suspensions or soil suspensions were incubated briefly in 0.7% phenol (carbolic acid) just before application to a sucrose density gradient. This phenol incubation decreased the number of contaminating eubacteria and fungi but more importantly increased the number of Frankia developing on the isolation plates. If the phenol incubation was used solely without sucrose fractionation no Frankia were isolated, suggesting the death of the organisms due to phenol toxicity. The use of selective nitrogen-deficient media proved important for the isolation of frankiae from soils.