In vitro inhibition of nonsymbiotic nitrogen-fixing bacteria by rhizosphere actinomycetes associated with grasses

Eighty-five isolates of actinomycetes were obtained from the rhizospheres of 10 indigenous grasses of central Texas. Of these, 10 were classified as members of the genus Nocardia; the remainder were members of the genus Streptomyces. To determine whether antibiotic production might be a significant factor in the establishment of associative symbioses between grasses and diazotrophic bacteria, 12 strains of free-living nitrogen-fixing bactria were cross streaked against each actinomycete isolate. This in vitro assay indicated that 78% of the 85 isolates were antagonistic to at least one of the 12 nitrogen-fixing bacterial strains.