Objective This study aimed to isolate and identify actinomycetes in rhizospheric soil of Casuarina equisetifolia, determine the antibacterial activity of the rhizospheric actinomycetes, and screen out the actinomycetes isolates with antimicrobial activities.

MethodsThe actinomycetes in the rhizospheric soil of C. equisetifolia were isolated by heat strike dilution method. The actinomycetes isolates were further identified by combining morphological observation with molecular biology method. The antibacterial activities of secondary metabolites produced by the rhizospheric actinomycetes against different bacteria were determined using TLC-bioautography assay. The secondary metabolites in the ethyl acetate layer of rhizospheric actinomycetes were analyzed by HPLC (high performance liquid chromatography).

ResultsA total of twelve actinomycetes strains were isolated and identified from the rhizospheric soil of C. equisetifolia, all of which were the genus of Streptomyces sp., and the maximum similarities were all above 97%. S. spinosus Ceaf-4 and S. chattanoogensis Ceaf-12 had stronger antibacterial activity against Ralstonia solanacearum, and the diameters of antibacterial spots were more than 10 mm. The antibacterial spot diameters of strain S. spinosus Ceaf-4 against Escherichia coli and Staphylococcus haemolyticus were also greater than 10 mm. In addition, S. spinosus Ceaf-4 and S. chattanoogensis Ceaf-12 both contained abundant secondary metabolites, but there were significant difference in the types and relative contents of secondary metabolites.

Conclusion There are actinomycetes with Streptomyces sp. as the dominant population in the rhizospheric soil of C. equisetifolia. S. spinosus Ceaf-4 and S. chattanoogensis Ceaf-12 show the stronger inhibitory activity against R. solanacearum and contain abundant secondary metabolites. They can be used as candidate active strains to further explore their antibacterial active components.