Clove endophyte Bacillus vallismortis DX78: isolation, identification, and preliminary study of antimicrobial components

Abstract:

[Background] Endophytic microorganisms often produce the same, similar or novel secondary metabolites as host plants. Clove has excellent antimicrobial activities, from which the endophytic bacteria with strong antimicrobial effects could be isolated. [Objective] To screen out clove endophytic bacteria against Ralstonia solanacearum and isolate active components. [Methods] The antimicrobial endophytic bacteria were screened out by oxford-cup tests and identified by 16S rRNA sequence analysis. The active components were isolated by organic solvent extraction, silica-gel column chromatography, and preparative thin layer chromatography, and identified by NMR. The inhibitory effects of the active component on pathogens were determined by filtering paper method and mycelial growth rate method. [Results] A total of 112 strains of endophytic bacteria were isolated, and most strains were obtained from leaves, accounting for 37.4%. Seventeen strains showed inhibitory effect on R. solanacearum. Among these strains, DX78 demonstrated the strongest activity and was identified as Bacillus vallismortis. Dibutyl phthalate (DBP) was isolated from the fermentation broth of DX78. The minimum inhibitory concentrations of DBP against R. solanacearum and Pseudomonas syringae were 0.3 mg/disc and 0.25 mg/disc, respectively. Furthermore, DBP inhibited plant pathogenic fungi, with the EC₅₀ values of 3.751, 18.568, and 22.019 µg/mL against Glomerella cingulata, Botrytis cinerea, and Valsa mali, respectively. With G. cingulata as the target fungus, the inhibitory toxicity of tebuconazole was about 4.5 times that of DBP, and the inhibitory toxicity of DBP was about 12.5 times that of carbendazim. [Conclusion] Clove harbors rich antimicrobial endophytic bacteria, and DBP isolated from B. vallismortis DX78 has strong inhibitory effect on G. cingulata, which is worthy of further study.