LAUSR

The antagonistic potential of bacteria obtained from the nest of Odontotermes formosanus was assessed against Fusarium oxysporum f. sp. cucumerinum (FOC). Of 30, seven termite nest-associated bacteria strains had biocontrol potential. Among them, the strain YC-9 showed the strongest antifungal activity toward FOC. Phylogenetic analysis of the 16S rRNA amplified product of YC-9 revealed its identification as Bacillus siamensis. The in vivo antifungal activity experiment showed that the application of YC-9 at 108 cfu/ml significantly reduced the cucumber wilt incidence with a control efficacy of 73.2%. Furthermore, plant growth parameters such as fresh weight, dry weight, plant height, and root height were significantly improved by 42.6, 53.0, 20.8, and 19.3%, respectively. We found that inoculation with B. siamensis YC-9 significantly increased the activity of defensive enzymes such as peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) in diseased cucumber roots, thereby raising the resistance. PCR using gene-specific primers revealed that B. siamensis YC-9 contains biosynthetic genes for known antibiotics, including bacillomycin, iturin, and surfactin. Chemical analysis of the cultivation of B. siamensis YC-9 resulted in the isolation of five metabolites, including hexadecanoic acid (1), cyclo-(L-phenylalanylglycine) (2), cyclo-(L-trans-Hyp-L-Leu) (3), C15-surfactin (4), and macrolactin A (5), the structures of which were identified by the analysis of NMR spectroscopic data and MS. Among them, the compound 4 showed significant antifungal activity against conidial germination of FOC with an IC50 value of 5.1 μg/ml, which was comparable to that of the positive control, cycloheximide (IC50 value of 2.6 μg/ml). Based on these findings, this study suggests that termite-nest associated B. siamensis YC-9 could be a potential biological control agent for integrated control of soil-borne diseases like cucumber Fusarium wilt.