Antagonistic effects and action mechanism of Bacillus velezensis HX-6 against Ralstonia solanacearum

In this study, we investigated the bacteriostatic activity and underlying mechanisms of the biocontrol bacterial strain HX-6 against Ralstonia solanacearum. The bacteriostatic activity of strain HX-6 fermentation broth and its cell-free supernatant (CFS) on R. solanacearum were evaluated using antagonistic plate assays and agar well diffusion assays. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of CFS were determined using the micro-dilution broth method. Strain HX-6 was identified based on colony morphological characteristics, along with 16S rRNA and gyrA gene sequencing. Its growth-promoting functions were assessed using selective media. Additionally, the inhibitory effects of CFS on R. solanacearum and its impact on cell integrity were further evaluated through changes in OD_{600 nm}, conductivity, nucleic acid leakage, and malondialdehyde content, as well as observations using scanning electron microscopy. The results demonstrated that both the HX-6 fermentation solution and its CFS exhibited significant inhibitory activity on R. solanacearum. The fermentation solution produced an inhibition zone with a diameter of 3.03 cm, while CFS achieved an inhibitory rate of 70.48%. The MIC and MBC of CFS were determined to be 12.5% and 50.0%, respectively. Strain HX-6 was identified as Bacillus velezensis and possesses functions such as indole-3-acetic acid (IAA) production, siderophore synthesis, biofilm formation, and phosphate solubilization. Furthermore, CFS significantly impaired R. solanacearum activity and damaged its cellular integrity. These effects were evidenced by changes in OD_{600 nm} value, conductivity, nucleic acid leakage, and malondialdehyde content. The results of scanning electron microscopy further revealed CFS caused significant damage to the integrity of R. solanacearum. The findings demonstrate that strain HX-6 exhibits strong antibacterial activity and shows promise as a novel biocontrol agent for bacterial wilt, warranting further investigation for agricultural use.