Antibacterial and Virulence-Modulating Activities of Bacillus velezensis JM3 Against Pectobacterium carotovorum subsp. carotovorum PCC21

Bacterial soft-rot caused by Pectobacterium carotovorum subsp. carotovorum (Pcc) is associated with quorum sensing (QS), motility, and plant cell wall degrading enzymes. In this study, strain JM3, isolated from the rhizosphere of Chinese cabbage, was identified as Bacillus velezensis based on gyrB, 16S rRNA and rpoB phylogenetic analyses. JM3 showed strong antibacterial activity against Pcc strain PCC21 and significantly reduced soft-rot lesions in Chinese cabbage leaf sections. An ethyl acetate extract of the JM3 cell-free supernatant (CFS) showed concentration-dependent antibacterial activity, with a minimum inhibitory concentration (MIC) of 1 mg/mL. The acid-treated cell-free supernatant (AT-CFS), following acid treatment of the JM3 CFS and removal of the resulting precipitate, showed markedly reduced antibacterial activity, with an MIC of 8 mg/mL. At 0.5 mg/mL (1/16 MIC), AT-CFS did not markedly affect PCC21 growth but reduced N-acyl homoserine lactone associated signals, swarming motility, and extracellular protease, pectate lyase, and cellulase activities. RT-qPCR analysis revealed reduced expression of QS-related genes (expI and expR), extracellular enzyme genes (pelB, pehA, celA, and prt1), and motilityassociated genes (motA and flhC). These findings suggest that JM3 antagonizes PCC21 through both direct growth inhibition and extracellular activities affecting QS and virulence-associated traits under growth-noninhibitory conditions. The results provide a basis for further characterization of JM3 derived extracellular metabolites involved in interactions with the soft-rot pathogen.