Biocontrol potential of the Leuconostoc mesenteroides strain MS4 against the black rot disease

In response to the increasing need for sustainable agricultural solutions, lactic acid bacteria have emerged as an alternative to chemical bactericides, demonstrating significant potential for managing bacterial plant diseases. In this study, we aimed to evaluate the potential of the Leuconostoc mesenteroides strain MS4 in controlling the black rot disease caused by Xanthomonas campestris pv. campestris (Xcc), the most damaging bacterial disease of vegetable brassicas worldwide. The in vitro assays demonstrated that MS4 exhibits significant antagonistic activity against Xcc. Notably, fluorescence and transmission electron microscopy analyses revealed that the cell-free precipitate (CFP) derived from strain MS4 cultured in MRS broth possesses bactericidal activity against Xcc cells, leading to cell membrane alterations and cytoplasmic condensation. This activity of CFP was lost after its treatment with proteinase K, indicating the proteinaceous nature of the active compounds. Furthermore, spot assays showed the involvement of three MS4-derived bacteriocins (MK-45, MR-53, and MW-56) in anti-Xcc activity at minimal lethal concentrations ranging from 8 mg/mL to 0.1 mg/mL, with MK-45 being the most potent bacteriocin. In in planta experiments, preventive application of MS4 to broccoli plants by foliar spraying resulted in an 83% reduction in black rot symptoms. This study represents the first investigation into the antagonistic activity of Leuconostoc mesenteroides against Xcc, demonstrating its efficacy and potential applicability for the prevention of black rot disease in Brassica vegetables.