Exploration of antimicrobial activities and mechanisms of biocontrol agent Serratia nematodiphila BC-SKRU-1 against Penicillium digitatum in tangerine fruit

The isolate BC-SKRU-1, identified as Serratia nematodiphila from fungal culture contamination, was investigated for its biocontrol potential against Penicillium digitatum in vitro and on tangerine fruit, with a focus on its mechanisms of action. In vitro tests on PDA plates revealed BC-SKRU-1's broad-spectrum antifungal activity, achieving up to 74.68% inhibition against seven plant pathogenic fungi, including P. digitatum NKP4321. Bacterial culture filtrates from BC-SKRU-1 (BCF BC-SKRU-1) at 5–10% concentrations inhibited NKP4321 mycelial growth in PDB medium (78.41–83.68%) more effectively than in PDA medium (46.20–72.61%), with complete suppression at 15% (v/v) in both media. BCF BC-SKRU-1's efficacy was comparable to chemical fungicides such as propiconazole®, prochloraz®, and mancozeb®. Notably, BCF BC-SKRU-1 retained its antifungal activity after dilution (1/1000), autoclaving (at 121 °C), and storage (at −20 °C). In vivo evaluations on tangerine fruit showed significant reduction in the severity of postharvest green mold disease with BC-SKRU-1 treatments, especially at a concentration of 10⁸ CFU mL⁻¹. Both preventive and curative applications were effective, with curative treatments being more successful. Mechanistic studies indicated that BCF BC-SKRU-1 reduces intracellular ergosterol content, compromises plasma membrane integrity, and attenuates antioxidant defense activities (SOD, CAT, GSH, GSSG, and GSH/GSSG ratio). These findings highlight BC-SKRU-1 and its metabolites as promising biocontrol agents against green mold in tangerine fruits and provide insights into their antifungal mechanisms.