Rhizosphere-Derived Biosurfactant-Producing Bacteria from Olive Trees as Biocontrol Agents Against Fusarium napiforme

Abstract

The increasing demand for sustainable alternatives to synthetic agrochemicals has intensified interest in rhizosphere microorganisms producing bioactive metabolites for crop protection. In this study, five biosurfactant-producing bacterial strains were isolated from the olive (Olea europaea L.) rhizosphere and their antifungal properties were evaluated against Fusarium napiforme F1. Based on phenotypic traits and 16S rRNA gene sequencing, the isolates were identified as Lysinibacillus macroides SY1, Lysinibacillus fusiformis SY2, Bacillus paramycoides SY3, Bacillus safensis SY4, and Pseudomonas sp. SY5. All strains exhibited surface-active properties with marked inter-strain variability. The highest emulsification index (E24) was observed for Pseudomonas sp. SY5 (67.9%), followed by B. paramycoides SY3 (57.1%) and L. macroides SY1 (53.6%). Surface tension was reduced from approximately 72 to 32.8–35.8 mN m⁻¹, while critical micelle concentration values ranged from 0.010 to 0.040 g L⁻¹, confirming efficient biosurfactant production. Biosurfactants from SY3 and SY5 remained stable under broad temperature, pH, and salinity conditions, indicating favourable formulation potential. The biochemical and chromatographic analyses of extracted biosurfactants suggested cyclic lipopeptide-type compounds in Gram-positive isolates and rhamnolipid-like glycolipids in Pseudomonas sp. SY5. Antifungal assays revealed strain-dependent inhibition profiles suggesting the involvement of multiple mechanisms of action. In dual culture, B. safensis SY4 showed the highest inhibition of F. napiforme (94.8%), whereas purified biosurfactants were most active for SY5 (59.8%) and volatile-mediated inhibition was strongest for SY2 (71.7%). These findings demonstrate that the olive rhizosphere is a valuable source of taxonomically diverse bacteria producing structurally distinct biosurfactants with significant antifungal potential. SY3 and SY5 represent promising candidates for the development of sustainable biocontrol formulations for crop protection.