Antibacterial Activity and Action Mechanism of Synthetic Interleukin‐8 Derived Peptides Against <i>Flavobacterium psychrophilum</i>

In Chile, Atlantic salmon and rainbow trout face significant production challenges due to the presence of Flavobacterium psychrophilum, which generates severe disease issues and economic losses. To address this, the salmon industry relies on vaccines and antibiotics, the latter raising concerns about bacterial resistance. For that reason, our study explores an alternative strategy for controlling F. psychrophilum infections based on host defence peptides. We previously identified and characterised IL-8-derived salmonid peptides (ssIL-8α and omIL-8α) with potential antimicrobial properties. In the current study, we further investigated the antibacterial activity and mechanism of action of these peptides against F. psychrophilum. First, we demonstrated the antibacterial activity of ssIL-8α and omIL-8α synthetic peptides. Then we evaluated the effects of these peptides on membrane fluidity and localisation on bacterial cells by fluorescence microscopy as well as its impact on bacterial morphology and ultrastructure by electron microscopy. The results indicate that the ssIL-8α at 30 μM exhibits superior efficacy in inhibiting the growth of F. psychrophilum. Also, both ssIL-8α and omIL-8α can bind to pathogen membrane, but ssIL-8α exhibits a higher binding capacity compared to omIL-8α against F. psychrophilum. omIL-8α exhibited the ability to induce early membrane alterations within 15 min, at concentrations of 15 or 30 μM. The SEM and TEM micrographs showed membrane disruption of the bacteria after incubation with ssIL-8α or omIL-8α. However, the damage was more pronounced in the ssIL-8α treatment, as evidenced by a complete detachment of the outer membrane after a 20-min exposure of F. psychrophilum. This study reveals that these peptides significantly alter bacterial membrane morphology, leading to bacterial death, highlighting their potential as alternative treatments in flavobacterial disease control. This work contributes to understanding host defence peptides' role in combating bacterial infections and reducing antibiotic resistance in aquaculture.