Exploring the multifaceted effects of dietary encapsulated piscidin 1 on gilthead seabream (Sparus aurata): from immunomodulation to muscle quality.

Host defense peptides are promising alternatives to conventional antimicrobials used in aquaculture. This study examined the in vitro stability and folding of piscidin 1 (P1) in gilthead seabream (Sparus aurata) and evaluated the immunomodulatory and product quality effects of dietary encapsulated P1 in vivo. Synthetic non-amidated P1 (FIGWLIKGAISAGKAIHGAIRRRFG) incubated with seabream serum retained high integrity after 3 h. Circular dichroism revealed a random-coil structure in water, which shifted to an α-helical conformation in membrane-mimetic media. For the feeding trial, P1 was encapsulated in carrageenan/pectin microparticles and administered at 0% (control), 0.05% (D1), and 0.1% (D2) for 17 days. Dietary P1 increased serum peroxidase activity at day 7 in D1 (p<0.05) without altering antiprotease or lysozyme, while respiratory burst of head kidney leucocytes decreased (p<0.05) in both D1 and D2. Phagocytic ability was higher (p<0.05) in D2 than in D1 on days 7 and 17, whereas phagocytic capacity increased in D1 and decreased (p<0.05) in D2 on day 7. RT-qPCR revealed tissue- and diet-dependent modulation along the gut-skin-brain axis, with prominent statistically significant gut responses and brain activation in D2 at day 17. PERMANOVA showed a diet effect between the control and D2 groups on day 17. Beyond immunity, the P1 diet produced darker eyes, a cooler/blue skin hue, increased muscle lightness in D2, and increased adhesiveness in both P1 groups and resilience in D2. Collectively, encapsulated P1 remained structurally competent and biologically active after oral delivery, modulating systemic immunity, gene expression, pigmentation, and texture without compromising nutritional composition, thereby supporting its potential as a multifunctional feed additive.