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Simple Summary: Probiotic supplementation plays a vital role in European sea bass wellbeing. Accordingly, it is important to increase our knowledge of and experience on their mechanisms of action and host effects. Although information on these aspects is available, further studies are needed to achieve optimal European sea bass aquaculture. European sea bass production has increased in recent decades. This increase is associated with an annually rising demand for sea bass, which encourages the aquaculture industries to increase their production to meet that demand. However, this intensification has repercussions on the animals, causing stress that is usually accompanied by dysbiosis, low feed-conversion rates, and immunodepression, among other factors. Therefore, the appearance of pathogenic diseases is common in these industries after immunodepression. Seeking to enhance animal welfare, researchers have focused on alternative approaches such as probiotic application. The use of probiotics in European sea bass production is presented as an ecological, safe, and viable alternative in addition to enhancing different host parameters such as growth performance, feed utilization, immunity, disease resistance, and fish survival against different pathogens through inclusion in fish diets through vectors and/or in water columns. Accordingly, the aim of this review is to present recent research findings on the application of probiotics in European sea bass aquaculture and their effect on growth performance, microbial diversity, enzyme production, immunity, disease resistance, and survival in order to help future research. [ABSTRACT FROM AUTHOR]

Bacillus spp. supplementation as probiotics in cultured fish diets has a long history of safe and effective use. Specifically, B. velezensis show great promise in fine-tuning the European sea bass disease resistance against the pathogenicity caused by several members of the Vibrio family. However, the immunomodulatory mechanisms behind this response remain poorly understood. Here, to examine the inherent immune variations in sea bass, two equal groups were fed for 30 days with a steady diet, with one treatment supplemented with B. velezensis. The serum bactericidal capacity against live cells of Vibrio anguillarum strain 507 and the nitric oxide and lysozyme lytic activities were assayed. At the cellular level, the phagocytic response of peripheral blood leukocytes against inactivated Candida albicans was determined. Moreover, head-kidney (HK) total leukocytes were isolated from previously in vivo treated fish with LPS of V. anguillarum strain 507. Mechanistically, the expression of some essential proinflammatory genes (interleukin-1 (il1b), tumor necrosis factor-alpha (tnfa), and cyclooxygenase 2 (cox2) and the sea bass specific antimicrobial peptide (AMP) dicentracin (dic) expressions were assessed. Surprisingly, the probiotic supplementation significantly increased all humoral lytic and cellular activities assayed in the treated sea bass. In addition, time-dependent differences were observed between the control and probiotic treated groups for all the HK genes markers subjected to the sublethal LPS dose. Although the il1b was the fastest responding gene to a significant level at 48 h post-injection (hpi), all the other genes followed 72 h in the probiotic supplemented group. Finally, an in vivo bacteria challenge against live V. anguillarum was conducted. The probiotic fed fish observed a significantly higher survival. Overall, our results provide clear vertical evidence on the beneficial immune effects of B. velezensis and unveil some fundamental immune mechanisms behind its application as a probiotic agent in intensively cultured European sea bass. • B. velezensis strain D-18 as probiotic generates protection in the European sea bass. • Innate humoral and cellular mechanisms show significant enhancements in the animals fed with the probiotic. • In vivo challenge against a pathogenic strain of Vibrio anguillarum was better resolved by the probiotic fortified fish. • The regular use of B. velezensis is suggested as a viable eco-friendly strategy to prevent bacterial diseases in fish culture. [ABSTRACT FROM AUTHOR]

In this work, we evaluated the effect of the probioticEnterococcus gallinarumL-1 on the cellular immune system of four different fish species of great interest in aquaculture such as gilthead sea bream (Sparus aurata), European sea bass (Dicentrarchus labrax), meagre (Argyrosomus regius) and red porgy (Pagrus pagrus). Phagocytic activity, respiratory burst and peroxidase content of leucocytes were observed 30 minutes after incubation with the probioticE. gallinarumstrain L-1, alive or inactivated with heat shock or ultraviolet (UV) light at different concentrations of 107, 108and 109cfu mL−1(final concentration 106, 107and 108cfu mL−1).E. gallinarumproduced dose-dependent increments in respiratory burst in red porgy, sea bream and sea bass leucocytes. About 106and 107cfu mL−1of live and inactivated bacteria with no stimulation of the respiratory burst activity of sea bream and red porgy head kidney leucocytes was shown. The highest values of peroxidase content were observed in red porgy cells with stimulation indexes higher than 1 in each treatment. Statistical analysis revealed that differences were only significant in sea bream where UV light-inactivated bacteria denote statistically significant differences (P< 0.05) with respect to other treatments. Highest values of phagocytic activity were obtained in sea bream leucocytes incubated with live bacteria (26% ± 1.88), where significant differences (P< 0.05) with other species were detected. Our results suggest that thein vitroassays may be useful in optimising their effective dose and viability for the immunomodulatory effects of probiotic bacteria, althoughin vivostudies are necessary to confirm the immunomodulatory effect of this strain. [ABSTRACT FROM PUBLISHER]