Lvvibriocin-GK effectively reduced skin ulcer syndrome of Apostichopus japonicus by eliminating surface bacteria, modulating gut microbiota, and enhancing host immune responses.

Skin ulceration syndrome (SUS) is a major threat to the aquaculture of Apostichopus japonicus, particularly in southern China, where it has shown high mortality rates and infectious potential. Traditional antibiotic treatments often lead to challenges such as antibiotic resistance. Antimicrobial peptides (AMPs), which are vital elements of innate immunity, represent a promising alternative for treating SUS. In the study, a novel AMP named Lvvibriocin-GK identified in Litopenaeus vannamei was found to have a strong antibacterial activity against multiple Vibrio species that possibly cause SUS. Through constructing a Vibrio harveyi-induced SUS model, we evaluated the efficacy of a 7-day Lvvibriocin-GK immersion treatment to SUS. Compared to doxycycline hydrochloride at the same concentration, Lvvibriocin-GK treatments could have ulcer area and numbers reduced, mortality decreased, the DAI index significantly lowered, as well as intestinal inflammatory cell infiltration decreased but no significant effect on body weight. The therapeutic effects of Lvvibriocin-GK were accompanied by significantly enhancing the activities of trypsin, lysozyme, T-NOS, and T-SOD and reducing Vibrio harveyi load in tissues. qPCR results indicated that Lvvibriocin-GK upregulated the expression of intestinal barrier proteins ZO-1 and Occludin, and downregulated pro-inflammatory factors such as IL17, p105, NLRP3, Rel, and Stat5. Furthermore, 16S rRNA sequencing revealed that the beneficial effects of Lvvibriocin-GK might be linked to favorable changes in A. japonicus 's gut microbiota, including increased microbial diversity, enhanced abundance of potential probiotics (Rhodobacteraceae, Bacillus, Serratia liquefaciens), and reduced the abundance of opportunistic pathogens (Acinetobacter and Bacteroides vulgatus). These changes resulted in a more complex microbial network and improved immune-associated functions, particularly through pathways such as NF-κB signaling. Mantel tests indicated stronger correlations between Lvvibriocin-GK-treated gut microbiota and disease phenotypes (gut pathology), enzymatic activities (lipase, lysozyme, T-NOS, T-SOD), intestinal barrier markers (Occludin), and immune-related genes (Stat5, Rel, FoxP, VEGF). Taken together, this study proposes a novel, environmentally friendly AMP immersion treatment for severe cases of SUS. The therapeutic effects are closely to effectively eliminate pathogens, modulate the gut microbiota and enhance host immunity. A comprehensive evaluation of the efficacy and mechanisms of AMP treatment in A. japonicus SUS will contribute to assessing its advantages and potential applications as an antibiotic alternative, promoting A. japonicus health and improving aquaculture practices.