Optimizing Extracellular Products from Vibrio proteolyticus for Their Use as Postbiotics in Aquaculture.

Vibrio proteolyticus DCF12.2 has demonstrated its ability to be used as a probiotic for fish species. This study investigates how different culture conditions influence the activity of its extracellular products (ECPs) in aquaculture, focusing on enzymatic and antibacterial activity, cytotoxicity, biofilm modulation, short-chain fatty acid (SCFA) profiles, and effects on Photobacterium damselae subsp. piscicida virulence. Enzymatic assays showed a variety of hydrolytic activities, including amylase, caseinase, and collagenase, which can enhance digestion and nutrient absorption in fish. Antibacterial assays revealed that ECPs from V. proteolyticus grown in an experimental aquafeed and a partial replacement of that aquafeed by 25% of a blend of microalgae inhibited P. damselae subsp. piscicida and P. damselae subsp. damselae. Cytotoxicity assays indicated variable effects across fish cell lines, with increased viability in SAF-1 and DLB-1 cells under specific conditions, and decreased viability in PLHC-1 cells, suggesting potential antitumor properties. Biofilm assays showed that certain ECP conditions reduced biofilm formation by Vibrio anguillarum, Aeromonas hydrophila, and Tenacibaculum maritimum. SCFA profiling detected acetic, iso-valeric, butyric, and valeric acids, which may contribute to antimicrobial activity and gut health. The ECPs significantly downregulated aip56 gene transcription, reducing the virulence of P. damselae subsp. piscicida. These findings suggest that ECPs from V. proteolyticus could be valuable aquafeed additives for enhancing fish nutrition, health, and disease resistance. Future research should aim to isolate and characterize the specific bioactive compounds responsible for these effects and elucidate their mechanisms of action for optimized application in aquaculture and other biotechnological fields.