Antagonistic activity of Phaeobacter piscinae against the emerging fish pathogen Vibrio crassostreae in aquaculture feed algae

Aquaculture provides a rich resource of high-quality protein; however, the production is challenged by emerging pathogens such as . While probiotic bacteria have been proposed as a sustainable solution to reduce pathogen load in aquaculture, their application requires a comprehensive assessment across the aquaculture food chain. The purpose of this study was to determine the antagonistic effect of the potential probiotic bacterium against the emerging fish pathogen in aquaculture feed algae that can be an entry point for pathogens in fish and shellfish aquaculture. strain S26 produces the antibacterial compound tropodithietic acid (TDA). In a plate-based assay, S26 was equally to more effective than the well-studied DSM17395 in its inhibition of the fish pathogens 90-11-286 and DMC-1. When co-cultured with the microalgae and , S26 reduced the maximum cell density of DMC-1 by 2 log and 3-4 log fold, respectively. A TDA-deficient mutant of S26 inhibited DMC-1 to a lesser extent than the wild type, suggesting that the antagonistic effect involves TDA and other factors. TDA is the prime antagonistic agent of the inhibition of 90-11-286. Comparative genomics of 90-11-286 and DMC-1 revealed that DMC-1 carries a greater arsenal of antibiotic resistance genes potentially contributing to the reduced effect of TDA. In conclusion, S26 is a promising new candidate for inhibition of emerging pathogens such as DMC-1 in algal feed systems and could contribute to a more sustainable aquaculture industry.IMPORTANCEThe globally important production of fish and shellfish in aquaculture is challenged by disease outbreaks caused by pathogens such as . These outbreaks not only lead to substantial economic loss and environmental damage, but treatment with antibiotics can also lead to antibiotic resistance affecting human health. Here, we evaluated the potential of probiotic bacteria, specifically the newly identified strain S26, to counteract these threats in a sustainable manner. Through a systematic assessment of the antagonistic effect of S26 against DMC-1, particularly within the context of algal feed systems, the study demonstrates the effectiveness of S26 as probiotic and thereby provides a strategic pathway for addressing disease outbreaks in aquaculture. This finding has the potential of significantly contributing to the long-term stability of the industry, highlighting the potential of probiotics as an efficient and environmentally conscious approach to safeguarding aqua