Vibriosis is a major epizootic disease that impacts free-living and farmed fish species worldwide. Use of probiotics is a promising approach for prevention of Vibrio infections in aquaculture. A probiotic anti-Vibrio strain, Bacillus pumilus H2, was characterized, and the mechanism of its effect was investigated. All 29 Vibrio strains tested were growth-inhibited by H2. The anti-Vibrio substance present in cell-free supernatant of H2 was purified and characterized by reversed-phase HPLC. Minimum inhibitory concentrations of the purified substance, determined in liquid media for various Vibrio strains, ranged from 0.5 to 64 µg/ml. Addition of the purified substance to Vibrio vulnificus culture inhibited cell growth (estimated by OD600). Confocal microscopy and scanning electron microscopy analyses showed that surface structure of V. vulnificus cells was damaged by the purified substance, as reflected by presence of membrane holes, disappearance of cellular contents, and formation of cell cavities. The major mechanism of this anti-Vibrio activity appeared to involve disruption of cell membranes, and consequent cell lysis. The purified anti-Vibrio substance was shown to be structurally identical to amicoumacin A by MS and NMR analysis. Our findings indicate that B. pumilus H2 has strong potential for prevention or treatment of fish vibriosis in the aquaculture industry.

Microplastics have emerged as a significant global concern, particularly in marine ecosystems. While extensive research has focused on the toxicological effects of microplastics on marine animals and/or their associated microorganisms as two separate entities, the holistic perspective of the adaptability and fitness of a marine animal metaorganism-comprising the animal host and its microbiome-remains largely unexplored. In this study, mussel metaorganisms subjected chronic PS-MPs exposure experienced acute mortality but rapidly adapted. We investigated the response of innate immunity, digestive enzymes and their associated microbiomes to chronic PS-MPs exposure. We found that PS-MPs directly and indirectly interacted with the host and microbe within the exposure system. The adaptation was a joint effort between the physiological adjustments of mussel host and genetic adaptation of its microbiome. The mussel hosts exhibited increased antioxidant activity, denser gill filaments and increased immune cells, enhancing their innate immunity. Concurrently, the gill microbiome and the digestive gland microbiome respective selectively enriched for plastic-degrading bacteria and particulate organic matter-utilizing bacteria, facilitating the microbiome's adaptation. The microbial adaptation to chronic PS-MPs exposure altered the ecological roles of mussel microbiome, as evidenced by alterations in microbial interactions and nutrient cycling functions. These findings provided new insights into the ecotoxicological impact of microplastics on marine organisms from a metaorganism perspective.