Metatranscriptomic analyses coupled with phenotypic and metabolic verification revealed competition mechanisms between grass carp spoilage bacteria in synthetic communities.

Bacterial competition is a major factor influencing microbiota succession in fish. However, the competition mechanisms among dominant spoilage bacteria in grass carp remain to be systematically investigated. Therefore, this study aimed to elucidate the mutual competition relationships and mechanisms among five representative spoilage bacteria isolated from grass carp. To this end, we constructed synthetic microbial communities (SynComs) with these five strains and employed metatranscriptomics coupled with phenotypic verification to demonstrate their interactions at both transcriptional and phenotypic levels. The results showed that bacterial competitiveness largely determined the microbiota composition in SynComs. Aeromonas rivipollensis ranked the most competitive bacterium by both inhibiting the virulence phenotypes (siderophore production, biofilm, and swimming motility) and disrupting the carbohydrate/amino acid metabolisms of Shewanella putrefaciens and Pseudomonas putida. Concurrently, A. rivipollensis enhanced its own siderophore production and motility when interacting with these competitors, which further promoted its advantage. In contrast, S. putrefaciens was the second most competitive, as it potently utilized amino acids and enhanced its siderophore production and motility in response to P. putida. Collectively, this study reveals that bacterial competition in fish is mediated by both phenotype interference and metabolism disruption, suggests siderophore as a new target for controlling microbiota succession and quality deterioration in fish, and therefore provides theoretical guidance for developing new fish preservation strategies.