Abstract: In order to figure out the function of the chaperone Hfq in Shewanella baltica SB-19 isolated from large yellow croaker, a hfq-knockout (KO) strain and a wild type (WT) strain of this spoilage bacterium were investigated for differences in their growth rate, quorum sensing, stress resistance, and ability to spoil sterilized fish juice. There was only one hfq gene in S. baltica SB-19, and its expression level increased with the increase of cell density. Although the growth rates of the WT and KO strains were similar at 30 ℃, the KO strain was significantly delayed and the cell density at the stationary phase was much lower at 4 ℃. Both strains had similar ability to produce the quorum sensing (QS) signal molecules of diketopiperazines (DKPs) and autoinducer-2 (AI-2), but the KO strain showed lower biofilm formation ability and extracellular protease activity, and sterilized fish juice inoculated with the KO strain produced less total volatile basic nitrogen (TVB-N) and trimethylamine (TMA). Besides, the KO strain was more susceptible to salt, nutritional, heavy metal, and disinfectant stress. The above results showed that Hfq was involved in regulating a variety of metabolic pathways in S. baltica SB-19, and affected the growth, quorum sensing, spoilage ability, and adaptability to environmental stress of the bacterium. Therefore, Hfq is a global regulatory factor that can regulate many physiological activities of S. baltica SB-19. This study provides a theoretical basis for the biological research of S. baltica and understanding the spoilage mechanism of seafood.

Key words: large yellow croaker; Shewanella baltica; Hfq; spoilage; functional analysis