Significance and Impact of the Study: Shewanella baltica is one of the commonest spoilage microbial species, predominantly found in chilling seafood. Understanding mechanism of spoilage caused by S. baltica is critical to prevent spoilage of seafoods. This study explored the role of rpoS, a key regulator, in S. baltica. It revealed that rpoS takes part in stress adaptation such as heat, ethanol, H 2 O 2 and NaCl. Biofilm formation of S. baltica was affected by rpoS and incubation temperature. RpoS actively participated in quorum sensing system of S. baltica. This study provides evidence that RpoS is an important coordinator for environmental adaptation in S. baltica.

Aeromonas and Pseudomonas are important bacterial species involved in spoilage of refrigerated freshwater fish. In this study, 10 Aeromonas and seven Pseudomonas bacterial strains were isolated from spoiled grass carp and identified. Twelve of seventeen bacterial strains showed high potential of biofilm formation and 14 of 17 can produce extracellular protease. In order to explore the spoilage capacity of dual‐species, the sterile grass carp fillets were inoculated with mono‐ and dual‐species of Aeromonas salmonicida and Pseudomonas azotoformans strains. The results revealed significantly higher levels of the total viable count and total volatile basic nitrogen in dual‐species as compared to mono‐species from day 6. The higher contents of histamine, cadaverine and serious degradation in muscles tissue were also observed in dual‐species after 10 days of storage. Results of in vitro experiments showed that the co‐culture of A. salmonicida and P. azotoformans significantly increased the bacterial maximum growth rate, promoted the biofilm formation and improved the spoilage capacity of bacterial strains. This study has revealed that the co‐culture of Aeromonas and Pseudomonas bacterial strains accelerated spoilage process of grass carp and increased biofilm formation. It indicates that the mixed‐cultures of spoilage micro‐organisms pose a huge threat to food industry.

Fish skin has both positive and negative effects on the shelf-life of the fish. This study aimed to investigate the spoilage process of the skin and flesh of refrigerated farmed turbot (Scophthalmus maximus) with vacuum packaging. Microbial community changes were analyzed by combining culture-dependent and culture-independent methods. The results indicated that the shelf-life of vacuum-packaged refrigerated turbot was 16 d; skin mucus was the interference factor of turbot quality. The culture-dependent analysis demonstrated that the total viable counts and the population of H S-producing bacteria, Pseudomonas spp., Enterobacteriaceae spp., and lactic acid bacteria in skin had a range of 0.45 to 1.40 log (CFU/g) higher than the microbial numbers in flesh after 16 d in storage. 16S high throughout sequencing results demonstrated that the compositions of spoilage microbes were similar in skin and flesh. Shewanella spp., followed by Carnobacterium spp., was the dominant spoilage organism at day 16. Quorum sensing (QS) signaling activity increased during the storage. Exogenous N-butanoyl-L-homoserinelactone(C4-HSL) and N-hexanoyl-Lhomoserine lactone (C6-HSL) significantly accelerated the spoilage process of refrigerated turbot, while the addition of 4, 5-Dihydroxypentane-2, 3-dione (DPD) prolonged the lag phase duration. Therefore, QS may be involved in the spoilage process of refrigerated turbot.

N-acyl homoserine lactone (AHLs)-based quorum sensing signal molecules involving in the behavior regulation in most of the Gram-negative bacteria have widely been reported. This study aims to evaluate the effect of AHLs on Shewanella putrefaciens Z4 at the proteome level. It provides the theoretic basis for elucidating the spoilage mechanism of Shewanella spp., the common spoilage micro-organism in refrigerated marine aquatic food products.

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