Identification And Regulation Of Spoilage Related Genes By Quorum-Sensing (QS) Signaling Moleculesin Shewanella

Because of the high content of nutrients and moisture, the large yellow crockar (Pseudosciaena croced) becomes not only the main maricultured fish worldwide, but also the most important economy marine fish in southeast China. With increasing demands of large yellow croaker, breeding industry has been developed rapidly. The late 90s of last century, Chinese Agriculture Ministry identified the large yellow croaker as one of the excellent exporting aquatic products. Microorganisms are the major cause of spoilage of most seafood products. However, only a few members of the microbial community, the specific spoilage organisms (SSOs), give rise to the offensive off-flavours and toxic compounds associated with seafood spoilage. During the periods of storage, SSOs often vary according to the different conditions. Generally, Pseudomonas and Shewanella spp. are the two major SSOs of fish stored at low-temperature.Quorum-sensing (QS) signaling molecules can regulate the expression of certain genes resulting in food corruption. And DKPs is a new signaling molecule, which can be used as a specific inhibitor to inhibit fish spoilage. However, there are few studies about how the quorum-sensing signaling molecules to regulate the spoilage. Thus, the aim of the work was to investigate the different spoilage ability and regulation of DKPs in the spoilage related genes of different Shewanella baltica strains. The main results are as follows:1. After treatment with signal molecules of DKPs(cyclo-(L-Leu-L-Leu)、cyclo-(L-Pro-L-Gly)-. cyclo-(L-Pro-L-Leu) and cyclo-(L-Pro-L-Phe))the growth rates of the highest and lowest spoilage potential Shewanella baltica strains all increased. Remarkably, treatment with cyclo-(L-Pro-L-Phe) increased the number of S. baltica OS 185 with the highest spoilage ability, while S. baltica OS223 greatly increased supplemented with cyclo-(L-Pro-L-Leu). We also found that the bio film formation of S. baltica strains increased gradually and declined to a platform status during 0-96 h. When adding signal molecules of DKPs(cyclo-(L-Leu-L-Leu)、cyclo-(L-Pro-L-Gly)、cyclo-(L-Pro-L-Leu) and cyclo-(L-Pro-L-Phe)) the abilities of biofilm formation increased compared with the control group.2. The spoilage abilities were determined by total number of colonies(TVC), sensory analysis, trimethylamine(TMA), total volatile base nitrogen (TVB-N) and biogenic amines. By comparison among 16 strains, we found that 9 strains (S. baltica 074, S. baltica 081, S. baltica 075, S. baltica 072, S. baltica 071, S. baltica 077, S. baltica 098, S. baltica 073 and S. baltica 100) had obviously higher spoilage abilities, of which, S. baltica 073 and S. baltica 100 were the highest, and S. baltica 034 was the lowest. By whole genome sequencing, the nucleotide sequences of TMAO reductase system periplasmic protein TorT in different strains are various. The homologies of S. baltica 077, S. baltica 098, S. baltica 073 and S. baltica OS 185 were 100%, while that of S. baltica 074, S. baltica 081, S. baltica 075, S. baltica 072 and S. baltica 071 were 93%. The homologies of the other strains were only 23%. The fitting degrees of phenotype and genotype of different strains were measured by R, which showed nicely fitting (R≈0.8).3. The relative mRNA expressions of TMAO reductase, cysteine synthase B and thioredoxin reductase can be regulated by signal molecules of DKPs. The four signal molecules of DKPs up regulated the expressions of TMAO reductase and thioredoxin reductase genes, while they down regulated the expression of cysteine synthase B gene.