Spoilage potential characterization of Shewanella and Pseudomonas isolated from spoiled large yellow croaker (Pseudosciaena crocea)

Ten strains were isolated from a spoiled large yellow croaker (Pseudosciaena crocea). All of them were able to grow aerobically from 4 to 30 degrees C, and reduce trimethylamine-N-oxide to trimethylamine (TMA) and produce H2S except SB01, PF05 and PF07. Biochemical characterization and phylogenetic analysis of 16S rRNA gene showed that eight H2S-producing isolates were closely related to Shewanella baltica, and two isolates PF05 and PF07 were identified as Pseudomonas fluorescens and Pseudomonas fragi respectively. However, of the eight Shewanella, seven isolates cluster with S.baltica and one with Shewanella glacialipiscicola based on the analysis of the gyrB gene. Shewanella baltica also had the ability to produce biogenic amines, while two Pseudomonas had high activities of proteinase and lipase, and failed to produce TMA and biogenic amines. In spoilage potential evaluation, the TVB-N value of S.baltica was significantly higher than that of Pseudomonas in sterile fish juice, although its growth was slower than Pseudomonas. Therefore, this work demonstrated that S.baltica was able to cause rapid and strong spoilage and was therefore identified as a specific spoilage organism in refrigerated P.crocea.