As a specific spoilage organism of seafood under refrigerated temperature conditions, Shewanella spp. tend to form biofilms that exacerbate the occurrence of seafood spoilage. Biofilm-promoting factor A (BpfA) has been reported to promote the adhesion and biofilm formation of Shewanella spp., but its role in adhesion and biofilm formation of S. putrefaciens under cold stress needs to be further investigated. To better comprehend the effect of BpfA on adhesion and biofilm formation of S. putrefaciens under cold stress (4 ℃), bacterial adhesion and biofilm phenotype of S. putrefaciens CN32 WT and ΔbpfA at 4 ℃ were analyzed and performed transcriptomics. The results showed that the deletion of bpfA had almost no effect on the growth of S. putrefaciens CN32 at 4 ℃, but weakened the unicellular adhesion capacity of S. putrefaciens CN32 and destabilized the stability of the multicellular adhesion layer. In addition, the biomass of the mature biofilm formed by ΔbpfA was merely around 50% of that observed in the mature biofilm of S. putrefaciens CN32 WT, the average thickness and volume of the biofilm decreased by 18% and 27%, respectively, and the composition of the biofilm changed. Transcriptome analysis demonstrated that the deletion of bpfA led to differential expression of genes involved in metabolic pathways such as bacterial chemotaxis, two-component system, tyrosine metabolism, drug metabolism-other enzymes and biofilm formation-Vibrio cholerae, which in turn influenced bacterial adhesion and biofilm formation. Those results advance our acknowledgment of the character of BpfA on adhesion and biofilm formation of S. putrefaciens CN32, which contributes to understanding bacterial adhesion and the control of biofilm formation.

Bigeye tuna is a protein-rich fish that is susceptible to spoilage during cold storage, however, there is limited information on untargeted metabolomic profiling of bigeye tuna concerning spoilage-associated enzymes and metabolites. This study aimed to investigate how cold storage affects enzyme activities, nutrient composition, tissue microstructures and spoilage metabolites of bigeye tuna. The activities of cathepsins B, H, L increased, while Na⁺/K⁺-ATPase and Mg²⁺-ATPase decreased, α-glucosidase, lipase and lipoxygenase first increased and then decreased during cold storage, suggesting that proteins undergo degradation and ATP metabolism occurs at a faster rate during cold storage. Nutrient composition (moisture and lipid content), total amino acids decreased, suggesting that the nutritional value of bigeye tuna was reduced. Besides, a logistic regression equation has been established as a food analysis tool and assesses the dynamics and correlation of the enzyme of bigeye tuna during cold storage. Based on untargeted metabolomic profiling analysis, a total of 524 metabolites were identified in the bigeye tuna contained several spoilage metabolites involved in lipid metabolism (glycerophosphocholine and choline phosphate), amino acid metabolism (L-histidine, 5-deoxy-5′-(methylthio)adenosine, 5-methylthioadenosine), carbohydrate metabolism (D-gluconic acid, α-D-fructose 1, 6-bisphosphate, D-glyceraldehyde 3-phosphate). The results of tissue microstructures of tuna showed a looser network and visible deterioration of tissue fiber during cold storage. Therefore, metabolomic analysis and tissue microstructures provide insight into the spoilage mechanism investigations on bigeye tuna during cold storage.