Sensory characteristics, the succession of bacterial communities and the contents of six biogenic amines during long-term fermentation (1, 2, 3 and 8 years) of shrimp paste were investigated by sensory evaluation, high-throughput sequencing and high-performance liquid chromatography (HPLC), respectively. Results showed that the color of shrimp paste turned dark brown, the L* value declined and the b* value increased then declined, and the grainy mouthfeel disappeared along with the prolongation of fermentation time. Meanwhile, the shrimp-like and ammonia-like smell decreased, and umami taste reached the maximum after two years of fermentation. The bacterial flora detected in shrimp paste comprised 600 species belonging to 894 genera in 42 phyla, with the dominant phyla being Firmicutes and Proteobacteria, and the dominant genera being Tetragenococcus, Psychrobacter, Pseudomonas, Enterococcus, Lactobacillus and Roseovarius, and the bacterial diversity reached a peak after two years of fermentation. The types and contents of biogenic amines in shrimp paste gradually increased as fermentation progressed. Putrescine, cadaverine, histamine, tyramine, spermidine and spermidine were detected in all samples after eight years fermentation. Pearson correlation analysis showed that histamine had a significantly positive correlation with Devosia (P < 0.05), but a significantly negative correlation with Bacillus, Ralstonia, Photobacterium and Staphylococcus (P < 0.01). Putrescine had a significantly positive correlation with Ralstonia (P < 0.01), buta significantly positive correlation with Pseudomonas, Roseicyclus and Sulfitobacter (P < 0.05). Acinetobacter was positively correlated with cadaverine (P < 0.01). The study provides a theoretical basis for screening functional strains and controlling biogenic amines in shrimp paste.

Bacillus subtilis JZXJ-7 isolated from shrimp paste can significantly degrade histamine under salt stress but the mechanism is unclear. This study aims to evaluate the effect of 170 and 340 mmol/L NaCl on B. subtilis JZXJ-7 growth, histamine degradation, antioxidant enzymes (catalase (CAT), superoxide dismutase (SOD) and glutathione S-transferase (GST)) activities and Na⁺/K⁺-ATPase activity. Furthermore, comparative metabolomics was used to investigate histamine biodegradation mechanism by B. subtilis JZXJ-7 subjected to salt stress. Both 170 and 340 mmol/L NaCl promoted B. subtilis JZXJ-7 growth in late stages of reproduction (32−48 h), increased histamine degradation rate by 64.85% and 79.87% (P < 0.05), Na⁺/K⁺-ATPase activity to 6.28 (P < 0.05) and 11.63 U/mg (P < 0.01) respectively. NaCl treatment significantly increased the activities of CAT, GST and SOD (P < 0.05), amino acids and its metabolites (33.39%), benzene and substituted derivatives (12.05%), heterocyclic compounds (10.62%), organic acids and derivatives (9.75%), aldehydes, ketones, esters (5.59%) and nucleotides and its metabolites (4.58%). Metabolite set enrichment analysis revealed NaCl induced differential metabolic pathways of D-glutamine, D-glutamate, L-arginine, L-proline, histidine and glycerophospholipids, L-lysine degradation, and aminoacyl-tRNA biosynthesis. Exposure to 340 mmol/L NaCl up-regulated carbohydrate, glutathione and glycerophospholipid metabolism. The new insights into the mechanism of salt stress to promote B. subtilis JZJX-7 growth, energy metabolic pathways and to degrade histamine provide the theoretical basis for application of B. subtilis JZXJ-7 in food fermentation industry.