Quorum Sensing Involved in the Spoilage Process of the Skin and Flesh of Vacuum-Packaged Farmed Turbot (Scophthalmus maximus) Stored at 4 °C

Role of RpoS in stress resistance, biofilm formation and quorum sensing of Shewanella baltica

Shewanella baltica is one of the most important bacterial species contributing to spoilage of seafood. Principally, RpoS has been recognized as the central regulator of stress resistance in many bacterial species. However, little is known about the role of RpoS in S. baltica. In this study, an rpoS mutant of S. baltica was constructed and analysed for its functions. The results showed that the survival rate of rpoS mutant decreased when treated with heat, ethanol and H₂ O_2, while increased the resistance to NaCl. Moreover RpoS promoted the biofilm formation of S. baltica at 30°C, while declined at 4°C. Interestingly, the rpoS-deficient mutant showed increased swimming motility. Furthermore, the results revealed that the production of quorum-sensing (QS) signals such as cyclo-(l-Pro-l-Leu) and cyclo-(l-Pro-l-Phe) reduced in rpoS mutant. Mainly, rpoS positively regulated QS response regulators, as the expression of all luxR genes in rpoS mutant significantly decreased relative to wild type. This study reveals that RpoS is a major regulator involved in stress responses, biofilm formation and quorum sensing system in S. baltica. The present work provides significant information for the control of microbiological spoilage of seafood.

High-CO2 Modified Atmosphere Packaging with Superchilling (-1.3 °C) Inhibit Biochemical and Flavor Changes in Turbot (Scophthalmus maximus) during Storage

The effects of modified atmosphere packaging (MAP) in combination with superchilling (−1.3 °C) on the physicochemical properties, flavor retention, and organoleptic evaluation of turbot samples were investigated during 27 days storage. Results showed that high-CO₂ packaging (70% or 60% CO₂) combined with superchilling could reduce the productions of off-flavor compounds, including total volatile basic nitrogen (TVB-N) and ATP-related compounds. Twenty-four volatile organic compounds were determined by gas chromatography–mass spectrometry (GC/MS) during storage, including eight alcohols, 11 aldehydes, and five ketones. The relative content of off-odor volatiles, such as 1-octen-3-ol, 1-penten-3-ol, (E)-2-octenal, octanal, and 2,3-octanedione, was also reduced by high-CO₂ packaging during superchilling storage. Further, 60% CO₂/10% O₂/30% N₂ with superchilling (−1.3 °C) could retard the water migration on the basis of the water holding capacity, low field NMR, and MRI results, and maintain the quality of turbot according to organoleptic evaluation results during storage

Involvement of Exogenous N-Acyl-Homoserine Lactones in Spoilage Potential of Pseudomonas fluorescens Isolated From Refrigerated Turbot

Some bacteria can modulate their spoilage potential by responding to environmental signaling molecules via the quorum sensing (QS) system. However, the ability of Pseudomonas fluorescens, the specific spoilage organism (SSO) of turbot, to response to environmental signaling molecules remains unclear. This study investigated the effects of six synthetic N-acyl homoserine lactones (AHLs) on typical behaviors mediated by QS in P. fluorescens, such as biofilm formation and extracellular protease activity. Total volatile basic nitrogen (TVB-N) was used as a spoilage indicator to evaluate quality changes in AHL-treated turbot filets during storage. The results confirm the enhancing effect of environmental AHLs on QS-dependent factors of P. fluorescens and quality deterioration of turbot filets, with C₄-HSL and C₁₄-HSL being the most effective. Moreover, the content decrease of exogenous AHLs was also validated by gas chromatography–mass spectrometry analysis. Further, changes in rhlR transcription levels in P. fluorescens suggest that this bacterium can sense environmental AHLs. Finally, molecular docking analysis demonstrates the potential interactions of RhlR protein with various exogenous AHLs. These findings strongly implicate environmental AHLs in turbot spoilage caused by P. fluorescens, suggesting preservation of turbot should not exclusively consider the elimination of SSO-secreted AHLs.

Characterization of Volatile Compounds by SPME-GC-MS During the Ripening of Kedong Sufu, a Typical Chinese Traditional Bacteria-Fermented Soybean Product

The purpose of the present study was to evaluate the volatile profile of Kedong sufu, which is a typical bacteria-fermented soybean product in China, using solid phase microextraction coupled to gas chromatography and mass spectrometry and to reveal the evolution and diversity of flavor substances for this specialty. A total of 75 compounds were identified, including 35 esters, 4 alcohols, 4 phenols, 4 aldehydes, 7 acids, 10 ketones, and 11 other compounds from sufu samples during ripening. Some volatile compounds increased with ripening time, especially hexadecenoic acid ethyl ester, methoxy acetic acid pentyl ester, benzene propanoic acid ethyl ester, ethyl 9-hexadecenoate, ethyl oleate, ethanol, 3-methyl-1-butanol, 5-methoxy-1-pentanol, and eugenol; these compounds enriched the flavors and provided the typical savory taste of Kedong sufu. PRACTICAL APPLICATION: This research elucidated the formation of flavor substances in sufu. For traditional fermented foods, this study provides a scientific basis for promoting the generation of typical flavor substances and for the precise determination of maturity time.

Spoilage microbial community profiling by 16S rRNA amplicon sequencing of modified atmosphere packaged live mussels stored at 4oC

There is little information on the microbial communities associated with modified atmosphere (MA)-packaged live mussels. There is also a dearth of information on how pre-packaging depuration modifies the microbial communities and spoilage of live mussels. Amplicon sequencing was used to describe spoilage microbial succession in MA-packaged live mussels during storage at 4 °C. Proteobacteria, Cyanobacteria and Firmicutes were the three major phyla observed in the mussel meat and pouch water of undepurated and depurated mussels. Among these phyla, Cyanobacteria was more predominant on day 0 in mussel meat of undepurated and depurated mussels while Proteobacteria was predominant in commercially-depurated mussels. Synechococcus was apparently dominant on days 0-7 in the meat of undepurated mussels and days 0-10 in depurated mussels. Shewanella was dominant on day 0 in commercially-depurated mussels and dominant on day 15 in undepurated while Acidaminococcus was dominant in depurated mussels on day 15. Psychromonas was observed to be dominant in commercially-depurated mussels on day 7 and further shifted to Acinetobacter by day 10 and 15. In the pouch water, Acinetobacter was dominant throughout the storage days in undepurated mussels while Psychrobacter was predominant in both depurated and commercially-depurated mussels. This study demonstrated the impact of depuration on the microbiota and the spoilage mechanism of MA-packaged live mussels. Shewanella was easily removed through depuration. However, spoilage bacteria such as Acidaminococcus could not be easily removed although they are not important at the beginning but grew at the end. Pouch water contributed suitable biological medium for the growth of Acinetobacter and Psychrobacter and both enhanced the growth of spoilage bacteria such as Shewanella and Acidaminococcus.

Acyl-homoserine-lactones receptor LuxR of Shewanella baltica involved in the development of microbiota and spoilage of refrigerated shrimp

Numerous bacterial species utilize quorum sensing molecules acyl-homoserine-lactones (AHLs) to communicate, however, crosstalk often complicates the dynamics and behaviors of mixed populations. In this study, we developed a luxR mutant of wild type Shewanella baltica SA03 (WT SA03), and aimed to investigate the role of S. baltica LuxR (AHLs receptor) involved in the spoilage of refrigerated shrimp (Litopenaeus vannamei) by inoculating WT SA03 and luxR mutant of S. baltica SA03 (ΔluxR SA03), respectively. The results indicated the maximum growth rate of total viable bacteria in shrimp inoculated with ΔluxR SA03 was 73.34% lower than that of WT SA03. The lag time of total bacteria in shrimp treated with ΔluxR SA03 were 87.6 h, significantly longer than that of WT SA03. Meanwhile, the total volatile basic nitrogen concentrations of shrimp treated with WT SA03 were significantly higher than that of ΔluxR SA03 after 2 days of storage, which were in agreement with the decrease of the content of AHLs of the shrimp. The results indicated S. baltica might utilize AHLs produced by other bacteria and accelerate the shrimp spoilage process through LuxR receptor system.

Spoilage of refrigerated Litopenaeus vannamei: eavesdropping on Acinetobacter acyl-homoserine lactones promotes the spoilage potential of Shewanella baltica

Shewanella baltica and Acinetobacter are among the predominant spoilage bacteria in refrigerated shrimp (Litopenaeus vannamei). S. baltica are incapable of producing acyl-homoserine lactone (AHL) quorum sensing signals, but can respond to environmental AHLs. In this paper, Acinetobacter was found to produce three AHLs, i.e. N-butanoyl-l-homoserine lactone (C4-HSL), N-(3-oxohexanoyl)-l-homoserine lactone (O-C6-HSL) and N-(3-oxooctanoyl)-l-homoserine lactone (O-C8-HSL), according to thin-layer chromatography using the bioreporter Agrobacterium tumefaciens A136. The agar diffusion and β-galactosidase assays revealed that S. baltica could eavesdrop on these three AHLs from Acinetobacter. Eavesdropping on Acinetobacter AHLs especially C4-HSL was found to boost the growth of S. baltica particularly under nutrient limiting conditions (up to 40-fold increase) in the co-culture experiments. The azocasein assay revealed that S. baltica produced fourfold more extracellular proteases in response to Acinetobacter AHLs. As demonstrated by the biofilm crystal violet staining assay and confocal laser scanning microscopy, eavesdropping also decreased the biofilm-forming capacity of Acinetobacter. By inoculation of S. baltica and Acinetobacter onto surface-sterilized shrimp, eavesdropping was found to endow a growth advantage to S. baltica in vivo, resulting in a 0.5 day shortened shelf life of shrimp according to total volatile basic nitrogen levels and sensory analysis. Overall, the AHL-dependent eavesdropping increased the spoilage potential of S. baltica, providing a fresh perspective on the spoilage process of refrigerated L. vannamei, and this may inspire the development of novel preservation techniques in the future to further reduce post-harvest loss of shrimp.