Quorum Sensing Regulation and Inhibition of Exoenzyme Production and Biofilm Formation in the Food Spoilage BacteriaPseudomonas psychrophilaPSPF19

Insights into the fish protein degradation induced by the fish-borne spoiler Pseudomonas psychrophila and Shewanella putrefaciens: From whole genome sequencing to quality changes

The aim of this study is evaluating the protein degradation capacity of specific spoilage organisms (SSOs) Pseudomonas psychrophila and Shewanella putrefaciens in fish flesh during chilled storage and revealing the underlying genes by whole-genome sequencing (WGS). Biochemical and physical tests were performed on fish flesh inoculated with P. psychrophila and S. putrefaciens individually, including textural properties, myofibrillar fragmentation index, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles, free amino acid composition, total volatile basic nitrogen (TVB-N), trichloroacetic acid (TCA) soluble peptides, and muscle microstructure. Results showed that P. psychrophila and S. putrefaciens exhibited a strong capacity for decomposing the fish protein, and the deterioration of fish flesh texture was primarily attributed to P. psychrophila. The genes from SSOs associated with the production of proteases were identified by whole genome sequencing and serine protease may be the primary enzyme secreted by SSOs involved in the degradation of fish protein. Therefore, the present study has shed light on the mechanisms of protein degradation induced by SSOs, thereby offering valuable insights for the development of effective quality control strategies.

Shotgun-metagenomics reveals a highly diverse and communal microbial network present in the drains of three beef-processing plants

Background

Multi-species biofilms pose a problem in various environments, especially food-processing environments. The diversity of microorganisms in these biofilms plays a critical role in their integrity and protection against external biotic and abiotic factors. Compared to single-species biofilms, mixed-species biofilms are more resistant to various stresses, including antimicrobials like sanitizers. Therefore, understanding the microbiome composition and diversity in biofilms and their metabolic potential is a priority when developing intervention techniques to combat foodborne pathogens in food processing environments.

Methods

This study aimed to describe and compare the microbiome profile of 75 drain biofilm samples obtained from five different locations (Hotscale, Hotbox, Cooler, Processing, & Grind room) of three beef-processing plants (Plant A, B & C) taken over two timepoints 2017-18 (T1) and 2021 (T2) by shotgun sequencing.

Results

Core microbiome analysis found Pseudomonas, Psychrobacter, and Acinetobacter to be the top three prevalent genera among the plants and locations. Alpha diversity analysis demonstrated a high diversity of microbiome present in all the plants and locations across the time points. Functional analysis showed the high metabolic potential of the microbial community with abundance of genes in metabolism, cell-adhesion, motility, and quorum sensing. Moreover, Quaternary Ammonium Compound (QAC) resistance genes were also observed, this is significant as QAC sanitizers are commonly used in many food processing facilities. Multi-functional genes such as transposases, polymerases, permeases, flagellar proteins, and Mobile Genetic Elements (MGEs) were found suggesting these are dynamic microbial communities that work together to protect themselves against environmental stresses through multiple defense mechanisms.

Conclusion

This study provides a framework for understanding the collective microbial network spanning a beef processing system. The results can be used to develop intervention strategies to best control these highly communicative microbial networks.

Potential of Syzygnium polyanthum as Natural Food Preservative: A Review

Food preservation is one of the strategies taken to maintain the level of public health. Oxidation activity and microbial contamination are the primary causes of food spoilage. For health reasons, people prefer natural preservatives over synthetic ones. Syzygnium polyanthum is widely spread throughout Asia and is utilized as a spice by the community. S. polyanthum has been found to be rich in phenols, hydroquinones, tannins, and flavonoids, which are potential antioxidants and antimicrobial agents. Consequently, S. polyanthum presents a tremendous opportunity as a natural preservative. This paper reviews recent articles about S. polyanthum dating back to the year 2000. This review summarizes the findings of natural compounds presented in S. polyanthum and their functional properties as antioxidants, antimicrobial agents, and natural preservatives in various types of food.

Current Advances in the Concept of Quorum Sensing-Based Prevention of Spoilage of Fish Products by Pseudomonads

Microbial spoilage of fish is attributed to quorum sensing (QS)-based activities. QS is a communication process between the cells in which microorganisms secrete and sense the specific chemicals (autoinductors, AIs) that regulate proteolysis, lipolysis, and biofilm formation. These activities change the organoleptic characteristics and reduce the safety of the products. Although the microbial community of fish is diverse and may consist of a range of bacterial strains, the deterioration of fish-based products is attributed to the growth and activity of Pseudomonas spp. This work summarizes recent advancements to assess the influence of QS mechanisms on seafood spoilage by Pseudomonas spp. The quorum sensing inhibition (QSI) in the context of fish preservation has also been discussed. Detailed recognition of this phenomenon is crucial in establishing effective strategies to prevent the premature deterioration of fish-based products.

Application of Biotechnology in Specific Spoilage Organisms of Aquatic Products

Aquatic products are delicious and have high nutritive value, however, they are highly perishable during storage due to the growth and metabolism of microorganisms. The spoilage process of aquatic products was demonstrated to be highly related to the composition of microorganisms, in which the specific spoilage organisms (SSOs) are the main factors. In this article, the spoilage indicators of SSOs were systematically described, which could make a comprehensive evaluation of the quality of aquatic products. Quorum sensing (QS) regulates the growth, metabolism and characteristics of SSOs, the common signaling molecules and the QS system in the major SSOs of aquatic products were discussed. Moreover, we compared various technologies for the analysis of SSOs in aquatic products. Besides, quality control techniques based on microbiota regulating of aquatic products, including physical, chemical and biological preservation strategies, were also compared. In conclusion, novel preservation technologies and hurdle techniques are expected to achieve comprehensive inhibition of SSOs.

Recent Advances in the Mechanisms and Regulation of QS in Dairy Spoilage by Pseudomonas spp

Food spoilage is a serious issue dramatically impacting the worldwide need to counteract food insecurity. Despite the very expensive application of low temperatures, the proper conservation of fresh dairy products is continuously threatened at different stages of production and commercialization by psychrotrophic populations mainly belonging to the Pseudomonas genus. These bacteria cause discolouration, loss of structure, and off-flavours, with fatal implications on the quality and shelf-life of products. While the effects of pseudomonad decay have been widely reported, the mechanisms responsible for the activation and regulation of spoilage pathways are still poorly explored. Recently, molecule signals and regulators involved in quorum sensing (QS), such as homoserine lactones, the luxR/luxI system, hdtS, and psoR, have been detected in spoiled products and bacterial spoiler species; this evidence suggests the role of bacterial cross talk in dairy spoilage and paves the way towards the search for novel preservation strategies based on QS inhibition. The aim of this review was to investigate the advancements achieved by the application of omic approaches in deciphering the molecular mechanisms controlled by QS systems in pseudomonads, by focusing on the regulators and metabolic pathways responsible for spoilage of fresh dairy products. In addition, due the ability of pseudomonads to quickly spread in the environment as biofilm communities, which may also include pathogenic and multidrug-resistant (MDR) species, the risk derived from the gaps in clearly defined and regulated sanitization actions is underlined.

Computation in bacterial communities

Bacteria across many scales are involved in a dynamic process of information exchange to coordinate activity and community structure within large and diverse populations. The molecular components bacteria use to communicate have been discovered and characterized, and recent efforts have begun to understand the potential for bacterial signal exchange to gather information from the environment and coordinate collective behaviors. Such computations made by bacteria to coordinate the action of a population of cells in response to information gathered by a multitude of inputs is a form of collective intelligence. These computations must be robust to fluctuations in both biological, chemical, and physical parameters as well as to operate with energetic efficiency. Given these constraints, what are the limits of computation by bacterial populations and what strategies have evolved to ensure bacterial communities efficiently work together? Here the current understanding of information exchange and collective decision making that occur in microbial populations will be reviewed. Looking toward the future, we consider how a deeper understanding of bacterial computation will inform future direction in microbiology, biotechnology, and biophysics.

Effect of quorum sensing and quorum sensing inhibitors on the expression of serine protease gene in Hafnia alvei H4

The serp gene codes for a protease that is considered to be an important factor associated with quorum sensing (QS)-based food spoilage caused by microorganisms. In this study, we evaluated the effect of temperature (4-37 °C) and QS inhibitors on the production of N-acyl-L-homoserine lactones (AHLs) and relative expression of the luxR/I, as well as serp in Hafnia alvei H4. Production of AHLs and expression of luxR/I were found to reach maximum levels at 10 °C, suggesting that the QS system of H. alvei H4 might have higher activity at low temperatures; similar result was also obtained for serp expression. Mutants of H. alvei H4 deficient in QS were used to identify the regulation of QS on serp expression. Significant reduction (P < 0.05) in serp expression was found in the mutants ∆luxR, ∆luxI, and ∆luxR/I, with ∆luxI and ∆luxR/I showing greater reduction than ∆luxR. Minimum inhibition concentrations (MIC) of Benzyl isothiocyanate and 3-Methylthiopropyl isothiocyanate for H. alvei H4 were determined to be 7.813 and 15.625 mM, respectively. Furthermore, the expression of serp, as well as that of luxR and luxI, was significantly repressed (P < 0.05) by the two QS inhibitors at 1/8 MIC and 1/16 MIC, indicating that these inhibitors might repress serp expression through affecting luxR and luxI expression in H. alvei H4. The findings of this study, therefore, suggested that food spoilage caused by H. alvei could be controlled through the application of QS inhibitors.

Tarragon essential oil as a source of bioactive compounds with anti-quorum sensing and anti-proteolytic activity against Pseudomonas spp. isolated from fish - in vitro, in silico and in situ approaches

The present study aimed to evaluate the anti-quorum sensing (anti-QS) and anti-proteolytic potentials of tarragon essential oil (TEO) and its major compounds against food-associated Pseudomonas spp. The activities were verified by in vitro, in silico and in situ approaches. In this work, methyl eugenol (ME)- and β-phellandrene (β-PH)-rich TEO was investigated. TEO at subMIC increased the percentage of saturated fatty acids in the bacterial membranes (from 7 to 22%) and exhibited anti-quorum sensing via decreasing the efficiency of QS autoinducer synthesis [3-oxo-C12-HSL (from 2.028 μg/mL to <LOD), C4-HSL (from 1.312 μg/mL to <LOD) and PQS (from 0.007625 μg/mL to <LOD)]. ME and β-PH were docked into LasR, RhlR and PqsR proteins, with docking scores comparable to native autoinductors. The subMICs of TEO, ME and β-PH decreased the proteolysis in the examined bacteria by 33, 29, and 21% (in TSB medium) and by 29, 26, and 19% (in fish juice medium), respectively. Almost all genes encoding proteases were downregulated by the applied agents. The ME- and β-PH-rich TEO acts as an anti-QS agent and significantly suppresses the proteolytic activity of food-associated pseudomonads. It might therefore increase the quality of fish-based products, where Pseudomonas spp. predominate.

A tandem GGDEF-EAL domain protein-regulated c-di-GMP signal contributes to spoilage-related activities of Shewanella baltica OS155

Cyclic diguanylate (c-di-GMP) is a second messenger involved in the regulation of various physiological processes in bacteria. However, its function in spoilage bacteria has not yet been addressed. Here, we studied the function of a tandem GGDEF-EAL domain protein, Sbal_3235, in the spoilage bacterium Shewanella baltica OS155. The deletion of sbal_3235 significantly reduced the c-di-GMP level, biofilm formation, and exopolysaccharide, trimethylamine (TMA), and putrescine production; sbal_3235 deletion also downregulated the expression of the torS and speF genes and affected membrane fatty acid composition. Site-directed mutagenesis in conserved GGDEF and EAL motifs abolished diguanylate cyclase (DGC) and phosphodiesterase (PDE) activity, respectively. These data indicate that Sbal_3235 is an essential contributor to the c-di-GMP pool with bifunctional DGC and PDE activity, which is involved in the biofilm formation and spoilage activity of S. baltica OS155. Our findings expand the biochemical role of c-di-GMP and uncover its link to spoilage activities, providing novel targets for food quality and safety controlling.

Antibiotic Resistant Pseudomonas Spp. Spoilers in Fresh Dairy Products: An Underestimated Risk and the Control Strategies

Microbial multidrug resistance (MDR) is a growing threat to public health mostly because it makes the fight against microorganisms that cause lethal infections ever less effective. Thus, the surveillance on MDR microorganisms has recently been strengthened, taking into account the control of antibiotic abuse as well as the mechanisms underlying the transfer of antibiotic genes (ARGs) among microbiota naturally occurring in the environment. Indeed, ARGs are not only confined to pathogenic bacteria, whose diffusion in the clinical field has aroused serious concerns, but are widespread in saprophytic bacterial communities such as those dominating the food industry. In particular, fresh dairy products can be considered a reservoir of Pseudomonas spp. resistome, potentially transmittable to consumers. Milk and fresh dairy cheeses products represent one of a few "hubs" where commensal or opportunistic pseudomonads frequently cohabit together with food microbiota and hazard pathogens even across their manufacturing processes. Pseudomonas spp., widely studied for food spoilage effects, are instead underestimated for their possible impact on human health. Recent evidences have highlighted that non-pathogenic pseudomonads strains (P. fluorescens, P. putida) are associated with some human diseases, but are still poorly considered in comparison to the pathogen P. aeruginosa. In addition, the presence of ARGs, that can be acquired and transmitted by horizontal genetic transfer, further increases their risk and the need to be deeper investigated. Therefore, this review, starting from the general aspects related to the physiological traits of these spoilage microorganisms from fresh dairy products, aims to shed light on the resistome of cheese-related pseudomonads and their genomic background, current methods and advances in the prediction tools for MDR detection based on genomic sequences, possible implications for human health, and the affordable strategies to counteract MDR spread.

Biochemical changes induced by dominant bacteria in chill-stored silver carp (Hypophthalmichthys molitrix) and GC-IMS identification of volatile organic compounds

To evaluate the spoilage potential of dominant bacteria (Aeromonas allosaccharophila, Pseudomonas psychrophila, and Shewanella putrefaciens) isolated from spoiled silver carp (Hypophthalmichthys molitrix) fillets, biochemical changes including protein degradation, trichloroacetic acid (TCA)-soluble peptides, total volatile basic nitrogen (TVB-N), biogenic amines, nucleotide catabolism, and volatile organic compounds were examined in single-species inoculated silver carp flesh for 14 days at 4 °C. P. psychrophila exhibited the strongest proteolytic activity, which resulted in the highest concentrations of TCA-soluble peptides and TVB-N. S. putrefaciens was responsible for the production of putrescine and cadaverine and led to the fastest degradation of hypoxanthine riboside (HxR). At the end of storage, P. psychrophila was the main producer of ketones, especially the C7-C9 ketones, while sulfur compounds were released primarily by S. putrefaciens. Moreover, 1-propanol, butanone, 2-hexanone, methyl isobutyl ketone, dimethyl sulfide, and dimethyl disulfide increased gradually with storage time, suggesting their potential as spoilage markers for freshness/spoilage monitoring. P. psychrophila possessed the strongest spoilage potential in the fish matrix, followed by S. putrefaciens, whereas A. allosaccharophila showed a very low spoilage potential. In conclusion, P. psychrophila and S. putrefaciens were identified as the specific spoilage organisms (SSOs) of silver carp, suggesting that preservation researchers should focus on these two spoilage contributors in future studies. This research contributes to a deeper understanding of silver carp spoilage and to the development of methods and tools to improve fish quality management.

Inhibition of Biogenic Amines in Shewanella baltica by Anthocyanins Involving a Quorum Sensing System

HIGHLIGHTS

Put was the main biogenic amine produced by Shewanella baltica. PL and PP were the main QS autoinducers of S. baltica. PL and PP displayed significant positive correlation with Put. DKPs and Put production and odc gene expression were inhibited by anthocyanins. Anthocyanins were proposed as new QSIs and seafood preservative candidates.

Positive Regulation of Spoilage Potential and Biofilm Formation in Shewanella baltica OS155 via Quorum Sensing System Composed of DKP and Orphan LuxRs

The spoilage potential and biofilm formation of Shewanella baltica are reported to be regulated by Quorum sensing (QS) system from the phenotype point of view, but the specific mechanism is not fully understood. In the present study, the QS autoinducers were detected by UHPLC-MS/MS, cell density-dependent luxR-type genes were obtained through autoregulation experiments among a series of candidates in S. baltica OS155 (The SSO of large yellow croaker). The direct interaction between cyclo-(L-Pro-L-Phe) (PP) and LuxR01 as well as LuxR02 proteins was revealed via in vitro binding assay. Deletion of luxR-type genes (luxR01 and luxR02) impaired spoilage potential and biofilm formation of S. baltica OS155 in various degrees. Transcriptional analysis and qRT-PCR validation showed that spoilage and biofilm-related genes torS, speF, and pomA were down-regulated in luxR01 and luxR02 deletion strains. In addition, exogenous PP promoted spoilage potential and biofilm formation, which could be attenuated by luxR01 or luxR02 deletion. Our results revealed an explicit QS system employing PP as autoinducer and two orphan LuxRs as receptors which positively regulated spoilage capacity and biofilm formation via transcriptional regulation of corresponding genes in S. baltica OS155, which provides potential specific targets for seafood preservation involving QS system.

Characterization of LuxI/LuxR and their regulation involved in biofilm formation and stress resistance in fish spoilers Pseudomonas fluorescens

Quorum sensing (QS) is crucial for adaption and development of foodborne bacteria in diverse environments. Pseudomonas fluorescens PF07 with QS mediated acylated homoserine lactones (AHLs) activity was isolated from spoiled large yellow croaker (Pseudosciaena crocea). In this study AHL-mediated QS system was characterized and their roles in biofilm formation, motility, stress response and spoilage of P. fluorescens were evaluated. A LuxI/LuxR homolog consisting of a conserved AHL synthase gene (luxI) and a transcriptional regulator gene (luxR) was identified in the strain. Two in-frame deletion mutants of luxI and luxR, ∆luxI and ∆luxR, were constructed to explore their QS signaling function in P. fluorescens. Three types of AHLs were detected in PF07 culture by LC-MS/MS, and N-butanoyl-l-homoserine lactone (C₄-HSL) was a major signal molecule. The C₄-HSL activities was almost abolished in ∆luxI, and decreased greatly in ∆luxR. Compared with wild type (WT) strain, both ∆luxI and ∆luxR showed the significant decrease of biofilm biomass and expolysaccharide production, resulting in thinner and incompact biofilm structure, but promoted swimming motility. The resistance of P. fluorescens to H₂O₂, heat, NaCl and crystal violet apparently declined in two mutants compared to WT. Spoilage factors, siderophore and protease, apparently attenuated due to deletion of luxI or luxR gene, while the growth and TVB-N production did not differ. Furthermore, the changes of the biofilm formation, motility and protease in ∆luxI strain were partially restored by the exogenous C₄-HSL. In agreement with the effect of two mutants on various phenotypes, the transcriptions of alg, lapA, flgA, rpoS, and aprX were significantly down-regulated, and flgA was up-regulated in ∆luxI and ∆luxR. Therefore, the present study highlighted that the co-operation of LuxI/LuxR homolog was an important QS regulator in biofilm formation, motility and spoilage potential, and hinted its positive regulation of stress resistance with RpoS in P. fluorescens.

Insights into Bacterial Milk Spoilage with Particular Emphasis on the Roles of Heat-Stable Enzymes, Biofilms, and Quorum Sensing

Milk spoilage caused by psychrotrophic bacteria and their heat-stable enzymes is a serious challenge for the dairy industry. In many studies, spoilage has been explored based on the simplistic view of undesirable enzymes produced by planktonic cells. Recently, biofilms and quorum sensing (QS) have been suggested as important factors in the deterioration of milk, which opens new avenues for investigation of the processes and challenges. Production and heat stability of enzymes are enhanced in biofilms, mainly because of inherent differences in physiological states and protective shielding by extracellular polymeric substances. QS plays a key role in modulating expression of hydrolytic enzymes and biofilm formation. To date, few studies have been conducted to investigate the complex interplays of enzyme production, biofilm formation, and QS. This review provides novel insights into milk spoilage with particular emphasis on the roles of biofilms and QS and summarizes potential effective strategies for controlling the spoilage of milk.

Changes in Quality and Microbial Succession of Lightly Salted and Sugar-Salted Blunt Snout Bream ( Megalobrama amblycephala) Fillets Stored at 4°C

The effect of a low concentration of salt and sugar on the quality and microbial succession in blunt snout bream ( Megalobrama amblycephala) fillets was assessed by sensory analysis, total volatile basic nitrogen, biogenic amines, K value, total viable counts, 16S rRNA gene analysis, and Illumina MiSeq PE300 high-throughput sequencing. Fish samples were left untreated (control), treated with 1.8% salt (T1), or treated with 1.8% salt plus 0.9% sugar (T2). Consequently, salted and sugar-salted treatments extended the shelf life of bream fillets by 2 days, which retarded the increase of total volatile basic nitrogen, putrescine, cadaverine, and total viable counts. The putrescine and cadaverine concentrations of T2 were significantly ( P < 0.05) higher than T1 after day 10. Brachybacterium was the major initial microbiota of bream fillets. As storage time progressed, Pseudomonas and Shewanella were major genera in the spoiled control group. Pseudomonas, Shewanella, and Pseudoalteromonas became the main spoilers in the T1 and T2 groups.

Characterization of the microbiota in lightly salted bighead carp (Aristichthys nobilis) fillets stored at 4 °C

The microbiota of unsalted and salted (dry-cured with 2% salt) bighead carp (Aristichthys nobilis) fillets during storage at 4 °C were identified by 16S rRNA gene analysis. Eleven genera were present in the initial microbiota of bighead carp fillets, where Acinetobacter, Aeromonas and Kocuria were the dominant bacteria. As storage time progressed, the microbial composition of both unsalted and salted fillets became less diverse. Additionally, differences in microbiota were observed between these two treatments. For unsalted bighead carp fillets, Aeromonas became the dominant genus at the end of storage and Pseudomonas was found less commonly. For salted fillets, Pseudomonas was the only bacteria identified at the end of storage.

Inhibition of biofilm development and spoilage potential of Shewanella baltica by quorum sensing signal in cell-free supernatant from Pseudomonas fluorescens

The objective of this study was to in vitro evaluate the effect of a cell-free supernatant (CFS) containing quorum sensing (QS) signal of Pseudomonas fluorescens on the growth, biofilm development and spoilage potential of Shewanella baltica, and preliminarily assess the interactive influences of various chemically synthesized autoinducers on spoilage phenotypes of S. baltica. PF01 strain isolated from spoiled Pseudosciaen crocea was identified P. fluorescens. The addition of 25% and 50% CFS to S. baltica culture had no effect on the growth rate during the lag and exponential phase, however, caused cell decline during the stationary phase. The presence of CFS from P. fluorescens significantly inhibited biofilm development, and greatly decreased the production of trimethylamine (TMA) and biogenic amino in S. baltica. Various signal molecules of QS in the CFS of P. fluorescens culture were detected, including seven N-acyl-l-homoserine lactones (AHLs), autoinducer-2 (AI-2) and two diketopiperazines (DKPs). Exogenous supplement of synthesized seven AHLs containing in the CFS decreased biofilm formation and TMA production in S. baltica, while exposure to exogenous cyclo-(l-Pro-l-Leu) was showed to promote spoilage potential, which revealed that S. baltica also sense the two QS molecules. Furthermore, the stimulating effect of cyclo-(l-Pro-l-Leu) was affected when AHL was simultaneously added, suggesting that the inhibitory activity of spoilage phenotypes in S. baltica might be attributed to a competitive effect of these QS compounds in the CFS of P. fluorescens. The present studies provide a good basis for future research on the role of QS in the regulation of spoilage microbial flora.