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

Microbial dynamics and Pseudomonas natural product production in milk and dairy products

Covering: 2000 up to the first half of 2024Milk and its derived dairy products have long been integral to the human diet, with evidence of consumption dating back over 9000 years. Milk's high nutritional value renders dairy products an important element of human diet while also offering a fertile environment for microbial growth. Beneficial microorganisms in dairy products are often associated with biogenic and probiotic effects, whereas spoilage or pathogenic microorganisms can pose health risks. Fermentation is a key method to preserve milk. Whereas dairying practices in most parts of the world have been highly altered by industrialization over the past century, nomadic pastoralists in Mongolia notably retain a rich tradition of household-level dairy fermentation that has been practiced since 3000 BC. Milk-associated microorganisms produce a vast number of low molecular weight natural products that can mediate beneficial and detrimental interactions. Bacteria of the genus Pseudomonas are found in traditional Mongolian dairy products and are common contaminants in commercial dairy products, and they can strongly impact the quality and shelf-life of dairy products. These bacteria are well known for their ability to produce a variety of secondary metabolites, including nonribosomal (lipo)peptides, which are both structurally and functionally diverse. Lipopeptides can have antimicrobial properties, act as quorum sensing molecules, and contribute to biofilm formation due to their amphiphilic nature. Although often associated with spoilage, some of these natural products can also exhibit positive effects with potential beneficial applications in the dairy industry. This review aims to provide a comprehensive overview of the interplay between culinary fermentation and the production and activities of microbial-derived natural products.

Diversity and spoilage potential of Pseudomonas spp. from Spanish milk and dairy products: Impact on fresh cheese and milk quality

Psychrotrophic Pseudomonas spp. contaminate milk and dairy products, producing heat-resistant enzymes and pigments that reduce their shelf life. Identifying spoiling strains is essential for tracing contamination and improving preservation. This study examines 208 Pseudomonas spp. isolates from Spanish dairy products, assessing their genetic diversity and spoilage potential in vitro and under simulated storage. Pulsed-field gel electrophoresis (PFGE) identified 108 distinct strains, clustered into 70 groups (≥ 80 % similarity), showing high diversity. Gene sequencing (ileS or rpoD) classified the strains into 20 species, with P. fluorescens (19 %), P. fragi (16 %), P. lundensis (12 %), and P. shahriarae (6 %) being predominant. P. shahriarae, P. atacamensis, P. salmasensis, P. solani, and P. canadensis were isolated from milk or dairy products for the first time. In fresh cheese, 89 % of Pseudomonas spp. caused discoloration (fluorescent yellow, creamy, orange, and blue) after 7 days of cold storage. In refrigerated milk, 48 % of strains showed significant proteolysis after 5 days, with OPA (o-Phthaldialdehyde-based method) values > 0.274, especially some P. fluorescens, P. gessardii, P. fulva, P. shahriarae, Pseudomonas spp., and P. koreensis strains. After simulated UHT treatment and accelerated storage, 75 % of strains retained thermostable proteolytic activity, especially P. fluorescens, P. proteolytica, P. shahriarae, and P. fulva strains. The aprX gene, coding for a thermostable protease, was present in 57 % of strains, suggesting other proteases may also be produced. Overall, the isolated Pseudomonas spp. led to different spoilage patterns during milk and fresh cheese storage, emphasizing the need for specific strain identification to improve preservation strategies.

Study of the release kinetics of Ethyl Lauroyl Arginate from poly(3-hydroxybutyrate-co-3-hydroxyvalerate) active films

This study investigates the underexplored area of the release mechanism and kinetics of the antimicrobial Ethyl Lauroyl Arginate (LAE®) from an innovative active packaging system based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). We evaluated the impact of food simulants and temperatures on LAE® release, diffusion, and partition coefficients. Mathematical modeling was used to elucidate LAE® release kinetics, offering understanding of the release behaviour in food matrices. Results highlighted that temperature notably affected LAE® release into simulant A (10% EtOH) unlike the release into simulant D1 (50% EtOH). Although the release was faster in the less polar simulant, a greater partition coefficient demonstrated greater LAE® retention within the polymer matrix at equilibrium. Weibull models ensured robust fits, suggesting their usefulness for future studies on LAE® release kinetic. Finally, the active films were validated in food, showing significant reduction in microbial counts. These findings contribute to the design of effective antimicrobial food packaging and the selection of suitable food applications.

Optimizing pectin lyase production using the one-factor-at-a-time method and response surface methodology

Pectinases are commonly industrially synthesized by molds. This study aimed to optimize pectin lyase synthesis by a bacterium, Pseudomonas fluorescens, using both the one-factor-at-a-time (OFAT) method and response surface methodology. First, on optimization of pectin lyase fermentation by the OFAT method, the effects of pectin, peptone, yeast extract, (NH4)2SO4, pH, and salts were investigated. The highest pectin lyase activity was determined to be 28.63 U/mL at pH 8, 30°C, with 1% (w/v) pectin and 0.14% (w/v) (NH4)2SO4 concentration at the 90th hour. The effect of substrate inhibition on the microbial growth was also investigated, and the results showed that the process can be described by noncompetitive inhibition model. The values of kinetic constants were determined as µm = 0.175 h-1, KS = 6.931 g/L, and, KI = 6.932 g/L by nonlinear regression analysis. It was reported that pectin lyase enzymes exhibited peak activity at 50°C and pH 8. Finally, response surface methodology (RSM) was utilized to optimize pH, concentrations of ammonium sulfate, and pectin, which were chosen as independent variables. The interactions between these variables were also examined. According to RSM, the optimum values of the parameters to achieve a maximum pectin lyase activity of 35.62 U/mL were determined to be pH 7.97, 1.25% (w/v) pectin concentration, and 0.25% (w/v) (NH4)2SO4 concentration.

Spoilage microbial groups in dairy industry

Milk and dairy products can be spoiled by microorganisms from the raw milk microbiota as well as resident microorganisms in dairy industries, with some being related to more than one type of spoilage due to their metabolic versatility. Different types of spoilage have harmed dairy production, including milk destabilization by enzymatic activity, cheese blowing, discolorations, acidification, off-flavors production, slime formation, and ropiness. Generally, the compounds most associated with these problems are enzymes, secondary metabolites, biogenic amines, lactic acid, volatile compounds, polysaccharides, among others. Additionally, many microorganisms that are beneficial in some dairy products also have the potential to cause spoilage. Here, the main groups of microorganisms associated with the spoilage of milk and dairy products are described, and measures for their control and prevention are presented.

Evaluating the efficacy of a phage cocktail against Pseudomonas fluorescens group strains in raw milk: microbiological, physical, and chemical analyses

The objective of this study was to investigate the effectiveness of a phage cocktail against Pseudomonas fluorescens group and its effect on the microbial, physical and chemical properties of raw milk during different storage conditions. A phage cocktail consisting of Pseudomonas fluorescens, Pseudomonas tolaasii, and Pseudomonas libanensis phages was prepared. As a result, reductions in fluorescent Pseudomonas counts of up to 3.44 log units for the storage at 4 °C and 2.38 log units for the storage at 25 °C were achieved. Following the phage application, it is found that there was no significant difference in the total mesophilic aerobic bacteria and Enterobacteriaceae counts. However, it was observed that the number of lactic acid bacteria was higher in phage-treated groups. The results also showed that pH values in the phage added groups were lower than the others and the highest titratable acidity was obtained only in the bacteria-inoculated group. As a future perspective, this study suggests that, while keeping the number of target microorganisms under control in the milk with the use of phages during storage, the microbiota and accordingly the quality parameters of the milk can be affected. This work contributes to the development of effective strategies for maintaining the quality and extending the shelf life of milk and dairy products.

An Evaluation of the Sensitivity and Applicability of a Droplet Digital Polymerase Chain Reaction Assay to Simultaneously Detect Pseudomonas aeruginosa and Pseudomonas fragi in Foods

Achieving effective control over microbial contamination necessitates the precise and concurrent identification of numerous pathogens. As a common bacterium in the environment, Pseudomonas is rich in variety. It not only has pathogenic strains, but also spoilage bacteria that cause food spoilage. In this research, we devised a remarkably sensitive duplex droplet digital PCR (dddPCR) reaction system to simultaneously detect pathogenic Pseudomonas aeruginosa (P. aeruginosa) and spoilage Pseudomonas fragi (P. fragi). By employing comparative genomics, we identified four genes of P. fragi. Through a specific analysis, the RS22680 gene was selected as the detection target for P. fragi, and the lasR gene was chosen for P. aeruginosa, which were applied to construct a dddPCR reaction. In terms of specificity, sensitivity and anti-interference ability, the constructed dddPCR detection system was verified and analyzed. The assay showed excellent sensitivity and applicability, as evidenced by a limit of detection of 100 cfu/mL. When the concentration of natural background bacteria in milk or fresh meat was 100 times that of the target detection bacteria, the method was still capable of completing the absolute quantification. In the simulation of actual sample contamination, P. aeruginosa could be detected after 3 h of enrichment culture, and P. fragi could be detected after 6 h. The established dddPCR detection system exhibits exceptional performance, serving as a foundation for the simultaneous detection of various pathogenic bacteria in food products.

Emerging trends in the inhibition of bacterial molecular communication: An overview

Quorum sensing (QS) is a molecular cell-cell communication utilized by several bacteria and some fungi. It involves cell density dependent gene expression that includes extra polymeric substance production, sporulation, antibiotic production, motility, competence, symbiosis and conjugation. These expressions were carried out by different signaling molecules like acyl homo-serine lactone (AHL) and auto-inducing peptides (AIPs) which was effluxed by gram negative and gram positive bacteria. Pathogenic bacteria and biofilms often exhibit high resistance to antibiotics, attributed to the presence of antibiotic efflux pumps, reduced membrane permeability, and enzymes that deactivate quorum sensing (QS) inhibitors. To counteract virulence and multi-drug resistance (MDR), novel strategies such as employing quorum sensing (QS) inhibitors and quorum quenchers are employed. It targets signaling molecules with synthesis and prevents the signal from binding to receptors. In this present review, the mechanisms of QS along with inhibitors from different sources are described. These strategies potentially interfere with QS and it can be applied in different fields, mainly in hospitals and marine environments where the pathogenic infections and biofilm formation are highly involved.

Dynamic Changes in the Microbial Composition and Spoilage Characteristics of Refrigerated Large Yellow Croaker (Larimichthys crocea) during Storage

The quality changes, dynamic changes in microbial composition, and diversity changes in large yellow croaker (Larimichthys crocea) during 4 °C refrigeration were studied using 16S rDNA high-throughput sequencing technology, and the total viable count (TVC), total volatile basic nitrogen (TVB-N), and thiobarbituric acid-reactive substances (TBARS) were determined. The results revealed a consistent increase in TVC, TVB-N, and TBARS levels over time. On the 9th day, TVC reached 7.43 lg/(CFU/g), while on the 15th day, TVB-N exceeded the upper limit for acceptable quality, reaching 42.56 mg/100 g. Based on the 16S rDNA sequencing results, we categorized the storage period into three phases: early storage (0th and 3rd days), middle storage (6th day), and late storage (9th, 12th, and 15th days). As the storage time increased, both the species richness and diversity exhibited a declining trend. The dominant genus identified among the spoilage bacteria in refrigerated large yellow croaker was Pseudomonas, accounting for a high relative abundance of 82.33%. A comparison was carried out of the spoilage-causing ability of three strains of Pseudomonas screened and isolated from the fish at the end of storage, and they were ranked as follows, from strongest to weakest: P. fluorescen, P. lundensis, and P. psychrophila. This study will provide a theoretical basis for extending the shelf life of large yellow croaker.