Identification of Pseudomonas jessenii and Pseudomonas gessardii as the most proteolytic Pseudomonas isolates in Iranian raw milk and their impact on stability of sterilized milk during storage

Isolation, Identification, and Fermentation Optimization of Phytase-Producing Bacteria and Their Effects on Soybean Seedlings

Phosphorus in soil mostly exists in complex compounds such as phytic acid, which reduces the effectiveness of phosphorus and limits agricultural production. Phytase has the activity of hydrolyzing phytate into phosphate. The mineralization of phytate in soil by phytase secreted by microorganisms is an effective way to improve the utilization rate of phytate. This study isolated a high-yield phytase strain, identified as Pseudomonas by 16S rDNA and named Pseudomonas sp. S3-10. The fermentation medium composition and conditions were optimized using the single-factor method, Plackett-Burman design (PBD), and response surface methodology (RSM). The results showed that cane molasses, MgCl2, and temperature significantly affected the fermentation biomass of the bacterium. The optimal fermentation conditions were cane molasses and MgCl2 concentrations of 61.80 g/L and 5.94 g/L, respectively, at 34.4 °C. Compared with the unoptimized fermentation conditions, the maximum biomass increased by 160.17 ± 6.26% under the optimized fermentation conditions, reaching 9.13 ± 0.09 × 109 CFU/mL. The pot experiment results showed that Pseudomonas sp. S3-10 has a significant promoting effect on soybean growth. The strain increased the fresh weight and length of soybean seedlings by 112.92 ± 28.41% and 74.02 ± 3.24%, respectively, and increased the phytase activity in the soil and available phosphorus concentration in the plant rhizosphere by 388.15 ± 24.24% and 365.05 ± 91.96%, respectively. This study provided a high-yield phytase strain and its optimal fermentation conditions. The bacterium has significant plant growth-promoting effects and can be used as a new type of biological fertilizer, which is of great significance for reducing phosphorus fertilizer usage, improving phosphorus utilization efficiency, and protecting the ecological environment in agricultural production.

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.

Evaluation of UHT milk spoilage caused by proteases from psychrophilic bacteria based on peptidomics

The quality issues of ultra-high-temperature (UHT) milk, such as protein hydrolysis and aging gels throughout shelf life, are caused by proteases from psychrophilic bacteria. However, existing enzyme activity detection techniques have low sensitivity and cannot accomplish the detection of product deterioration caused by low enzyme activity. In this study, an attempt was made to analyze the relationship between enzymatically cleaved peptides and product quality using peptidomics techniques. The impact of psychrophilic bacteria proteases on the quality of UHT milk was investigated based on peptidomics by exogenously adding proteases. The results indicated that the protease activity and protein hydrolysis increased significantly in UHT milk over the storage period. Through peptidomic analysis, 2479 peptides were identified, in which 32 proteins linked to the identified peptides. 17 potential marker peptides, including αS 1 -casein143-156, β-lactoglobulin99-108, and β-casein39-66, were screened. The correlation of all identified peptides with protease activity and protein hydrolysis was explored by the weighted gene co-expression network analysis (WGCNA) method. The peptidomics technology helps to study the release or degradation of peptides in UHT milk during storage, which can be applied to monitor the shelf life of UHT milk and predict spoilage.

The First Pseudomonas Phage vB_PseuGesM_254 Active against Proteolytic Pseudomonas gessardii Strains

Bacteria of the Pseudomonas genus, including the Pseudomonas gessardii subgroup, play an important role in the environmental microbial communities. Psychrotolerant isolates of P. gessardii can produce thermostable proteases and lipases. When contaminating refrigerated raw milk, these bacteria spoil it by producing enzymes resistant to pasteurization. One possible way to prevent spoilage of raw milk is to use Pseudomonas lytic phages specific to undesirable P. gessardii isolates. The first phage, Pseudomonas vB_PseuGesM_254, was isolated and characterized, which is active against several proteolytic P. gessardii strains. This lytic myophage can infect and lyse its host strain at 24 °C and at low temperature (8 °C); so, it has the potential to prevent contamination of raw milk. The vB_PseuGesM_254 genome, 95,072 bp, shows a low level of intergenomic similarity with the genomes of known phages. Comparative proteomic ViPTree analysis indicated that vB_PseuGesM_254 is associated with a large group of Pseudomonas phages that are members of the Skurskavirinae and Gorskivirinae subfamilies and the Nankokuvirus genus. The alignment constructed using ViPTree shows that the vB_PseuGesM_254 genome has a large inversion between ~53,100 and ~70,700 bp, which is possibly a distinctive feature of a new taxonomic unit within this large group of Pseudomonas phages.

Identification, proteolytic activity quantification and biofilm-forming characterization of Gram-positive, proteolytic, psychrotrophic bacteria isolated from cold raw milk

Psychrotrophic bacteria of raw milk face the dairy industry with significant spoilage and technological problems due to their ability to produce heat-resistant enzymes and biofilms. Despite extensive information about Gram-negative psychrotrophic bacteria in milk, little is known about Gram-positive psychrotrophic bacteria in milk, and their proteolytic activity and biofilm-forming characteristics. In the present study, Gram-positive, proteolytic, psychrotrophic bacteria of cold raw milk were identified, and their proteolytic activity and biofilm-forming capacity were quantified. In total, 12 genera and 22 species were represented among the bacterial isolates, however 50% belonged to three genera, namely Staphylococcus (19.4%), Bacillus (16.7%), and Enterococcus (13.9%). Different levels of proteolytic activity were detected in the identified isolates, even among the strains belonging to the same species. In addition, proteolytic activity was significantly higher at 25°C than at 7°C for all isolates. The crystal violet staining assay in polystyrene microtitre plates revealed a high level of variation in the biofilm-forming capacity at 7°C. After 72 hours of incubation, 11.1% of the strains did not produce a biofilm, while 27.8%, 52.8%, and 8.3% produced low, moderate, and high amounts of biofilm on polystyrene, respectively. The psychrotrophic bacteria were also able to produce biofilms on the surface of stainless steel coupons in ultra-high temperature milk after 72 h of incubation at 7°C; the number of attached cells ranged from 1.34 to 5.11 log cfu/cm2. These results expand the knowledge related to the proteolytic activity and biofilm-forming capacity of Gram-positive psychrotrophic milk bacteria.

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.