Potential harmful of extracellular proteases secreted by Pseudomonas fluorescens W3 on milk quality

Discovery of a temperature-dependent protease spoiling meat from Pseudomonas fragi: Target to myofibrillar and sarcoplasmic proteins rather than collagen

Chilled meat frequently suffered microbial spoilage because bacteria can secrete various proteases that break down the proteins. In this study, Pseudomonas fragi NMC 206 exhibited a temperature-dependent secretion pattern, with the ability to release the specific protease only below 25 °C. It was identified as alkaline protease AprA by LC-MS/MS, with the molecular weight of 50.4 kDa, belonging to the Serralysin family metalloprotease. Its significant potential for meat spoilage in situ resulted in alterations in meat color and sensory evaluation, as well as elevated pH, total volatile basic nitrogen (TVB-N) and the formation of volatile organic compounds (VOCs). The hydrolysis of meat proteins in vitro showed that AprA possessed a considerable proteolysis activity and degradation preferences on meat proteins, especially its ability to degrade myofibrillar and sarcoplasmic proteins, rather than collagen. These observations demonstrated temperatures regulated the secretion of AprA, which was closely related to chilled chicken spoilage caused by bacteria. These will provide a new basis for the preservation of meat products at low temperatures.

Effect of Thermostable Enzymes Produced by Psychrotrophic Bacteria in Raw Milk on the Quality of Ultra-High Temperature Sterilized Milk

Ultra-high temperature sterilized milk (UHT) is a popular dairy product known for its long shelf life and convenience. However, protein gel aging and fat quality defects like creaming and flavor deterioration may arise during storage. These problems are primarily caused by thermostable enzymes produced by psychrotrophic bacteria. In this study, four representative psychrotrophic bacteria strains which can produce thermostable enzymes were selected to contaminate UHT milk artificially. After 11, 11, 13, and 17 weeks of storage, the milk samples, which were contaminated with Pseudomonas fluorescens, Chryseobacterium carnipullorum, Lactococcus raffinolactis and Acinetobacter guillouiae, respectively, demonstrated notable whey separation. The investigation included analyzing the protein and fat content in the upper and bottom layers of the milk, as well as examining the particle size, Zeta potential, and pH in four sample groups, indicating that the stability of UHT milk decreases over time. Moreover, the spoiled milk samples exhibited a bitter taste, with the dominant odor being attributed to ketones and acids. The metabolomics analysis revealed that three key metabolic pathways, namely ABC transporters, butanoate metabolism, and alanine, aspartate, and glutamate metabolism, were found to be involved in the production of thermostable enzymes by psychrotrophic bacteria. These enzymes greatly impact the taste and nutrient content of UHT milk. This finding provides a theoretical basis for further investigation into the mechanism of spoilage.

Rapid Identification of Pseudomonas fluorescens Harboring Thermostable Alkaline Protease by Real-Time Loop-Mediated Isothermal Amplification

ABSTRACT

Thermostable alkaline protease (TAP) harbored by Pseudomonas fluorescens decomposes protein in milk and dairy products, leading to milk and dairy product spoilage during storage. Thus, a specific, sensitive, rapid, and simple method is required to detect TAP-harboring P. fluorescens. Two sets of primers targeting the aprX and gyrB genes of P. fluorescens were designed. The detection system and conditions were optimized, and a real-time loop-mediated isothermal amplification (real-time LAMP) method was developed for the simultaneous detection of TAP-harboring P. fluorescens in two separate reaction tubes. The phylogenetic tree targeting aprX showed that P. fluorescens and Pseudomonas lurida clustered on the same branch. The phylogenetic tree targeting gyrB showed that P. fluorescens clustered on the same branch with 95% confidence value, whereas P. lurida clustered on different branches. DNA of 16 strains of P. fluorescens and 34 strains of non-P. fluorescens was detected by real-time LAMP. TAP-harboring P. fluorescens can only be identified when the real-time LAMP detection results of both aprX and gyrB are positive. The dissociation temperatures of aprX and gyrB in the real-time LAMP-amplified products were approximately 90.0 and 88.0°C, respectively. The detection limits of the real-time LAMP targeting aprX and gyrB were 4.9 CFU per reaction in pure culture and 2.2 CFU per reaction in skimmed milk. The coefficient of variation of the repeatability test was less than 2%, indicating that the established real-time LAMP of P. fluorescens targeting gyrB and aprX has good stability and repeatability. Real-time LAMP was used to test 200 raw milk samples for the presence of TAP-harboring P. fluorescens in 3 h, and the coincidence rate of the results with those obtained using the traditional method, which takes at least 5 to 7 days, was 100%. Real-time LAMP will be a practical and effective method for accurate and rapid identification of TAP-harboring P. fluorescens in raw milk.

HIGHLIGHTS

Proteolytic Traits of Psychrotrophic Bacteria Potentially Causative of Sterilized Milk Instability: Genotypic, Phenotypic and Peptidomic Insight

The proteolytic traits of the psychrotrophic strains Pseudomonas poae LP5, Pseudomonas fluorescens LPF3, Chryseobacterium joostei LPR1, Pseudomonas fulva PS1, Citrobacter freundii PS37, Hafnia alvei PS46, and Serratia marcescens PS92 were initially investigated by phenotypic and genotypic approaches. Six strains elicited extracellular proteolytic activity, and five expressed the thermostable AprX or (likely) Ser1 enzymes. Then, the strains were inoculated (10⁴ CFU/mL) in microfiltered pasteurized milk and kept at 4 °C for five days. All of the strains reached 10⁸ CFU/mL at the end of storage and five produced thermostable extracellular proteolytic enzymes. The freshly inoculated samples and the corresponding samples at 10⁸ CFU/mL were batch-sterilized (131 °C, 30 s) and kept at 45 °C up to 100 days. The former samples did not gel until the end of incubation, whereas the latter, containing P. poae, P. fluorescens, C. joostei, C. freundii, and S. marcescens, gelled within a few days of incubation. The thermostable proteolytic activity of strains affected the peptidomic profile, and specific proteolyzed zones of β-CN were recognized in the gelled samples. Overall, the results confirm some proteolytic traits of psychrotrophic Pseudomonas spp. strains and provide additional insights on the proteolytic activity of psychrotrophic bacteria potentially responsible for sterilized milk destabilization.