Identification and characterization of a heat-resistant protease from Serratia liquefaciens isolated from Brazilian cold raw milk

Phloretin Inhibits Quorum Sensing and Biofilm Formation in Serratia marcescens

This study investigated the antivirulence capacity and mechanism of apple-skin-derived phloretin against Serratia marcescens NJ01, a vegetable spoilage bacterium. At 0.5 to 2 mg/mL doses, phloretin considerably inhibited the secretion of acyl homoserine lactones (AHLs), indicating that phloretin disrupted quorum sensing (QS) in S. marcescens NJ01. The dysfunction of QS resulted in reduced biofilms and the decreased production of protease, prodigiosin, extracellular polysaccharides (EPSs), and swimming and swarming motilities. Dysfunctional QS also weakened the activity of antioxidant enzymes and improved oxidative injury. The improved oxidative injury changed the composition of the membrane, improved membrane permeability, and eventually increased the susceptibility of biofilm cells to amikacin, netilmicin, and imipenem. The disrupted QS and enhanced oxidative stress also caused disorders of amino acid metabolism, energy metabolism, and nucleic acid metabolism, and ultimately attenuated the ability of S. marcescens NJ01 to induce spoilage. Our results indicated that phloretin can act as a potent drug to defend against spoilage by S. marcescens.

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.

High-throughput sequencing provides new insights into the roles and implications of core microbiota present in pasteurized milk

Residual microorganisms in dairy products are closely related to their quality deterioration and safety. Based on the minimum sterilization conditions required by Grade A Pasteurized Milk Ordinance, this study explored the microbiota present in milk products that were high temperature short time pasteurized at 72, 75, 80, 83, or 85 °C for 15 s, 20 s, and 30 s separately. Based on high-throughput sequencing results, 6 phyla and 18 genera were identified as dominant microbiota. Proteobacteria and Firmicutes were the maior bacteria in phyla, and each comprising more than 50%. Pseudomonas was account for more than 42% of all the genera detected in all samples. Moreover, the changes in flavor substances in pasteurized milk, including 16 free amino acids, 9 fatty acids, and 17 volatile compounds, were detected using principal component and multi factor analyses. The Pearson correlation coefficient analysis identified six bacteria genera as the core functional microbiota that significantly affected the flavor compounds and the safety and quality of pasteurized milk. Interestingly, Pseudomonas, Omithimimicrobium, Cyanobacteria and Corynebacterium had positive correlations with the flavor substances, whereas Streptococcus and Paeniclostridium had significant negative correlations with these substances. The results may help enhance the quality control of dairy products and can be used as indicators of microbial contamination of pasteurized dairy products.

Phenolic extract of Eugenia uniflora L. and furanone reduce biofilm formation by Serratia liquefaciens and increase its susceptibility to antimicrobials

Serratia liquefaciens is a spoilage microorganism of relevance in the dairy industry because it is psychrotrophic, able to form biofilm, and produces thermoresistant proteases and lipases. Phenolic compounds and furanones have been studied as inhibitors of biofilm formation. In this study, the potential of the pulp phenolic extract of Eugenia uniflora L. orange fruits, also called pitanga, and furanone C30 on the inhibition of biofilm formation by S. liquefaciens L53 and the susceptibility to different antimicrobials were evaluated. The pulp phenolic extract of pitanga had a high total phenolic content, being mainly composed of glycosylated quercetins and ellagitannins. Sub-inhibitory concentrations of this extract and furanone reduced biofilm formation by S. liquefaciens on polystyrene and the amount of polysaccharides, proteins and extracellular DNA in the biofilms. These biofilms were also more susceptible to kanamycin. The combinations of furanone with phenolic extract of pitanga or kanamycin showed a synergistic effect with total growth inhibition of S. liquefaciens.

Game animals as a reservoir of rarely recorded opportunistic bacteria

In Poland, the population of game animals, including wild boars, roe and deer, is constantly increasing. The tonsils of wild game animals are a place of large variety of microorganisms. The pathogenic bacteria present in them, such as E. coli, Y. enterocolitica and Salmonella sp. are the most common source of meat infection. The opportunistic bacteria R. aquatilis, P. fluorescens, S. liquefaciens, S. fonticola and S. plymuthica also play an important role in public health. S. liquefaciens may be an opportunistic pathogen and in people causes sepsis and inflammation of the cornea. The protease, nuclease, phospholipase A, flagella, elastase and toxins haemolysin and proteolysin are responsible for its ability to cause disease symptoms. S. plymuthica, a rod commonly found in the environment, produces a red dye. It is most often isolated from soil, water and food. In humans, it causes infections of burn wounds, soft leg tissue as well as post-operative wounds and bacteraemia. Typically, the environmental rod S. fonticola differs from other Serratia sp. by the lack of extracellular enzymes. It causes wound infections and bacteraemia resulting from urinary tract infections. Bacteria from the genus Serratia sp. show a natural resistance to glycopeptides, rifampicin and erythromycin. The aquatic rod R. aquatilis in humans causes opportunistic infections of the urinary tract, wounds, bacteraemia and endocarditis. It exhibits natural resistance to antibiotics from the group of cephalosporins, penicillins and macrolides and quinolones. It is conditioned by the presence of class 1 integrons and class A β-lactamases. The virulence factors of R. aquatilis are also LPS and adhesin. P. fluorescens in humans is responsible for bacteraemia. It also exhibits resistance to human serum thanks to outer membrane proteins and has T3SS, which effectively infects the host.

Insights into Psychrotrophic Bacteria in Raw Milk: A Review

HIGHLIGHTS

Levels of psychrotrophic bacteria in raw milk are affected by to habitats and farm hygiene. Biofilms formed by psychrotrophic bacteria are persistent sources of contamination. Heat-stable enzymes produced by psychrotrophic bacteria compromise product quality. Various strategies are available for controlling dairy spoilage caused by psychrotrophic bacteria.

Purification and characterization of the thermostable protease produced by Serratia grimesii isolated from channel catfish

BACKGROUND

Microbial spoilage of fishery products accounts for significant financial losses, yearly on a global scale. Psychrotrophic spoilage bacteria often secrete extracellular enzymes to break down surrounding fish tissue, rendering the product unsuitable for human consumption. For a better understanding of bacterial spoilage due to enzymatic digestion of fish products, proteases in Serratia grimesii isolated from North American catfish fillets (Ictalurus punctatus) were investigated.

RESULTS

Mass spectrometric evidence demonstrated that S. grimesii secretes two distinct extracellular proteases and one lipase. Protease secretion displayed broad thermostability in the 30-90 °C range. The major protease-secretion (O-1) was most active under alkaline conditions and utilized manganese as a co-factor. Organic solvents significantly disrupted the efficacy of S. grimesii extracellular enzymes and, in a series of bactericidal detergents, protease activity was highest when treated with Triton X-100. Ethylenediaminetetraacetic acid (EDTA) and phenylmethylsulfonyl fluoride (PMSF) significantly inhibited the enzyme activity, while protease was moderately stable under freeze-thaw and refrigerated storage.

CONCLUSION

The influence of fish spoilage-related enzymes, depending on various factors, is discussed in this paper. This study will provide new insight into enzymatic spoilage and its control, which can be exploited to enhance food safety and the shelf-life of fishery products worldwide. © 2018 Society of Chemical Industry.

Spoilage potential of psychrotrophic bacteria isolated from raw milk and the thermo-stability of their enzymes

The storage and transportation of raw milk at low temperatures promote the growth of psychrotrophic bacteria and the production of thermo-stable enzymes, which pose great threats to the quality and shelf-life of dairy products. Though many studies have been carried out on the spoilage potential of psychrotrophic bacteria and the thermo-stabilities of the enzymes they produce, further detailed studies are needed to devise an effective strategy to avoid dairy spoilage. The purpose of this study was to explore the spoilage potential of psychrotrophic bacteria from Chinese raw milk samples at both room temperature (28 °C) and refrigerated temperature (7 °C). Species of Yersinia, Pseudomonas, Serratia, and Chryseobacterium showed high proteolytic activity. The highest proteolytic activity was shown by Yersinia intermedia followed by Pseudomonas fluorescens (d). Lipolytic activity was high in isolates of Acinetobacter, and the highest in Acinetobacter guillouiae. Certain isolates showed positive β-galactosidase and phospholipase activity. Strains belonging to the same species sometimes showed markedly different phenotypic characteristics. Proteases and lipases produced by psychrotrophic bacteria retained activity after heat treatment at 70, 80, or 90 °C, and proteases appeared to be more heat-stable than lipases. For these reasons, thermo-stable spoilage enzymes produced by a high number of psychrotrophic bacterial isolates from raw milk are of major concern to the dairy industry. The results of this study provide valuable data about the spoilage potential of bacterial strains in raw milk and the thermal resistance of the enzymes they produce.

A Tropical Composting Operation Unit at São Paulo Zoo as a Source of Bacterial Proteolytic Enzymes

Composting operation systems are valuable sources of microorganisms and enzymes. This work reports the assessment of proteolytic enzymes from cultivable bacteria isolated from a composting facility of the São Paulo Zoo Park (SPZPF), São Paulo, Brazil. Three hundred bacterial isolates were obtained and identified based on 16S rRNA gene as belonging to 13 different genera. The most common genus among the isolates was Bacillus (67%); some of which show high proteolytic activity in their culture media. Biochemical assays of hydrolytic activities using FRET peptides as substrates allowed the characterization of a repertoire of serine proteases and metalloproteases with different molecular weights secreted by Bacillus strains isolated from composting. Furthermore, thermostable serine and metalloproteases were detected in the composting leachate, which might be of interest for industrial applications.

The Biodiversity of the Microbiota Producing Heat-Resistant Enzymes Responsible for Spoilage in Processed Bovine Milk and Dairy Products

Raw bovine milk is highly nutritious as well as pH-neutral, providing the ideal conditions for microbial growth. The microbiota of raw milk is diverse and originates from several sources of contamination including the external udder surface, milking equipment, air, water, feed, grass, feces, and soil. Many bacterial and fungal species can be found in raw milk. The autochthonous microbiota of raw milk immediately after milking generally comprises lactic acid bacteria such as Lactococcus, Lactobacillus, Streptococcus, and Leuconostoc species, which are technologically important for the dairy industry, although they do occasionally cause spoilage of dairy products. Differences in milking practices and storage conditions on each continent, country and region result in variable microbial population structures in raw milk. Raw milk is usually stored at cold temperatures, e.g., about 4°C before processing to reduce the growth of most bacteria. However, psychrotrophic bacteria can proliferate and contribute to spoilage of ultra-high temperature (UHT) treated and sterilized milk and other dairy products with a long shelf life due to their ability to produce extracellular heat resistant enzymes such as peptidases and lipases. Worldwide, species of Pseudomonas, with the ability to produce these spoilage enzymes, are the most common contaminants isolated from cold raw milk although other genera such as Serratia are also reported as important milk spoilers, while for others more research is needed on the heat resistance of the spoilage enzymes produced. The residual activity of extracellular enzymes after high heat treatment may lead to technological problems (off flavors, physico-chemical instability) during the shelf life of milk and dairy products. This review covers the contamination patterns of cold raw milk in several parts of the world, the growth potential of psychrotrophic bacteria, their ability to produce extracellular heat-resistant enzymes and the consequences for dairy products with a long shelf life. This problem is of increasing importance because of the large worldwide trade in fluid milk and milk powder.

Ser2 from Serratia liquefaciens L53: A new heat stable protease able to destabilize UHT milk during its storage

The heat-stable protease Ser2 is secreted by the species Serratia liquefaciens, a psychrotrophic bacteria frequently found in raw milk. To understand the physicochemical modifications of casein micelles induced by Ser2 and to confirm its implication in UHT milk destabilization, the enzyme was purified and added to microfiltered raw milk before UHT treatment. UHT milk destabilization was investigated during 90days of storage. A visual destabilization appeared after 8days of storage with the presence of sediment. Zeta potential increase and formation of aggregates were observed during the storage. Using tandem mass spectrometry, numerous released peptides from the four caseins were identified at the end of storage. Caseins were hydrolyzed in the preferential order β->α_s1->κ->α_s2. No specific peptidic hydrolysed bond was detected. The present study confirmed that the presence of the protease Ser2 in raw milk can be one of the main causes of UHT milk destabilization.