Recombinase polymerase amplification with polymer flocculation sedimentation for rapid detection of Staphylococcus aureus in food samples.

Currently, rapid, sensitive, and convenient visual detection methods for Staphylococcus aureus (S. aureus) are scarce. In this study, a novel detection method based on recombinase polymerase amplification (RPA) and polymer flocculation sedimentation (PFS) was developed. Twelve effective primer combinations derived from four forward primers F1, F2, F3, F4, and three reverse primers R1, R2, R3 targeting the nuc gene of S. aureus were designed and screened by a polymerase chain reaction and RPA methods. RPA reaction conditions, including temperature, time, and volume as well as PEG8000 and NaCl concentrations range, were optimized. Moreover, the specificity and sensitivity of the RPA-PFS assay were further analyzed. Finally, the potential use of the RPA-PFS assay was evaluated using artificially S. aureus contaminated food samples, including pork, beef, shrimp, fish, cheese, cabbage, leftover rice, egg, milk, and orange juice. Results showed that the SA5 (F2/R2) combination was the optimal primer candidate. The optimal temperature range, the shortest time and the minimal volume of RPA reaction were 40-42 °C, 10 min and 10 µL, respectively and the optimal PEG8000/NaCl concentrations were 0.2 g/mL and 2.5 M, respectively, for the adsorption between magnetic beads and RPA products. The RPA-PFS method could detect as little as 13 fg genomic DNA of S. aureus and was also specific for five target S. aureus as well as twenty-seven non-target foodborne bacteria. The limit of detection of RPA-PFS for S. aureus in artificially contaminated food samples was 38 CFU/mL (g). Besides, RPA-PFS has directly been judged by the naked eye and has totally taken less than 20 min. In short, the assay RPA-PFS developed in this study is a rapid, sensitive, and specific visual detection method for S. aureus.

Sensitive and rapid visual detection of Salmonella Typhimurium in milk based on recombinase polymerase amplification with lateral flow dipsticks.

Salmonella Typhimurium (S. Typhimurium) can cause serious foodborne diseases. In this study, an assay combining recombinase polymerase amplification (RPA) with lateral flow dipsticks (LFD) was developed to detect S. Typhimurium in milk. The RPA forward primers STF1, STF2, STF3, the reverse primer STR labeled with digoxin, and the probe STProb labeled with FAM were designed and screened to produce RPA products for LFD detection. The RPA reaction volume, temperature, and time were then optimized, and the sensitivity and specificity of the developed method were analyzed. Finally, the RPA-LFD method was evaluated using milk artificially contaminated with S. Typhimurium. Results indicated that the primer pair STF1/STR is the optimal combination for detecting the bacterium. The minimum volume, shortest time, and optimal temperature of the RPA reaction were 10 µL, 10 min, and 40-42 °C, respectively. The limit of detection of RPA-LFD for detecting the genomic DNA of S. Typhimurium was 1 fg, which is 5 and 10 times lower than the corresponding limits of RPA-agarose gel electrophoresis (AGE) and PCR-AGE, respectively. Testing with 29 other foodborne bacteria as controls revealed that RPA-LFD was highly specific for S. Typhimurium. RPA-LFD can detect S. Typhimurium at concentrations as low as 1.95 CFU/mL in artificially inoculated milk samples and is thus 10 times more sensitive than PCR. Hence, the RPA-LFD assay established in this study could be a potential point-of-care/need test for S. Typhimurium, especially in areas with limited resources.

Development of duplex PCR-ELISA for simultaneous detection of Salmonella spp. and Escherichia coli O157: H7 in food.

In the current study, a duplex PCR-ELISA method was developed targeting the specific genes, invA of Salmonella spp. and rfbE of Escherichia coli O157: H7, to detect one or both bacteria in food. In brief, PCR product amplified by PCR primer labeled with digoxin at the 5'-end and a probe labeled with biotin at the 3'-end can form dimer by nucleic acid hybridization which can be captured by binding of biotin to streptomycin coated in ELISA plate before using enzyme-labeled anti-digoxin antibody and substrate to develop color. Also, evaluation of the duplex PCR-ELISA method was conducted in different food samples including milk, juice, cabbage, shrimp, chicken, pork and beef. Results indicated that the duplex PCR-ELISA developed here was specific when using 25 non-target bacteria strains as controls and was sensitive with a limit of detection (LOD) of 1 CFU/mL, 1, 000 times higher than that of the duplex PCR method and was repeatable regardless of inter- and intra-batch variations. The duplex PCR-ELISA method established in the present study has proven to be highly specific, sensitive and repeatable. It has the potential to be applied in such fields as clinical diagnosis of food-borne diseases, food hygiene monitoring and pathogen detection in food.

Physicochemical Properties and Fatty Acid Profiles of Elaeagnus mollis Diels Nut Oils.

The physicochemical properties, fatty acid profiles, content of tocopherol and sterol of the oils extracted from the nuts of Elaeagnus mollis Diels grown in different regions of China were studied in this work. The results indicated that the Elaeagnus mollis Diels nut oils contained about 0.2% sterols and the tocopherol contents were in the range of 119.6-128.6mg/100g. The nut oils were all rich in unsaturated fatty acids, especially oleic acid and linoleic acid. Furthermore, the main triacylglycerols species of the nut oils were all dilinoleoyl-monoolein (LOL), dioleoyl-monolinoleoyl (OLO) and trilinoleate (LLL). This work might be useful for developing applications for Elaeagnus mollis Diels nut oil.

High copy numbers and N terminal insertion position of influenza A M2E fused with hepatitis B core antigen enhanced immunogenicity.

The extra domain of influenza M2 protein (M2e) is almost completely conserved among all influenza A virus subtypes. M2e is a promising candidate target for the development of a broad-spectrum recombinant influenza A vaccine. However, the immunogenicity of M2e needs to be improved. Copy numbers of M2e and its fusion expression with different carrier proteins may affect its immunopotency. In this study, we designed and created different constructs through genetic fusion of M2e (MSLLTEVETPTRSEWECRCSDSSD) (A/California/05/2009 (H1N1)) with the N-terminus (HBcAg1-149aa + Cys) by insertion in the N-terminus Hepatitis B Core (HBc) antigen 1-149aa and Middle 78-81aa of HBcAg1-149aa to construct a recombinant M2e-based vaccine candidate. These chimeric sequences were expressed in Escherichia coli. We constructed fusion proteins containing influenza A H1N1 influenza virus (2009), as well as one, two, and three copies of M2e and hepatitis B core antigen1-149aa amino acid-optimized codon inserted N and its intermediate. The recombinant protein was expressed and purified. Western blot analysis was employed to evaluate the expression of the M2e recombinant protein containing different copy numbers of M2e. Mice were immunized for two times with the purified fusion protein HBc/M2e BALB/c. Serum levels of M2e antibody gradually increased along with increase in immunity. The levels of different fusion protein M2e antibodies increase with increasing M2e copy number. In addition, the protein antibody level in the N terminal fusion protein is higher than that in intermediate fusion.

Expressional induction of Paralichthys olivaceus cathepsin B gene in response to virus, poly I:C and lipopolysaccharide.

Cathepsin B is a lysosomal cysteine protease of the papain-like enzyme family with multiple biological functions. In this study, Paralichthys olivaceus cathepsin B (PoCatB) cDNA was isolated from flounder embryonic cells (FEC) treated with UV-inactivated grass carp hemorrhage virus (GCHV) and subsequently identified as a virally induced gene. The full length cDNA of PoCatB is 1801bp encoding 330-amino acids. The deduced protein has high homology to all known cathepsin B proteins, containing an N-terminal signal peptide, cysteine protease active sites, the occluding loop segment and a glycosylation site, all of which are conserved in the cathepsin B family. PoCatB transcription of FEC cells could be induced by turbot (Scophthalmus maximus) rhabdovirus (SMRV), UV-inactivated SMRV, UV-inactivated GCHV, poly I:C or lipopolysaccharide (LPS), and SMRV or poly I:C was revealed to be most effective among the five inducers. In normal flounder, PoCatB mRNA was detectable in all examined tissues. Moreover, SMRV infection could result in significant upregulation of PoCatB mRNA, predominantly in spleen, head kidney, posterior kidney, intestine, gill and muscle with 18.2, 10.9, 24.7, 12, 31.5 and 18 fold increases at 72h post-infection respectively. These results provided the first evidence for the transcriptional induction of cathepsin B in fish by virus and LPS, indicating existence of a novel function in viral defense.

Characterization and expression analysis of Paralichthys olivaceus voltage-dependent anion channel (VDAC) gene in response to virus infection.

Voltage-dependent anion channel (VDAC, also known as mitochondrial porin) is acknowledged to play an important role in stress-induced mammalian apoptosis. In this study, Paralichthys olivaceus VDAC (PoVDAC) gene was identified as a virally induced gene from Scophthalmus Maximus Rhabdovirus (SMRV)-infected flounder embryonic cells (FEC). The full length of PoVDAC cDNA is 1380 bp with an open reading frame of 852 bp encoding a 283 amino acid protein. The deduced PoVDAC contains one alpha-helix, 13 transmembrane beta-strands and one eukaryotic mitochondrial porin signature motif. Constitutive expression of PoVDAC was confirmed in all tested tissues by real-time PCR. Further expression analysis revealed PoVDAC mRNA was upregulated by viral infection. We prepared fish antiserum against recombinant VDAC proteins and detected the PoVDAC in heart lysates from flounder as a 32 kDa band on western blot. Overexpression of PoVDAC in fish cells induced apoptosis. Immunofluoresence localization indicated that the significant distribution changes of PoVDAC have occurred in virus-induced apoptotic cells. This is the first report on the inductive expression of VDAC by viral infection, suggesting that PoVDAC might be mediated flounder antiviral immune response through induction of apoptosis.

Molecular characterisation and inductive expression of a fish protein arginine methyltransferase 1 gene in response to virus infection.

Protein arginine methyltransferase 1 (PRMT1) is currently thought as an effector to regulate interferon (IFN) signalling. Here Paralichthys olivaceus PRMT1 (PoPRMT1) gene was identified as a virally induced gene from UV-inactivated Scophthalmus maximus Rhabdovirus (SMRV)-infected flounder embryonic cells (FEC). PoPMRT1 encodes a 341-amino-acid protein that shares the conserved domains including post-I, motif I, II and III. Homology comparisons show that the putative PoPMRT1 protein is the closest to zebrafish PMRT1 and belongs to type I PRMT family (including PRMT1, PRMT2, PRMT3, PRMT4, PRMT6, PRMT8). Expression analyses revealed an extensive distribution of PoPMRT1 in all tested tissues of flounder. In vitro induction of PoPRMT1 was determined in UV-inactivated SMRV-infected FEC cells, and under the same conditions, flounder Mx was also transcriptionally up-regulated, indicating that an IFN response might be triggered. Additionally, live SMRV infection of flounders induced an increased expression of PoPRMT1 mRNA and protein significantly in spleen, and to a lesser extent in head kidney and intestine. Immunofluorescence analysis revealed a major cyptoplasmic distribution of PoPRMT1 in normal FEC but an obvious increase occurred in nucleus in response to UV-inactivated SMRV. This is the first report on in vitro and in vivo expression of fish PRMT1 by virus infection, suggesting that PoPRMT1 might be implicated in flounder antiviral immune response.

Molecular cloning and stress-induced expression of paralichthys olivaceus heme-regulated initiation factor 2alpha kinase.

The heme-regulated initiation factor 2alpha kinase (HRI) is acknowledged to play an important role in translational shutoff in reticulocytes in response to various cellular stresses. In this study, we report its homologous cDNA cloning and characterization from cultured flounder embryonic cells (FEC) after treatment with UV-inactivated grass carp haemorrhagic virus (GCHV). The full-length cDNA of Paralichthys olivaceus HRI homologue (PoHRI) has 2391 bp and encodes a protein of 651 amino acids. The putative PoHRI protein exhibits high identity with all members of eIF2alpha kinase family. It contains 12 catalytic subdomains located within the C-terminus of all Ser/Thr protein kinases, a unique kinase insertion of 136 amino acids between subdomains IV and V, and a relatively conserved N-terminal domain (NTD). Upon heat shock, virus infection or Poly I:C treatment, PoHRI mRNA and protein are significantly upregulated in FEC cells but show different expression patterns in response to different stresses. In healthy flounders, PoHRI displays a wide tissue distribution at both the mRNA and protein levels. These results indicate that PoHRI is a ubiquitous eIF2alpha kinase and might play an important role in translational control over nonheme producing FEC cells under different stresses.

Differential expression of three Paralichthys olivaceus Hsp40 genes in responses to virus infection and heat shock.

Heat shock proteins (Hsps) are a family of highly conserved cellular proteins present in all organisms, mediating a range of essential housekeeping and cytoprotective functions as well-known molecular chaperones and recently as regulators of the immune response. By subtractive suppression hybridization, three Hsp40 homologues have been identified in the flounder (Paralichthys olivaceus) embryonic cells (FEC) after treatment with UV-inactivated turbot (Scophthalmus maximus L.) rhabdovirus (SMRV), termed PoHsp40A4, PoHsp40B6 and PoHsp40B11, whose encoded proteins all possess the conserved DnaJ domain, a signature motif of the Hsp40 family. Based on different protein structure and phylogenetic analysis, they can be categorized into two subfamilies, PoHsp40A4 for Type I Hsp40, PoHsp40B6 and PoHsp40B11 for Type II Hsp40. Further expression analysis revealed two very different types of kinetics in response either to heat shock or to virus infection, with a marked induction for PoHsp40A4 and a weak one for both PoHsp40B6 and PoHsp40B11. A very distinct tissue distribution of mRNA was also revealed among the three genes, even between PoHsp40B6 and PoHsp40B11. This is the first report on the transcriptional induction of Hsp40 in virally stimulated fish cells, and the differential expressions might reflect their different roles in unstressed and stressed cells.