BackgroundMagnesium oxide nanoparticles (MgO nanoparticles, with average size of 20 nm) have considerable potential as antimicrobial agents in food safety applications due to their structure, surface properties, and stability. The aim of this work was to investigate the antibacterial effects and mechanism of action of MgO nanoparticles against several important foodborne pathogens.ResultsResazurin (a redox sensitive dye) microplate assay was used for measuring growth inhibition of bacteria treated with MgO nanoparticles. The minimal inhibitory concentrations of MgO nanoparticles to 104 colony-forming unit/ml (CFU/ml) of Campylobacter jejuni, Escherichia coli O157:H7, and Salmonella Enteritidis were determined to be 0.5, 1 and 1 mg/ml, respectively. To completely inactivate 108−9 CFU/ml bacterial cells in 4 h, a minimal concentration of 2 mg/ml MgO nanoparticles was required for C. jejuni whereas E. coli O157:H7 and Salmonella Enteritidis required at least 8 mg/ml nanoparticles. Scanning electron microscopy examination revealed clear morphological changes and membrane structural damage in the cells treated with MgO nanoparticles. A quantitative real-time PCR combined with ethidium monoazide pretreatment confirmed cell membrane permeability was increased after exposure to the nanoparticles. In a cell free assay, a low level (1.1 μM) of H2O2 was detected in the nanoparticle suspensions. Consistently, MgO nanoparticles greatly induced the gene expression of KatA, a sole catalase in C. jejuni for breaking down H2O2 to H2O and O2.ConclusionsMgO nanoparticles have strong antibacterial activity against three important foodborne pathogens. The interaction of nanoparticles with bacterial cells causes cell membrane leakage, induces oxidative stress, and ultimately leads to cell death.

A total of 108 S. aureus isolates from 16 major hospitals located in 14 different provinces in China were characterized for the profiles of 18 staphylococcal enterotoxin (SE) genes, 3 exfoliatin genes (eta, etb and etd), and the toxic shock syndrome toxin gene (tsst) by PCR. The genomic diversity of each isolate was also evaluated by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and accessory gene regulator (agr) typing. Of these strains, 90.7% (98/108) harbored toxin genes, in which tsst was the most prevalent toxin gene (48.1%), followed by sea (44.4%), sek (42.6%) and seq (40.7%). The see and etb genes were not found in any of the isolates tested. Because of high-frequency transfer of toxin gene-containing mobile genetic elements between S. aureus strains, a total of 47 different toxin gene combinations were detected, including a complete egc cluster in 19 isolates, co-occurrence of sea, sek and seq in 38 strains, and sec and sel together in 11 strains. Genetic typing by PFGE grouped all the strains into 25 clusters based on 80% similarity. MLST revealed 25 sequence types (ST) which were assigned into 16 clonal complexes (CCs) including 2 new singletons. Among these, 11 new and 6 known STs were first reported in the S. aureus strains from China. Overall, the genotyping results showed high genetic diversity of the strains regardless of their geographical distributions, and no strong correlation between genetic background and toxin genotypes of the strains. For genotyping S. aureus, PFGE appears to be more discriminatory than MLST. However, toxin gene typing combined with PFGE or MLST could increase the discriminatory power of genotyping S. aureus strains.

Rapid detection of the foodborne pathogen Escherichia coli O157:H7 is of vital importance for public health worldwide. Among detection methods, reporter phages represent unique and sensitive tools for the detection of E. coli O157:H7 from food as they are host-specific and able to differentiate live cells from dead ones. Upon infection, target bacteria become identifiable since reporter genes are expressed from the engineered phage genome. The E. coli O157:H7 bacteriophage ΦV10 was modified to express NanoLuc luciferase (Nluc) derived from the deep-sea shrimp Oplophorus gracilirostris. Once infected by the ΦV10 reporter phage, E. coli O157:H7 produces a strong bioluminescent signal upon addition of commercial luciferin (Nano-Glo®). Enrichment assays using E. coli O157:H7 grown in LB broth with a reporter phage concentration of 1.76 × 102 pfu ml−1 are capable of detecting approximately 5 CFU in 7 hours. Comparable detection was achieved within 9 hours using 9.23 × 103 pfu ml−1 of phage in selective culture enrichments of ground beef as a representative food matrix. Therefore we conclude that this NanoLuc reporter phage assay shows promise for detection of E. coli O157:H7 from food in a simple, fast and sensitive manner.

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Incidental contamination of foods by pathogenic bacteria and/or their toxins is a serious threat to public health and the global economy. The presence of food-borne pathogens and toxins must be rapidly determined at various stages of food production, processing, and distribution. Producers, processors, regulators, retailers, and public health professionals need simple and cost-effective methods to detect different species or serotypes of bacteria and associated toxins in large numbers of food samples. This review addresses the desire to replace traditional microbiological plate culture with more timely and less cumbersome rapid, biosensor-based methods. Emphasis focuses on high-throughput, multiplexed techniques that allow for simultaneous testing of numerous samples, in rapid succession, for multiple food-borne analytes (primarily pathogenic bacteria and/or toxins).

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Microfiltration of chicken extracts has the potential to significantly decrease the time required to detect Salmonella, as long as the extract can be efficiently filtered and the pathogenic microorganisms kept in a viable state during this process. We present conditions that enable microfiltration by adding endopeptidase from Bacillus amyloliquefaciens to chicken extracts or chicken rinse, prior to microfiltration with fluid flow on both retentate and permeate sides of 0.2 μm cutoff polysulfone and polyethersulfone hollow fiber membranes. After treatment with this protease, the distribution of micron, submicron, and nanometer particles in chicken extracts changes so that the size of the remaining particles corresponds to 0.4-1 μm. Together with alteration of dissolved proteins, this change helps to explain how membrane fouling might be minimized because the potential foulants are significantly smaller or larger than the membrane pore size. At the same time, we found that the presence of protein protects Salmonella from protease action, thus maintaining cell viability. Concentration and recovery of 1-10 CFU Salmonella/mL from 400 mL chicken rinse is possible in less than 4 h, with the microfiltration step requiring less than 25 min at fluxes of 0.028-0.32 mL/cm(2) min. The entire procedure-from sample processing to detection by polymerase chain reaction-is completed in 8 h.