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.

Autoinducer-2 (AI-2) is a quorum-sensing signal molecule that controls a variety of cellular activities in response to cell density in both gram-negative and gram-positive bacteria. The production of AI-2 is dependent upon LuxS, the last enzyme in the AI-2 biosynthesis pathway. For this study, we constructed a luxS null mutation (Delta luxS) in Campylobacter jejuni strain 81-176, and showed that it abolished AI-2 production. The Delta luxS mutant had a longer doubling time in Mueller-Hinton (MH) broth and reduced swarming on MH soft agar at 37 degrees C compared to the wild type (wt), whereas growth rate or swarming at 42 degrees C was not affected. The Delta luxS mutant was also more sensitive to hydrogen peroxide (H(2)O(2)) and cumene hydroperoxide than the wt by disc inhibition assays at 42 degrees C, though minimum inhibitory concentration comparisons were inconclusive. Differences in genome-wide gene expression between wt and Delta luxS mutant with and without H(2)O(2) treatments were compared using DNA microarrays. The genes that showed differential expressions (wt/Delta luxS) include operons/pathways involved in AI-2 synthesis and S-adenosylmethionine (SAM) metabolism (metE, metF, and pfs), flagellar assembly/regulation, stress response (ahpC, tpx, and groES), ABC transporters/efflux systems, and two genes of unknown function located downstream of luxS (Cj1199 and Cj1200). The wt/Delta luxS expression ratios of ahpC (encoding alkyl hydroperoxide reductase) and tpx (encoding thiol peroxidase) were increased only with H(2)O(2) treatment, consistent with our finding that the Delta luxS mutant exhibits higher sensitivity to oxidative stress than wt. Our microarray results agreed with the Delta luxS mutant phenotypes, and suggested that LuxS plays a role in central metabolism involving SAM metabolism, but it is uncertain whether AI-2 functions as a true quorum-sensing signal in C. jejuni.

The firefly (Photinus pyralis) luciferase (luc) gene on plasmid vector pBESTluc and the Aequorea victoria green fluorescent protein (gfp) gene on plasmid vector pGFP were introduced into strains of Escherichia coli O157:H7. The recombinant E. coli strains were indistinguishable from their parent strains in biochemical and immunological assays and in a multiplex PCR reaction. There was no significant difference in the growth kinetics of the luc-bearing recombinants and the parent strains. At 37°C all of the recombinant strains maintained the vectors and expressed luciferase and the green fluorescent protein when grown both with and without antibiotic selection. Individual colonies of luc-bearing E. coli strains were readily luminescent in the dark after being sprayed with a solution of 1 mM beetle luciferin. The recombinants containing pGFP emitted bright green fluorescence when excited with UV light and the addition of any other proteins, substrates, or cofactors was not required. The green fluorescent protein-expressing E. coli O157:H7 strains were used in studies examining the survival of the organism in apple cider and in orange juice. In apple cider the organism declined to undetectable levels in 24 days at refrigeration temperature while in orange juice the strains survived with only small decreases in number during the 24-day sampling period. These recombinant E. coli O157:H7 strains, containing readily identifiable and stable markers, could be useful as positive controls in microbial assays as well as in studies monitoring bacterial survival and the behavior of E. coli O157:H7 in foods and in a food processing environment.