The antibacterial effect of zinc oxide (ZnO) nanoparticles on Campylobacter jejuni was investigated for inhibition and inactivation of cell growth. The results showed that C. jejuni was extremely sensitive to treatment with ZnO nanoparticles. The MIC of ZnO nanoparticles for C. jejuni was determined to be 0.05 to 0.025 mg/ml, which is 8-to 16-fold lower than that for Salmonella enterica serovar Enteritidis and Escherichia coli O157:H7 (0.4 mg/ml). The action of ZnO nanoparticles against C. jejuni was determined to be bactericidal, not bacteriostatic. Scanning electron microscopy examination revealed that the majority of the cells transformed from spiral shapes into coccoid forms after exposure to 0.5 mg/ml of ZnO nanoparticles for 16 h, which is consistent with the morphological changes of C. jejuni under other stress conditions. These coccoid cells were found by ethidium monoazide-quantitative PCR (EMA-qPCR) to have a certain level of membrane leakage. To address the molecular basis of ZnO nanoparticle action, a large set of genes involved in cell stress response, motility, pathogenesis, and toxin production were selected for a gene expression study. Reverse transcription-quantitative PCR (RT-qPCR) showed that in response to treatment with ZnO nanoparticles, the expression levels of two oxidative stress genes (katA and ahpC) and a general stress response gene (dnaK) were increased 52-, 7-, and 17-fold, respectively. These results suggest that the antibacterial mechanism of ZnO nanoparticles is most likely due to disruption of the cell membrane and oxidative stress in Campylobacter.

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The interaction of a range of Salmonella serovars with pre- and postharvest tomatoes was evaluated. Serovars were selected on the basis of previous association in tomato-linked outbreaks of salmonellosis (Salmonella Javiana, Salmonella Montevideo, and Salmonella Newport) or those typically isolated from animal or clinical infections (Salmonella Dublin, Salmonella Enteritidis, Salmonella Hadar, Salmonella Infantis, Salmonella Typhimurium, and Salmonella Senftenberg). Salmonella serovars introduced onto the flowers of growing plants were recovered on and within the developing tomato fruit. Of all the Salmonella serovars tested, Montevideo appeared to be more adapted to survival within tomatoes and was recovered from 90% of the fruit screened. All of the Salmonella serovars could persist and grow when introduced onto unripened (green) tomato fruit. In general, growth (internal and external) was promoted at the high incubation temperature (25 degrees C) and high relative humidity (95%), although this was serovar dependent. The growth and persistence of Salmonella introduced on and into ripened (red) tomatoes was serovar dependent. Salmonella serovars Enteritidis, Typhimurium, and Dublin were less adapted to grow in or on intact red tomatoes than were serovars Hadar, Montevideo, or Newport. The results illustrated that a diverse range of Salmonella serovars can become established within and/or on preharvest tomatoes. The majority of Salmonella can grow and become established both on and within unripened tomatoes, but growth on ripened fruit was serovar dependent. The results provide a possible explanation why only a narrow range of Salmonella serovars are associated with foodborne illness outbreaks linked to tomatoes.