Persistence and growth of different Salmonella serovars on pre- and postharvest tomatoes

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.

Mode of Salmonella and Escherichia coli O157:H7 inactivation by a stabilized oxychloro-based sanitizer

AIM

To determine the mechanisms by which a stabilized oxychloro (SOC)-based sanitizer, applied to decontaminate seeds destined for sprout production, inactivates Escherichia coli O157:H7 ph1 and Salmonella serotype Meleagridis.

MATERIALS AND RESULTS

The action of SOC on the metabolism, membrane and DNA integrity of Salmonella and E. coli O157:H7 was studied. In both pathogens, there was an oxidative burst and depletion of intracellular glutathione (GSH) upon initial exposure to 200 ppm SOC. Metabolic activity, measured via bioluminescence, decreased over a 4-h period in E. coli O157:H7 ph1 cells exposed to SOC. Membrane integrity, assessed through viability staining, decreased progressively over 23 h when exposed to SOC. The appearance of auxotrophic mutants suggested that DNA damage had also occurred. Enzymes rich in disulfide bonds (alkaline phosphatase and protease) were sensitive to the chlorite-based sanitizer. Through challenging other microbial types, it was found that Gram positive had higher tolerance to SOC than Gram negatives with the exception of Salmonella. MS2 bacteriophage was highly sensitive; however, Bacillus endospores were not inactivated by SOC.

CONCLUSIONS

SOC inactivates E. coli O157:H7 and Salmonella through GSH oxidation and disruption of disulfide bonds. Ultimately, membrane damage resulting from prolonged exposure to SOC leads to the loss of cell viability.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide a basis for understanding why extended treatment times are required to inactivate bacteria using SOC.

Inactivation of Escherichia coli O157:H7 and Salmonella on artificially or naturally contaminated mung beans (Vigna radiata L) using a stabilized oxychloro-based sanitizer

AIMS

To evaluate the efficacy of a stabilized oxychloro-based (SOC) sanitizer to decontaminate mung beans artificially or naturally contaminated with Escherichia coli O157:H7 or Salmonella.

METHODS AND RESULTS

Naturally contaminated beans were produced by introducing a five-strain cocktail of E. coli O157:H7 or Salmonella onto the flowers of growing mung bean plants. Escherichia coli O157:H7 was only sporadically recovered from sprout lots (three testing positive from 10 tested) derived from harvested beans. In contrast, Salmonella was recovered from 18 of 20 lots screened. Pathogens present on naturally contaminated seed could be successfully inactivated with SOC applied at 200 ppm for 24 h at 28 degrees C. SOC treatment could also decontaminate artificially inoculated mung bean batches containing different levels of contaminated seed. SOC inactivated E. coli O157:H7, but not Salmonella introduced onto damaged (scarified) beans.

CONCLUSIONS

SOC sanitizer could inactivate Salmonella or E. coli O157:H7 naturally or artificially introduced onto mung beans. However, the SOC treatment failed to inactivate Salmonella introduced onto damaged mung beans.

SIGNIFICANCE AND IMPACT OF THE STUDY

SOC sanitizer represents an effective method for decontaminating undamaged mung beans.

Inactivation of escherichia coli 0157:H7 and Salmonella on mung beans, alfalfa, and other seed types destined for sprout production by using an oxychloro-based sanitizer

The efficacy of a stabilized oxychloro-based food grade sanitizer to decontaminate seeds destined for sprout production has been evaluated. By using mung bean seeds as a model system, it was demonstrated that the sanitizer could be used to inactivate a five-strain cocktail of Escherichia coli O157:H7 or Salmonella introduced onto beans at 10(3) to 10(4) CFU/g. Salmonella was more tolerant to stabilized oxychloro than was E. coli O157:H7, with sanitizer levels of >150 and >50 ppm, respectively, being required to ensure pathogen-free sprouts. The decontamination efficacy was also found to be dependent on treatment time (>8 h optimal) and the seed-to-sanitizer ratio (>1:4 optimal). Stabilized oxychloro treatment did not exhibit phytotoxic effects, as germination and sprout yields were not significantly (P > 0.05) different as compared with untreated controls. Although human pathogens could be effectively eliminated from mung beans, the aerobic plate count of native microflora on sprouts grown from treated seed was not significantly (P > 0.05) different from the controls. The diversity of microbial populations (determined through 16S rRNA denaturing gradient gel electrophoresis analysis) associated with bean sprouts was not significantly affected by the sanitizer treatment. However, it was noted that Klebsiella and Herbasprillum (both common plant endophytes) were absent in sprouts derived from decontaminated seed but were present in control sprouts. When a further range of seed types was evaluated, it was found that alfalfa, cress, flax, and soybean could be decontaminated with the stabilized oxychloro sanitizer. However, the decontamination efficacy with other seed types was less consistent. It appears that the rate of seed germination and putative activity of sanitizer sequestering system(s), in addition to other factors, may limit the efficacy of the decontamination method.

Spatial distribution of Salmonella, Escherichia coli O157:H7, and other bacterial populations in commercial and laboratory-scale sprouting mung bean beds

The reliability of testing spent irrigation water to assess the microbiological status of sprouting mung bean beds has been investigated. In commercial trials, the distribution of opportunistic contaminants within 32 bean sprout beds (25 kg of mung beans per bin) was assessed 48 h after germination. The prevalence of generic Escherichia coli, thermotolerant coliforms, and Aeromonas in sprouts (n = 288) was 5, 11, and 39%, respectively, and 57, 70, and 79% in the corresponding spent irrigation water samples (n = 96). Contamination was heterogeneously distributed within the seedbed. In laboratory trials, beans inoculated with a five-strain cocktail of either Salmonella or E. coli O157:H7 (10(3) to 10(4) CFU/g) were introduced (1 g/500 g of noninoculated seeds) at defined locations (top, middle, or base), and the beans were then sprouted for 48 h. When seeds inoculated with pathogens were introduced at the base or top of the seedbed, the pathogens were typically restricted to these sites and resulted in 44% of the spent irrigation water samples returning false-negative results. Introducing inoculated beans into the middle or at the presoak stage enhanced the distribution of both pathogens within the subsequent sprout bed and resulted in comparable levels recovered in spent irrigation water. The study demonstrated that even though screening a single sample of spent irrigation water is more reliable than testing sprouts directly, it does not provide an accurate assessment of the microbiological status of sprouting mung bean beds. Such limitations may be addressed by ensuring that bean batches are mixed prior to use and by taking spent irrigation water samples from multiple sites at the latter stages of the sprouting process.

Silicon nitride thin films by low voltage reactive ion plating: optical properties and composition

Silicon nitride films were synthesized in a Balzers BAP 800 coating plant by low voltage reactive ion plating. Transparent, stoichiometric films free of hydrogen, oxygen, argon, and heavy metals were obtained. The properties of the silicon nitride films were investigated in terms of optical constants, chemical composition, and hydrogen content.