Influence of diluent on antimicrobial activity of cinnamon bark essential oil vapor against Staphylococcus aureus and Salmonella enterica on a laboratory medium and beef jerky

The influence of chemical diluents on the antimicrobial activity of plant essential oil (EO) vapors was evaluated. We first determined if vapors generated from 22 chemical diluents not containing EO had antimicrobial activities. Ethyl ether vapor retarded the growth of S. aureus. The minimal inhibitory concentrations (MICs) and the minimal lethal concentrations (MLCs) of cinnamon bark EO vapor, which was diluted in and generated from 21 diluents, against S. aureus and S. enterica were determined. Cinnamon bark EO vapor showed significantly (P ≤ 0.05) lower MICs against S. aureus when diluted in dimethyl sulfoxide (DMSO), ethanol, ethyl acetate, or jojoba oil, and against S. enterica when diluted in DMSO, ethanol, or jojoba oil, compared to those in other diluents. We compared antimicrobial activities of cinnamon bark EO vapor diluted in DMSO, ethanol, ethyl acetate, or jojoba oil against S. aureus and S. enterica on beef jerky as a food model. Antimicrobial activity was significantly (P ≤ 0.05) higher when vaporized from DMSO. These results indicate that antimicrobial activity of cinnamon bark EO vapor may vary significantly (P ≤ 0.05) depending on the type of diluent from which it is vaporized. These observations provide basic information when developing food and food-contact surface decontamination strategies using EO vapors.

Effects of temperature, pH, and sodium chloride on antimicrobial activity of magnesium oxide nanoparticles against Escherichia coli O157:H7

AIM

This study was done to determine the effects of temperature, pH, and sodium chloride (NaCl) on antimicrobial activity of magnesium oxide (MgO) nanoparticles (NPs) against Escherichia coli O157:H7.

METHODS AND RESULTS

Culture conditions were established by varying the pH (5.0, 7.2, and 9.0), NaCl concentration (0.5, 2.0, 3.5, and 5.0%, w/v), and incubation temperatures (4, 12, 22, and 37°C). At each condition, the antimicrobial activities of MgO-NPs (0, 1, 2, and 4 mg/mL) against E. coli O157:H7 were measured. Four-way analysis of variance indicated interactions among all factors had a significant effect (P ≤ 0.05) on the antimicrobial activity of MgO-NPs. The concentration of MgO-NPs necessary to cause a 5-log reduction of E. coli O157:H7 under the most inhibitory conditions (37°C, pH 9.0, and 5.0% NaCl) was 0.50 mg/mL of MgO-NPs.

CONCLUSION

The antimicrobial activity of the MgO-NPs increased significantly (P ≤ 0.05) with increased temperature, pH, and NaCl concentration in TSB.

SIGNIFICANCE AND IMPACT OF THE STUDY

The influence of intrinsic and extrinsic factors on antimicrobial activity of MgO-NPs we found will contribute to the development of microbial decontamination strategies using MgO in the food industry.

Antimicrobial activities of plant essential oil vapours against Acidovorax citrulli and Xanthomonas campestris on Cucurbitaceae, Brassicaceae and Solanaceae seeds

AIM

This study was done to develop a seed decontamination treatment for organic seeds against plant pathogens (Acidovorax citrulli and Xanthomonas campestris) using essential oil (EO) vapours without affecting the seeds' germination rate.

METHODS AND RESULTS

By using a diffusion assay and determining minimum inhibitory and lethal concentrations, we screened two EO vapours which were most inhibitory to A. citrulli (cinnamon bark and garlic EO vapours) and X. campestris (onion and garlic EO vapours). After 48 h of exposure to EO vapours at 25°C and 43% or 85% relative humidity (RH), no significant decrease (p > 0.05) in germination rates was observed compared with those of control seeds. It was observed that EO vapour treatment at 25°C and 43% or 85% RH for 48 h caused significant population reductions (p ≤ 0.05) (ca. 0.3-2.6 log colony forming unit/g) compared to those of untreated seeds.

CONCLUSION

Applications of EO vapours showed significant (p ≤ 0.05) antimicrobial effects against A. citrulli and X. campestris on both laboratory mediums and plant seeds without decreasing the germination rate of seeds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information for the development of natural seed sterilization treatments using EO vapours.

Survival of Salmonella and Shiga Toxin-Producing Escherichia coli and Changes in Indigenous Microbiota during Fermentation of Home-Brewed Kombucha

ABSTRACT

Survival and growth of Salmonella and Shiga toxin-producing Escherichia coli (STEC) were investigated in kombucha prepared from four brands of commercially available kombucha kits intended for use by home brewers. Changes in populations of the indigenous microbiota responsible for fermentation of kombucha were also determined. An initial population of Salmonella (6.77 log CFU/mL) decreased to below the detection limit (0.30 log CFU/mL) within 10 days in kombucha prepared from two of the test brands. Populations of 1.85 and 1.20 log CFU/mL were detected in two brands fermented for 14 days. An initial STEC population of 7.02 log CFU/mL decreased to <0.30 log CFU/mL in two brands within 14 days; 0.77 and 0.87 log CFU/mL were detected in kombucha prepared from the other two brands. Salmonella and STEC increased within 1 day in three brands of base tea used to prepare kombucha and were stable throughout 14 days of incubation. Both pathogens steadily declined in base tea prepared from one brand of kombucha kit. Inactivation of the pathogens occurred as the pH of the kombucha decreased, but a clear correlation between rates of inactivation among different brands of kits and decrease in pH was not evident. Growth and peak populations of mesophilic aerobic microorganisms, yeasts, lactic acid bacteria, and acetic acid bacteria varied depending on the kombucha kit brand. No strong evidence was found of a correlation between the behavior of Salmonella or STEC and that of any of these groups of indigenous microbiota. Results of this study show that survival of Salmonella and STEC in kombucha and base tea used to prepare kombucha is dependent on inherent differences in commercially available kombucha kits intended for use in home settings. Strict application of hygienic practices is essential for preventing contamination with Salmonella or STEC and reducing the risk of illness associated the consumption of kombucha.

HIGHLIGHTS

Inactivation of Escherichia coli O157:H7 on radish and cabbage seeds by combined treatments with gaseous chlorine dioxide and heat at high relative humidity

This study was done to develop a method to inactivate Escherichia coli O157:H7 on radish and cabbage seeds using simultaneous treatments with gaseous chlorine dioxide (ClO₂) and heat at high relative humidity (RH) without decreasing seeds' viability. Gaseous ClO₂ was spontaneously vaporized from a solution containing hydrochloric acid (HCl, 1 N) and sodium chlorite (NaClO₂, 100,000 ppm). Using a sealed container (1.8 L), an equation (y = 5687×, R² = 0.9948) based on the amount of gaseous ClO₂ generated from HCl-NaClO₂ solution at 60 °C and 85% RH was developed. When radish or cabbage seeds were exposed to gaseous ClO₂ at concentrations up to 3,000 ppm for 120 min, germination rates did not significantly decrease (P > 0.05). When seeds inoculated with E. coli O157:H7 were treated with 2,000 or 3,000 ppm of gaseous ClO₂ in an atmosphere with 85% RH at 60 °C, populations (6.8-6.9 log CFU/g) on both types of seeds were decreased to below the detection limit for enrichment (-0.5 log CFU/g) within 90 min. This study provides useful information for developing a decontamination method to control E. coli O157:H7 and perhaps other foodborne pathogens on plant seeds by simultaneous treatment with gaseous ClO₂ and heat at high RH.

Synergistic activities of gaseous oregano and thyme thymol essential oils against Listeria monocytogenes on surfaces of a laboratory medium and radish sprouts

We investigated combinations of gaseous essential oils (EO gases) for their synergistic inhibitory activities against Listeria monocytogenes on a laboratory medium and radish sprouts. The minimum inhibitory concentrations and minimum lethal concentrations of oregano, thyme thymol, and cinnamon bark EO gases against L. monocytogenes were 0.0781 μL/mL on nutrient agar supplemented with glucose and bromocresol purple (NGBA). A checkerboard assay showed that combinations of oregano and thyme thymol EO gases and of oregano and cinnamon bark EO gases exert the strongest synergistic antilisterial activity (fractional inhibitory concentration index [FICI] = 0.3750). A combination of thyme thymol and cinnamon bark EO gases also had a synergistic effect (FICI = 0.5000) on L. monocytogenes on NGBA. Combinations of oregano and thyme thymol EO gases were tested for synergistic antimicrobial activity against L. monocytogenes on radish sprouts. A combination of these gases, each at 0.313 μL/mL, caused a significant (P ≤ 0.05) reduction in the number of L. monocytogenes on radish sprouts compared with reductions caused by treatment with oregano or thyme thymol EO gas alone at the same concentration. Our findings provide information that will be useful when developing antimicrobial applications using EO gases to control L. monocytogenes in the food industry.

Development of a decontamination method to inactivate Acidovorax citrulli on Cucurbitaceae seeds without loss of seed viability

BACKGROUND

Acidovorax citrulli is a plant pathogen causing bacterial fruit blotch in Cucurbitaceae family. Applying high concentration of disinfectants to seeds containing plant pathogen may substantially decrease the germination rate of seeds. Therefore, it is necessary to develop a hurdle technology which can inactivate plant pathogens without decreasing seed viability. This study was conducted to develop a decontamination method to inactivate the plant pathogen Acidovorax citrulli on Cucurbitaceae seeds by sequential treatments with aqueous chlorine dioxide (ClO₂ ), drying, and dry heat.

RESULTS

The maximum ClO₂ concentration that did not lower germination rates of cucumber, honeydew melon, and watermelon seeds was ca. 100 μg mL^-1 of ClO₂ for 5 min. Optimal incubation conditions for drying seeds that had been treated with aqueous ClO₂ were determined as 25 °C and 43% relative humidity (RH) for 48 h. The maximum dry-heat temperature that did not reduce germination rates of seeds, which had been treated with ClO₂ and dried at 25 °C, was 60 °C at 43% RH for 24 h. When seeds containing A. citrulli (6.4-7.0 log CFU g^-1 ) were treated with aqueous ClO₂ (50 μg mL^-1 , 5 min), dried (25 °C, 43% RH, 24 h), and dry heated (60 °C, 43% RH, 24 h), the pathogen was inactivated to below the detection limit from all three seed types (<-0.5 log CFU g^-1 ).

CONCLUSION

The decontamination conditions to inactivate A. citrullii from Cucurbitaceae seeds without decreasing the seed viability were determined (sequential treatment with ClO₂ [50 μg mL^-1 , 5 min], dried [25 °C, 43% RH, 24 h], and dry heated [60 °C, 43% RH, 24 h]). The results of this study may also be applicable to other plant pathogens on other types of seeds. © 2019 Society of Chemical Industry.

Inhibition of Listeria monocytogenes using biofilms of non-pathogenic soil bacteria (Streptomyces spp.) on stainless steel under desiccated condition

Of the 1648 microbial isolates from 133 soil samples collected from 30 diverse locations in the Republic of Korea, two isolates exhibited strong antilisterial activity and ability to grow to high populations (>8.0 log CFU/ml) in Bennett's broth. Isolates were identified as Streptomyces lactacystinicus (strain Samnamu 5-15) and Streptomyces purpureus (strain Chamnamu-sup 4-15). Both isolates formed biofilms on the surface of stainless steel coupons (SSCs) immersed in Bennett's broth within 24 h at 25 °C. Cells retained antilisterial activity after biofilm formation and showed significantly (P ≤ 0.05) enhanced resistance against dry conditions (43% relative humidity [RH]) compared to the cells not in biofilm. An initial population (ca. 3.2 log CFU/cm²) of Listeria monocytogenes inoculated on SSCs lacking Streptomyces biofilm decreased to 1.4 log CFU/cm² within 48 h at 25 °C and 43% RH. In contrast, L. monocytogenes (3.3 log CFU/cm²) inoculated on SSCs containing Streptomyces biofilm decreased to populations below the theoretical detection limit (0.5 log CFU/cm²) within 24 h. The results indicate that biofilms formed by the Streptomyces spp. inhibitory to L. monocytogenes showed enhanced resistance to desiccation condition (43% RH) and effectively inhibited the growth of L. monocytogenes on the surfaces of SSCs. Antilisterial biofilms developed in this study may be applicable on desiccated environmental surfaces in food related environments such as food storage, handling, and processing facilities to enhance the microbiological safety of foods.

Inhibition of Escherichia coli O157:H7 on stainless steel using Pseudomonas veronii biofilms

We produced a Pseudomonas veronii biofilm on the surface of a stainless steel that is inhibitory to Escherichia coli O157:H7. Pseudomonas veronii strain KACC 81051BP, isolated from lettuce, readily formed biofilm on the surface of stainless steel coupons (SSCs) immersed in tryptic soy broth at 25°C. Cells showed significantly (P ≤ 0·05) enhanced tolerance to desiccation stress (43% relative humidity (RH)) and retained antimicrobial activity against E. coli O157:H7. The number of E. coli O157:H7 (control; 4·1 ± 0·1 log CFU per coupon) on sterile SSCs decreased to 2·7 ± 0·2 log CFU per coupon after exposure to 43% RH at 25°C for 48 h, while the population of E. coli O157:H7 (4·1 ± 0·0 log CFU per coupon) on SSCs containing P. veronii biofilm decreased to below the theoretical detection limit (1·5 log CFU per coupon) within 24 h. The antimicrobial biofilm produced on stainless steel may have application in preventing cross-contamination by E. coli O157:H7 on other abiotic surfaces in food-contact environments.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of Escherichia coli O157:H7 on environmental surfaces of food manufacturing, transportation and storage facilities is a significant food safety concern because it can result in cross-contamination of food products. In this study, we developed a Pseudomonas veronii biofilm on the surface of a stainless steel that inhibits the growth of E. coli O157:H7. Since P. veronii in biofilm resists desiccation, it provides persistent antimicrobial activity. Information presented here provides novel and practical insights to developing biological strategies to inactivate E. coli O157:H7 on diverse surfaces in food processing and handling environments.

Retention of Viability of Salmonella in Sucrose as Affected by Type of Inoculum, Water Activity, and Storage Temperature

Outbreaks of salmonellosis have been associated with consumption of high-sugar, low-water activity (a_w) foods. The study reported here was focused on determining the effect of storage temperature (5 and 25°C) on survival of initially high and low levels of Salmonella in dry-inoculated sucrose (a_w 0.26 ± 0.01 to 0.54 ± 0.01) and wet-inoculated sucrose (a_w 0.24 ± 0.01 to 0.44 ± 0.04) over a 52-week period. With the exception of dry-inoculated sucrose at a_w 0.26, Salmonella survived for 52 weeks in dry- and wet-inoculated sucrose stored at 5 and 25°C. Retention of viability was clearly favored in sucrose stored at 5°C compared with 25°C, regardless of level or type of inoculum or a_w. Survival at 5°C was not affected by a_w. Initial high-inoculum counts of 5.18 and 5.25 log CFU/g of dry-inoculated sucrose (a_w 0.26 and 0.54, respectively) stored for 52 weeks at 5°C decreased by 0.56 and 0.53 log CFU/g; counts decreased by >4.18 and >4.25 log CFU/g in samples stored at 25°C. Inactivation rates in wet-inoculated sucrose were similar to those in dry-inoculated sucrose; however, a trend toward higher persistence of Salmonella in dry- versus wet-inoculated sucrose suggests there was a higher proportion of cells in the wet inoculum with low tolerance to osmotic stress. Survival patterns were similar in sucrose initially containing a low level of Salmonella (2.26 to 2.91 log CFU/g). The pathogen was recovered from low-inoculated sucrose stored at 5°C for 52 weeks regardless of type of inoculum or a_w and from dry-inoculated sucrose (a_w 0.54) and wet-inoculated sucrose (a_w 0.24) stored at 25°C for 12 and 26 weeks, respectively. Results emphasize the importance of preventing contamination of sucrose intended for use as an ingredient in foods not subjected to a treatment that would be lethal to Salmonella.

Comparison of New and Traditional Culture-Dependent Media for Enumerating Foodborne Yeasts and Molds

Fifty-six foods and food ingredients were analyzed for populations of naturally occurring yeasts and molds using Petrifilm rapid yeast and mold (RYM) count plates, Petrifilm yeast and mold (YM) count plates, dichloran rose bengal chloramphenicol (DRBC) agar plates, acidified potato dextrose agar (APDA) plates, and dichloran 18% glycerol (DG18) agar plates. Colonies were counted after incubating plates for 48, 72, and 120 h at 25°C. Of 56 foods in which either yeasts or molds were detected on at least one medium incubated for 120 h, neither yeasts nor molds were detected in 55.4, 73.2, 21.4, 19.6, and 71.4% of foods plated on the five respective media and incubated for 48 h; 10.7, 14.3, 3.6, 1.8, and 19.6% of foods were negative after 72 h, and 3.6, 1.8, 0, 0, and 0% of foods were negative after 120 h. Considering all enumeration media, correlation coefficients were 0.03 to 0.97 at 48 h of incubation; these values increased to 0.75 to 0.99 at 120 h. Coefficients of variation for total yeasts and molds were as high as 30.0, 30.8, and 27.2% at 48, 72, and 120 h, respectively. The general order of performance was DRBC = APDA > RYM Petrifilm > YM Petrifilm ≥ DG18 when plates were incubated for 48 h, DRBC > APDA > RYM Petrifilm > YM Petrifilm ≥ DG18 when plates were incubated for 72 h, and DRBC > APDA > RYM Petrifilm > YM Petrifilm > DG18 when plates were incubated for 120 h. Differences in performance among media are attributed to the diversity of yeasts and molds likely to be present in test foods and differences in nutrient, pH, and water activity requirements for resuscitation of stressed cells and colony development.

Survival of Salmonella in Cookie and Cracker Sandwiches Containing Inoculated, Low-Water Activity Fillings

A study was done to determine the rate of inactivation of Salmonella in cookie and cracker snack sandwiches. Two cookie bases (chocolate and vanilla) and cheese crackers, along with high-sugar chocolate and peanut butter-based crème cookie fillings and peanut butter- and cheese-based cracker fillings, were obtained from commercial sources. Fillings and sandwiches containing fillings that had been dry- or wet-inoculated with Salmonella were stored at 25°C for 1, 6, 21, 35, 70, 112, and 182 days (6 months). At initial populations of 3.4 and 3.6 log CFU/g of cookie sandwiches containing chocolate crème and peanut butter crème fillings, respectively, Salmonella survived for at least 182 days; initially at 0.36 log CFU/g, the pathogen survived for at least 35 and 70 days. Initially at 2.9 and 3.4 log CFU/g of cracker sandwiches containing peanut butter- and cheese-based fillings, respectively, Salmonella survived for at least 182 and 112 days; initially at 0.53 log CFU/g, the pathogen survived for at least 6 and 35 days. Inactivation of Salmonella was more rapid in wet-inoculated peanut butter crème cookie filling than in dry-inoculated filling but was less affected by type of inoculum in peanut butter-based cracker filling. Chocolate cookie base (water activity [aw] 0.39) and chocolate crème filling (aw 0.30) components of sandwiches equilibrated to aw 0.38 within 15 days at 25°C; vanilla cookie base (aw 0.21) and peanut butter-based crème filling (aw 0.27) equilibrated to aw 0.24 between 50 and 80 days. Cheese cracker (aw 0.14) and peanut butter-based filling (aw 0.31) or cheese-based filling (aw 0.33) components of sandwiches equilibrated to aw 0.33 in 80 days. The ability of Salmonella to survive for at least 182 days in fillings of cookie and cracker sandwiches demonstrates a need to assure that filling ingredients do not contain the pathogen and that contamination does not occur during manufacture.

Behaviour of Aspergillus flavus and Fusarium graminearum on rice as affected by degree of milling, temperature, and relative humidity during storage

We investigated the survival and growth patterns of Aspergillus flavus and Fusarium graminearum, as well as mycotoxin production, on Korean rice as affected by the degree of milling (rough, brown, and white rice) and storage conditions (21 °C/85% relative humidity [RH], 21 °C/97% RH, and 30 °C/85% RH). When rice was stored at 21 °C/85% RH, the population of A. flavus remained constant and aflatoxin was not produced, regardless of the degree of milling. At 21 °C/97% RH and 30 °C/85% RH, the populations of A. flavus increased significantly (P ≤ 0.05) and aflatoxins were produced. The highest population of A. flavus and highest amount of aflatoxin B1 were observed on brown rice stored at 21 °C/97% RH. For F. graminearum, when stored at 85% RH, the populations were reduced to less than a detectable level (5 CFU/g of rice) within 120 days and no deoxynivalenol (DON) was produced, regardless of the degree of milling and storage temperature. However, at 21 °C/97% RH, the population of F. graminearum increased significantly (P ≤ 0.05) and DON was produced on all types of rice. Findings from this study provide insights concerning storage conditions necessary to prevent growth and mycotoxin production by A. flavus and F. graminearum on Korean rice with different degrees of milling.

Comparison of the microbiological quality of environmentally friendly and conventionally grown vegetables sold at retail markets in Korea

Fresh produce is usually eaten raw without cooking or heating, which may increase the probability of foodborne infection. The microbiological quality of 11 types of fresh, raw vegetables (romaine lettuce, sesame leaves, crown daisy, garlic chives, iceberg lettuce, cabbage, broccoli, leek, chili pepper, capsicum, and zucchini) purchased at retail markets in Iksan, Korea as affected by cultivation method (environmentally friendly vegetables [organic, pesticide-free, and low-pesticide vegetables] and conventionally grown vegetables) and harvest season was determined. Escherichia coli O157:H7 and Salmonella were not detected in all samples of vegetables tested. Aerobic mesophiles (>6 log cfu/g) were detected in environmentally friendly romaine lettuce and crown daisy and environmentally friendly and conventionally grown garlic chives, which also contained coliforms (>3 log cfu/g). Sesame leaf and crown daisy (regardless of cultivation method), as well as conventionally grown romaine lettuce and leek, contained >1 log cfu/g of E. coli. The overall microbiological quality of environmentally friendly and conventionally grown vegetables was not significantly different (P > 0.05). However, there were seasonal effects on populations of coliforms and generic E. coli on vegetables. The greatest numbers of microorganisms were isolated from environmentally friendly or conventionally grown vegetables purchased in winter. The vegetables, regardless of cultivation method or season, should be subjected to appropriate antimicrobial treatment to enhance their microbial safety.

Survival of salmonella on dried fruits and in aqueous dried fruit homogenates as affected by temperature

A study was done to determine the ability of Salmonella to survive on dried cranberries, raisins, and strawberries and in date paste, as affected by storage temperature. Acid-adapted Salmonella, initially at 6.57 to 7.01 log CFU/g, was recovered from mist-inoculated cranberries (water activity [aw] 0.47) and raisins (aw 0.46) stored at 25°C for 21 days but not 42 days, strawberries (aw 0.21) for 42 days but not 84 days, and date paste (aw 0.69) for 84 days but not 126 days. In contrast, the pathogen was detected in strawberries stored at 4°C for 182 days (6 months) but not 242 days (8 months) and in cranberries, date paste, and raisins stored for 242 days. Surface-grown cells survived longer than broth-grown cells in date paste. The order of rate of inactivation at 4°C was cranberry > strawberry > raisin > date paste. Initially at 2.18 to 3.35 log CFU/g, inactivation of Salmonella on dry (sand)&ndash inoculated fruits followed trends similar to those for mist-inoculated fruits. Survival of Salmonella in aqueous homogenates of dried fruits as affected by fruit concentration and temperature was also studied. Growth was not observed in 10% (aw 0.995 to 0.999) and 50% (aw 0.955 to 0.962) homogenates of the four fruits held at 4°C, 50% homogenates at 25°C, and 10% cranberry and strawberry homogenates at 25°C. Growth of the pathogen in 10% date paste and raisin homogenates stored at 25°C was followed by rapid inactivation. Results of these studies suggest the need to subject dried fruits that may be contaminated with Salmonella to a lethal process and prevent postprocess contamination before they are eaten out-of-hand or used as ingredients in ready-to-eat foods. Observations showing that Salmonella can grow in aqueous homogenates of date paste and raisins emphasize the importance of minimizing contact of these fruits with high-moisture environments during handling and storage.

Fate of Escherichia coli O157:H7 and Salmonella in soil and lettuce roots as affected by potential home gardening practices

BACKGROUND

The survival and distribution of enteric pathogens in soil and lettuce systems were investigated in response to several practices (soil amendment supplementation and reduced watering) that could be applied by home gardeners.

RESULTS

Leaf lettuce was grown in manure compost/top soil (0:5, 1:5 or 2:5 w/w) mixtures. Escherichia coli O157:H7 or Salmonella was applied at a low or high dose (10(3) or 10(6) colony-forming units (CFU) mL(-1) ) to the soil of seedlings and mid-age plants. Supplementation of top soil with compost did not affect pathogen survival in the soil or on root surfaces, suggesting that nutrients were not a limiting factor. Salmonella populations on root surfaces were 0.7-0.8 log CFU g(-1) lower for mid-age plants compared with seedlings. E. coli O157:H7 populations on root surfaces were 0.8 log CFU g(-1) lower for mid-age plants receiving 40 mL of water compared with plants receiving 75 mL of water on alternate days. Preharvest internalization of E. coli O157:H7 and Salmonella into lettuce roots was not observed at any time.

CONCLUSION

Based on the environmental conditions and high pathogen populations in soil used in this study, internalization of Salmonella or E. coli O157:H7 into lettuce roots did not occur under practices that could be encountered by inexperienced home gardeners.

Postharvest handling conditions affect internalization of Salmonella in baby spinach during washing

Internalization of foodborne pathogens in fruits and vegetables is an increasing safety concern. The aim of this research was to assess the potential for internalization of an enteric pathogen (Salmonella enterica serotype Typhimurium) in a leafy vegetable (baby spinach) during washing as influenced by three postharvest handling conditions: (i) illumination, (ii) negative temperature differential, and (iii) relative humidity (RH). To compare these potential postharvest handling conditions, leaves were exposed to different levels of illumination (0, 1,000, and 2,000 lx), temperature differential (5, 11, 14, 20, and 26°C), and RH (99, 85, and 74%) for a short time before or during washing. Washing of baby spinach was carried out in water containing green fluorescent protein-tagged Salmonella Typhimurium (6.5 log CFU/ml) at 5°C for 2 min, followed by surface disinfection with chlorine (10,000 μg/ml) for 1 min, two rinses in water for 10 s, and spin drying for 15 s. Internalization was assessed by enumerating the pathogen on Salmonella-Shigella agar and by confocal laser scanning microscopy. Illumination of spinach leaves before and during washing and a negative temperature differential during washing did not significantly (P > 0.05) increase the number of internalized bacteria. However, exposure of leaves to low-RH conditions before washing, which reduced the tissue water content, decreased internalization of Salmonella compared with internalization in baby spinach exposed to high RH (P ≤ 0.05). Green fluorescent protein-tagged Salmonella Typhimurium was visualized by confocal laser scanning microscopy at a depth of up to 30 m m beneath the surface of spinach leaves after exposure to a high inoculum level (8 log CFU/ml) for an extended time (2 h). Results show that internalization of Salmonella into baby spinach leaves can occur but can be minimized under specific postharvest handling conditions such as low RH.

Efficacy of sanitizers in reducing Salmonella on pecan nutmeats during cracking and shelling

Studies were done to evaluate the efficacy of chlorine (200 to 1,000 μg/ml), lactic acid (0.5 to 2%), levulinic acid (0.5 to 2%), sodium dodecyl sulfate (SDS, 0.05%), lactic acid plus SDS, levulinic acid plus SDS, and a mixed peroxyacid sanitizer (Tsunami 200, 40 and 80 μg/ml) in killing Salmonella on or in immersion- and on surface-inoculated pecan nutmeats (U.S. Department of Agriculture medium pieces and mammoth halves). The addition of SDS to treatment solutions containing lactic acid or levulinic acid resulted in generally higher reductions of Salmonella, but differences in these reductions were not always significant. Lactic and levulinic acids (2%) containing SDS (0.05%) were equivalent in killing Salmonella on immersion-inoculated nutmeats. Tsunami 200 (40 μg/ml) was less lethal or equivalent to 1 or 2% lactic and levulinic acids, with or without 0.05% SDS. Reductions did not exceed 1.1 log CFU/g of immersion-inoculated pieces and halves, regardless of sanitizer concentration or treatment time (up to 20 min). Reductions on surface-inoculated pieces and halves were 0.7 to 2.6 log CFU/g and 1.2 to 3.0 log CFU/g, respectively. Treatment with 2% lactic acid plus SDS (0.05%) and Tsunami (80 μg/ml) was most effective in killing Salmonella on surface-inoculated pieces; treatment of halves with chlorine (1,000 μg/ml) or lactic acid (1 or 2%), with or without SDS, was most efficacious. Exposure of immersion-inoculated pecan pieces to chlorine (200 μg/ml), lactic acid (2%) and levulinic acid (2%) with or without SDS, and Tsunami (80 μg/ml) during intermittent vacuum (18 ± 2 mbar) and ambient atmospheric pressure treatments for up to 20 min reduced Salmonella by only 0.1 to 1.0 log CFU/g. These studies emphasize the importance of preventing contamination of pecan nutmeats with Salmonella. Once nuts are contaminated, the lethality of sanitizers tested in this study is minimal.

Low-water activity foods: increased concern as vehicles of foodborne pathogens

Foods and food ingredients with low water activity (a(w)) have been implicated with increased frequency in recent years as vehicles for pathogens that have caused outbreaks of illnesses. Some of these foodborne pathogens can survive for several months, even years, in low-a(w) foods and in dry food processing and preparation environments. Foodborne pathogens in low-a(w) foods often exhibit an increased tolerance to heat and other treatments that are lethal to cells in high-a(w) environments. It is virtually impossible to eliminate these pathogens in many dry foods or dry food ingredients without impairing organoleptic quality. Control measures should therefore focus on preventing contamination, which is often a much greater challenge than designing efficient control measures for high-a(w) foods. The most efficient approaches to prevent contamination are based on hygienic design, zoning, and implementation of efficient cleaning and sanitation procedures in the food processing environment. Methodologies to improve the sensitivity and speed of assays to resuscitate desiccated cells of foodborne pathogens and to detect them when present in dry foods in very low numbers should be developed. The goal should be to advance our knowledge of the behavior of foodborne pathogens in low-a(w) foods and food ingredients, with the ultimate aim of developing and implementing interventions that will reduce foodborne illness associated with this food category. Presented here are some observations on survival and persistence of foodborne pathogens in low-a(w) foods, selected outbreaks of illnesses associated with consumption of these foods, and approaches to minimize safety risks.

Development of a random genomic DNA microarray for the detection and identification of Listeria monocytogenes in milk

We developed a DNA microarray that contains random genomic DNA fragments of Listeria monocytogenes, validated its diagnostic abilities using cells grown in laboratory media and milk, and established enrichment conditions for detection of a low population of L. monocytogenes in milk. Genomic DNA of L. monocytogenes strain ATCC 19111 was fractionated by agarose gel electrophoresis after being cleaved using several different pairs of restriction enzymes. Sixty DNA fragments of different sizes were randomly selected and spotted onto an amine-coated glass slide. To validate diagnostic ability, probes on the DNA microarray were hybridized with genomic DNA extracted from L. monocytogenes, other Listeria spp., and foodborne pathogenic bacteria belonging to other genera grown in laboratory media. The DNA microarray showed 98-100% positive hybridization signals for the 16 strains of L. monocytogenes tested, 7-85% positive signals for 9 strains of other Listeria spp., and 0-32% positive signals for 13 strains of other types of foodborne pathogens. In milk, the detection limit of the DNA microarray was approximately 8 log CFU/mL. When milk contained L. monocytogenes (3-4 log CFU/mL) with other types of bacteria (Bacillus spp., B. cereus, Salmonella Montevideo, Peudomonas aeruginosa, and Yersinia enterocolitica; ca. 3 log CFU/mL each), L. monocytogenes enriched in UVM modified Listeria enrichment broth at 37°C for 24h was successfully detected by the DNA microarray. Results indicate that the DNA microarray can detect L. monocytogenes and distinguish it from other Listeria spp. and other foodborne pathogens in laboratory media and milk. This platform will be useful when developing a DNA microarray to rapidly and simultaneously detect and identify various foodborne pathogens in foods.

Evaluation of sanitizers for inactivating Salmonella on in-shell pecans and pecan nutmeats

Chlorine, organic acids, and water extracts of inedible pecan components were tested for effectiveness in killing Salmonella on pecans. In-shell pecans and nutmeats (U.S. Department of Agriculture medium pieces) were immersion inoculated with a mixture of five Salmonella serotypes, dried to 3.7% moisture, and stored at 4°C for 3 to 6 weeks. In-shell nuts were immersed in chlorinated water (200, 400, and 1,000 μg/ml), lactic acid (0.5, 1, and 2%), and levulinic acid (0.5, 1, and 2%) with and without 0.05% sodium dodecyl sulfate (SDS), and a mixed peroxyacid sanitizer (Tsunami 200, 40 μg/ml) for up to 20 min at 21°C. The rate of reduction of free chlorine in conditioning water decreased as the ratio of in-shell nuts/water was increased. The rate of reduction was more rapid when nuts were not precleaned before treatment. The initial population of Salmonella on in-shell nuts (5.9 to 6.3 log CFU/g) was reduced by 2.8 log CFU/g after treating with chlorinated water (1,000 μg/ml). Treatment with 2% lactic acid plus SDS or 2% levulinic acid plus SDS reduced the pathogen by 3.7 and 3.4 log CFU/g, respectively. Lactic and levulinic acids (2%) without SDS were less effective (3.3- and 2.1-log CFU/g reductions, respectively) than acids with SDS. Treatment with Tsunami 200 resulted in a 2.4-log CFU/g reduction. In-shell nuts and nutmeats were immersed in water extracts of ground pecan shucks (hulls), shells, a mixture of shells and pith, and pith. The general order of lethality of extracts to Salmonella was shuck < shell-pith ≤ shell ≤ pith < chlorine (400 μg/ml) and shuck < shell ≤ pith = shell-pith < chlorine (400 μg/ml). Results emphasize the importance of removing soil and dust on in-shell pecans before conditioning in chlorinated water and the need for sanitizers with increased effectiveness in killing Salmonella on pecans.

Viability of Salmonella and Listeria monocytogenes in delicatessen salads and hummus as affected by sodium content and storage temperature

A study was conducted to determine survival and growth behavior of Salmonella and Listeria monocytogenes in commercially prepared mayonnaise-based potato salad, macaroni salad, and coleslaw and in hummus (initial mean pH values were 4.80 to 4.94, 4.18 to 4.31, 3.87, and 4.50 to 4.52, respectively) as affected by sodium concentration (133 to 364, 190 to 336, 146 to 272, and 264 to 728 mg/100 g, respectively) and storage at 4 or 10°C for up to 27 days. Salmonella did not grow in any of the test products. Initial populations (2.02 to 2.38 log CFU/g) decreased in coleslaw to undetectable levels (<1 CFU/25 g) within 13 days and in most formulations of macaroni salad within 20 to 27 days. Salmonella survived in highest numbers in potato salad and hummus. The presence of added sodium in macaroni salad stored at 4°C and hummus stored at 4 or 10°C appeared to protect Salmonella against inactivation. L. monocytogenes, at an initial population of 1.86 to 2.23 log CFU/g, did not grow in test products, but with the exception of coleslaw containing sodium at a concentration used in the standard (control) recipe, this pathogen was detected by direct plating (≥ 1.0 log CFU/g) in all products stored at 4 or 10°C for 27 days. L. monocytogenes populations were significantly (P < 0.05) lower in potato salad and hummus with no added sodium than in test products with added sodium after storage at 4°C. Sodium concentration did not markedly affect aerobic plate counts over the 27-day storage period. Results confirm that the acidic pH of mayonnaise-based salads and hummus is a major factor preventing growth and influencing rates of inactivation of Salmonella and L. monocytogenes. In the absence of added sodium, death of these bacteria may be more rapid. However, in general decreasing or increasing the sodium concentration in selected delicatessen salad and hummus recipes does not markedly affect the behavior of Salmonella and L. monocytogenes when products are stored at 4 or 10°C for up to 27 days.

Controlled fermentation of kimchi using naturally occurring antimicrobial agents

Kimchi is a traditional Korean fermented food. Since it ferments continuously during distribution and storage, the extension of shelf life by preventing over-acidification is a major concern in the kimchi industry. One of the most frequently attempted ways to delay fermentation is to add naturally occurring antimicrobial agents. Many researchers have investigated ways to delay over-acidification by adding minor ingredients, fruits or fruit seed extracts, extracts of medicinal herbs, culinary herbs and spices, and other miscellaneous substances to kimchi. The addition of naturally occurring antimicrobial agents may enhance the acceptability of kimchi to consumers over a longer period of time but may also have a disadvantage in that it may cause changes in sensory quality, especially if added in large amounts. To avoid undesirable sensory changes, application of hurdle technologies (i.e., multifactor preservative systems) which involve using combinations of low amounts of various naturally occurring antimicrobial agents as ingredients should be explored with the goal of controlling fermentation. If synergistic or additive antimicrobial effects can be achieved using small amounts of a combination of natural agents, changes in sensory qualities will be minimized, thereby prolonging shelf life. Research findings summarized in this review provide a basis for developing effective hurdle technologies using naturally occurring antimicrobial agents to extend shelf life of kimchi and perhaps other types of traditional fermented foods.

An outbreak of Salmonella serotype Typhimurium infections with an unusually long incubation period

A 1998 investigation of an outbreak of Salmonella serotype Typhimurium infections among children tasting unpasteurized milk during tours of a dairy farm demonstrated a distribution of unusually long incubation periods (median, 8 days; interquartile range [IQR], 6-14 days). Bacterial isolates were highly acid tolerant and contained genes associated with protection against destructive phagocytic reactive oxygen intermediates. We hypothesize that exposure to low-dose oral inoculum of a pathogen with these properties could have contributed to cases of non-typhoidal salmonellosis with the longest incubation period reported to the Centers for Disease Control and Prevention (CDC).

Inactivation of Salmonella on pecan nutmeats by hot air treatment and oil roasting

Studies were done to determine the effectiveness of hot air drying, dry roasting, and oil roasting in killing Salmonella on pecan nutmeats. Pecan halves and pieces were inoculated by immersion in a five-serotype suspension of Salmonella or by surface application of powdered chalk containing the pathogen. Hot air treatment of low-moisture (2.8 to 4.1%) and high-moisture (10.5 to 11.2%) immersion-inoculated nutmeats (initial population, 6.18 to 7.16 log CFU/g) at 120°C for 20 min reduced the number of Salmonella by 1.18 to 1.26 and 1.89 to 2.04 log CFU/g, respectively. However, regardless of the moisture content, hot air treatment of pecan halves containing 0.77 log CFU/g at 120°C for 20 min failed to eliminate Salmonella. Reductions were >7 log CFU/g when dry pieces were dry roasted at 160°C for 15 min. Treatment of halves at 140°C for 20 min, 150°C for 15 min, or 170°C for 10 min reduced Salmonella by 5 log CFU/g. The pathogen was slightly more heat resistant in immersion-inoculated nutmeats than on surface-inoculated nutmeats. Exposure of immersion-inoculated pieces to peanut oil at 127°C for 1.5 min or 132°C for 1.0 min reduced the number of Salmonella by 5 log CFU/g. Treatment of halves at 138°C for 2.0 min reduced Salmonella by 5 log CFU/g; treatment at 132°C for 2.5 to 4.0 min did not always achieve this reduction. Hot air treatment cannot be relied upon to reduce Salmonella by 5 log CFU/g of raw pecan nutmeats without changing sensory qualities. Treatment temperatures and times typically used to oil roast nutmeats appear to be sufficient to reduce Salmonella by 5 log CFU/g.

Inactivation of Salmonella on in-shell pecans during conditioning treatments preceding cracking and shelling

Studies were done to determine the effectiveness of conditioning treatments for killing Salmonella in and on immersion-inoculated and surface-inoculated in-shell pecans. Treatment of immersion-inoculated, dried, stored pecans in chlorinated water (400 μg/ml) reduced Salmonella by not more than 1.6 log CFU/g. Treatment of immersion-inoculated, dried, stored pecans in chlorinated water (200 μg/ml, 1 min) followed by soaking in water for 2 h at 21°C and treating for 10 min in water at 85 to 95°C reduced Salmonella by >5.12 log CFU/g; treatment of nuts containing a low population of Salmonella (<0.60 log CFU/g) for 15 min at 90°C failed to eliminate the pathogen. Reductions of ≥6.42 log CFU/g were achieved by treating surface-inoculated nuts in water at 90 or 95°C for 80 s; treatment of nuts containing 1.78 log CFU/g at 95°C for 10 min did not eliminate the pathogen. Salmonella on surface-inoculated in-shell pecans (kernel moisture, 4.75%; water activity, 0.62) that had been dried and stored at 4°C for 3 to 5 weeks was more resistant to conditioning treatments than was Salmonella on surface-inoculated pecans (kernel moisture, 5.60%; water activity, 0.73) that were not thoroughly dried. Conditioning treatments were less effective for killing Salmonella on immersion-inoculated pecans than on surface-inoculated pecans. Response of Salmonella to conditioning treatments varied, depending on the method of inoculation and whether nuts were dried and stored between the time of inoculation and treatment, which emphasizes the importance of following practices commonly used by commercial pecan shellers when validating the lethality of conditioning treatments.

Identification of Yersinia enterocolitica using a random genomic DNA microarray chip

AIMS

To fabricate a DNA chip containing random fragments of genomic DNA of Yersinia enterocolitica and to verify its diagnostic ability.

METHODS AND RESULTS

A DNA microarray chip was fabricated using randomly fragmented DNA of Y. enterocolitica. Chips were hybridized with genomic DNA extracted from other Y. enterocolitica strains, other Yersinia spp. and bacteria in different genera. Genomic DNA extracted from Y. enterocolitica showed a significantly higher hybridization rate compared with DNA of other Yersinia spp. or bacterial genera, thereby distinguishing it from other bacteria.

CONCLUSIONS

A DNA chip containing randomly fragmented genomic DNA from Y. enterocolitica can detect Y. enterocolitica and clearly distinguish it from other Yersinia spp. and bacteria in different genera.

SIGNIFICANCE AND IMPACT OF THE STUDY

A microarray chip containing randomly fragmented genomic DNA of Y. enterocolitica was fabricated without sequence information, and its diagnostic ability to identify Y. enterocolitica was verified.

Survival and growth of salmonella in high-moisture pecan nutmeats, in-shell pecans, inedible nut components, and orchard soil

Outbreaks of salmonellosis associated with almonds have raised interest in better understanding the behavior of Salmonella on other tree nuts. We undertook a study to determine the survival and growth characteristics of Salmonella on high-moisture (water activity of 0.96 to 0.99) pecan nutmeats, in-shell pecans, and inedible components (shuck, shell, and middle septum tissue) of in-shell pecans. Salmonella did not grow on high-moisture nutmeat halves, pieces, or granules stored at 4°C for up to 48 h. Growth did occur, however, at 21, 30, and 37°C. Increases of 1.77 to 5.87 log CFU/g of nutmeats occurred within 48 h at 37°C; the order in which nutmeats supported growth was granules > pieces > halves. Populations of Salmonella on and in high-moisture in-shell pecans (kernel water activity of 0.94) stored at 4, 21, 30, and 37°C for 8 days decreased by 0.52 to 1.19 log CFU/g. The pathogen grew on the surface of high-moisture (water activity of 0.99) pecan shucks and shells but died on middle septum tissue stored at 21, 30, and 37°C for up to 6 days. Salmonella died in water extracts of shucks and in pecan orchard soil saturated with water or shuck extract, but survived well for at least 18 weeks in dry soil. The ability of the pathogen to grow on high-moisture nutmeats and some of the inedible components of pecans emphasizes the importance of controlling or limiting the time pecans are exposed to water in preharvest and postharvest environments.

Preharvest internalization of Escherichia coli O157:H7 into lettuce leaves, as affected by insect and physical damage

Environmental pests may serve as reservoirs and vectors of zoonotic pathogens to leafy greens; however, it is unknown whether insect pests feeding on plant tissues could redistribute these pathogens present on the surface of leaves to internal sites. This study sought to differentiate the degree of tissue internalization of Escherichia coli O157:H7 when applied at different populations on the surface of lettuce and spinach leaves, and to ascertain whether lettuce-infesting insects or physical injury could influence the fate of either surface or internalized populations of this enteric pathogen. No internalization of E. coli O157:H7 occurred when lettuce leaves were inoculated with 4.4 log CFU per leaf, but it did occur when inoculated with 6.4 log CFU per leaf. Internalization was statistically greater when spinach leaves were inoculated on the abaxial (underside) than when inoculated on the adaxial (topside) side, and when the enteric pathogen was spread after surface inoculation. Brief exposure (∼18 h) of lettuce leaves to insects (5 cabbage loopers, 10 thrips, or 10 aphids) prior to inoculation with E. coli O157:H7 resulted in significantly reduced internalized populations of the pathogen within these leaves after approximately 2 weeks, as compared with leaves not exposed to insects. Surface-contaminated leaves physically injured through file abrasions also had significantly reduced populations of both total and internalized E. coli O157:H7 as compared with nonabraded leaves 2 weeks after pathogen exposure.

Combined effects of chlorine dioxide, drying, and dry heat treatments in inactivating microorganisms on radish seeds

We determined the combined effectiveness of ClO(2) (200 and 500 μg/ml, 5 min), air drying [25 °C, 40% relative humidity (RH), 2 h], and mild dry heat (55 °C, 23% RH, up to 48 h) treatments in killing total aerobic bacteria (TAB), Escherichia coli O157:H7, and molds and yeasts (MY) on radish seeds. A 5.1-log reduction in the number of TAB was achieved on radish seeds treated with 200 or 500 μg/ml ClO(2) followed by air drying for 2 h and dry heat treatment for 48 h or 24 h, respectively. When radish seeds were treated with 200 and 500 μg/ml ClO(2), air dried, and heat treated for 12 h and 6 h, respectively, the initial population of E. coli O157:H7 (5.6 log CFU/g) on seeds was reduced to an undetectable level (<0.8 log CFU/g). However, the pathogen was detected in 5-day-old sprouts. The reduction of MY (1.2-1.0 log CFU/g) on radish seeds under similar experimental conditions was not changed significantly during subsequent heat treatment up to 48 h. Results show that treating radish seeds with 500 μg/ml ClO(2), followed by air dried at 25 °C for 2 h and heat treatment at 55 °C for 36 h achieved a >5-log CFU/g reduction of TAB and E. coli O157:H7. These observations will be useful when developing effective strategies and practices to enhance the microbiological safety of radish sprouts.

Factors affecting infiltration and survival of Salmonella on in-shell pecans and pecan nutmeats

A study was done to determine the infiltration and survival characteristics of Salmonella in pecans. The rate of infiltration of water into in-shell nuts varied among six varieties evaluated and was significantly (alpha = 0.05) affected by the extent of shell damage. The rate of infiltration at -20 or 4 degrees C was lower than the rate of infiltration into nuts at 21 or 37 degrees C when nuts were immersed in water at 21 degrees C. In-shell nuts immersed in a suspension of Salmonella (8.66 or 2.82 log CFU/ml) for 1 h contained populations of 6.94 to 6.99 and 1.85 to 1.95 log CFU/g, respectively. Salmonella that infiltrated in-shell nuts reached the kernel and remained viable after drying and during subsequent storage at 4 degrees C. Initially high (5.78 log CFU/g) and low (1.53 log CFU/g) populations of Salmonella did not significantly decrease in in-shell pecans stored at -20 and 4 degrees C for 78 weeks (18 months). Significant reductions of 2.49 and 3.29 log CFU/g occurred in in-shell nuts stored for 78 weeks at 21 and 37 degrees C, respectively. High (6.16 log CFU/g) and low (2.56 log CFU/g) populations on pecan halves and high (7.13 log CFU/g) and low (4.71 log CFU/g) populations on medium pieces stored for 52 weeks at -20 and 4 degrees C decreased slightly, but not always significantly. Significant reductions occurred on nutmeats stored for 52 weeks at 21 and 37 degrees C, but the pathogen was detectable, regardless of the initial inoculum level. Results emphasize the importance of applying process treatments that will inactivate Salmonella.

Synergistic effect of chlorine dioxide and drying treatments for inactivating Escherichia coli O157:H7 on radish seeds

Studies were done to determine whether calcium hypochlorite (Ca(OCl)(2)) and chlorine dioxide (ClO(2)) treatment followed by drying had a synergistic killing effect on microorganisms on radish seeds intended for sprout production. Uninoculated radish seeds and seeds inoculated with Escherichia coli O157:H7 were treated with water, Ca(OCl)(2) (free chlorine concentrations of 50 or 200 microg/ml), or ClO(2) (50 or 200 microg/ml) for 5 min and subsequently dried at 25 degrees C for up to 24 h. Populations of total aerobic bacteria (TAB), molds and yeasts (MY), and E. coli O157:H7 on the seeds treated with Ca(OCl)(2) were not significantly different (P = 0.05) than populations on seeds treated with ClO(2) at the same concentrations. However, populations of microorganisms on seeds treated with ClO(2) decreased more rapidly during drying. Treatment with ClO(2) (200 microg/ml) followed by drying caused reductions in TAB, MY, and E. coli O157:H7 of 3.1, 2.0, and 3.8 log CFU/g, respectively. When seeds were treated with water, Ca(OCl)(2) (50 or 200 microg/ml), and ClO(2) (50 microg/ml) and subsequently dried, reductions in TAB, MY, and E. coli O157:H7 were 0.2 to 2.0, 0.4 to 2.0, and 1.4 to 2.2 log CFU/g, respectively. Results indicate that inactivation of E. coli O157:H7 on radish seeds is greater after treatment with ClO(2) followed by drying than after treatment with Ca(OCl)(2) followed by drying, thus providing a synergistic treatment combination for reducing the safety risk associated with sprouts produced from these seeds.

Comparison of virulence of three strains of Cronobacter sakazakii in neonatal CD-1 mice

Cronobacter sakazakii (Enterobacter sakazakii) is an emerging pathogen that has been isolated from powdered infant formula and associated with outbreaks of infection in infants in neonatal intensive care units. In a previous study, we observed that neonatal CD-1 mice are susceptible to C. sakazakii infection and that the pathogen invades brain, liver, and cecum tissues. The study objective was to compare the virulence of three strains of C. sakazakii in neonatal CD-1 mice. The strains tested were MNW2 (a food isolate), SK81 (a clinical isolate), and 3290 (a clinical isolate). Timed-pregnant CD-1 mice were allowed to give birth on gestation day 19 or 20. Neonatal mice were sexed and culled to 10 per litter, each having five males and five females. Neonates were orally gavaged with C. sakazakii strains MNW2, SK81, or 3290 at doses ranging from 10(2.8) to 10(10.5) CFU on postnatal day 3.5. Pups surviving to postnatal day 10.5 were euthanized, and brain, liver, and cecum tissues were excised. C. sakazakii was isolated from all three tissues in mice treated with C. sakazakii, regardless of strain. C. sakazakii strain 3290 was significantly more invasive in brains (42.1% of mice) than were strains MNW2 (6.7%) and SK81 (15.9%). Mortality was observed for all strains of C. sakazakii tested, with SK81 being significantly more lethal (5.6%) than MNW2 (1.2%) or 3290 (0.6%). Our findings suggest that invasiveness does not necessarily correlate with mortality among different strains of C. sakazakii, and the clinical isolates are more virulent than the food isolate.