A multistate outbreak of Salmonella enterica serotype Baildon associated with domestic raw tomatoes

Multistate outbreak of Salmonella serotype Typhimurium infections associated with consumption of restaurant tomatoes, USA, 2006: hypothesis generation through case exposures in multiple restaurant clusters

Multiple salmonellosis outbreaks have been linked to contaminated tomatoes. We investigated a multistate outbreak of Salmonella Typhimurium infections among 190 cases. For hypothesis generation, review of patients' food histories from four restaurant-associated clusters in four states revealed that large tomatoes were the only common food consumed by patients. Two case-control studies were conducted to identify food exposures associated with infections. In a study conducted in nine states illness was significantly associated with eating raw, large, round tomatoes in a restaurant [matched odds ratio (mOR) 3·1, 95% confidence interval (CI) 1·3-7·3]. In a Minnesota study, illness was associated with tomatoes eaten at a restaurant (OR 6·3, mid-P 95% CI 1·05-50·4, P=0·046). State, local and federal regulatory officials traced the source of tomatoes to Ohio tomato fields, a growing area not previously identified in past tomato-associated outbreaks. Because tomatoes are commonly eaten raw, prevention of tomato contamination should include interventions on the farm, during packing, and at restaurants.

Internal colonization of Salmonella enterica serovar Typhimurium in tomato plants

Several Salmonella enterica outbreaks have been traced back to contaminated tomatoes. In this study, the internalization of S. enterica Typhimurium via tomato leaves was investigated as affected by surfactants and bacterial rdar morphotype, which was reported to be important for the environmental persistence and attachment of Salmonella to plants. Surfactants, especially Silwet L-77, promoted ingress and survival of S. enterica Typhimurium in tomato leaves. In each of two experiments, 84 tomato plants were inoculated two to four times before fruiting with GFP-labeled S. enterica Typhimurium strain MAE110 (with rdar morphotype) or MAE119 (without rdar). For each inoculation, single leaflets were dipped in 10(9) CFU/ml Salmonella suspension with Silwet L-77. Inoculated and adjacent leaflets were tested for Salmonella survival for 3 weeks after each inoculation. The surface and pulp of ripe fruits produced on these plants were also examined for Salmonella. Populations of both Salmonella strains in inoculated leaflets decreased during 2 weeks after inoculation but remained unchanged (at about 10(4) CFU/g) in week 3. Populations of MAE110 were significantly higher (P<0.05) than those of MAE119 from day 3 after inoculation. In the first year, nine fruits collected from one of the 42 MAE119 inoculated plants were positive for S. enterica Typhimurium. In the second year, Salmonella was detected in adjacent non-inoculated leaves of eight tomato plants (five inoculated with strain MAE110). The pulp of 12 fruits from two plants inoculated with MAE110 was Salmonella positive (about 10(6) CFU/g). Internalization was confirmed by fluorescence and confocal laser microscopy. For the first time, convincing evidence is presented that S. enterica can move inside tomato plants grown in natural field soil and colonize fruits at high levels without inducing any symptoms, except for a slight reduction in plant growth.

Cellulose mediates attachment of Salmonella enterica Serovar Typhimurium to tomatoes

Fresh fruit and vegetables are important components of a healthy and balanced diet. However, they are increasingly being recognized as important vehicles for transmission of human pathogens that were traditionally classified as zoonotic. There is a significant gap in our knowledge and understanding of the mechanisms by which human pathogens colonize and survive on or in fruits and vegetables. In this study we investigated the binding of Salmonella enterica to tomato fruits (Solanum lycopersicum), which is becoming a major source of human infection. We report that Salmonella enterica serovars Typhimurium and Senftenberg bound to the surface of unripe tomatoes in an aggregative pattern, while serovar Thompson adhered diffusely. We found that while flagella did not have a role in binding, bcsC S. Typhimurim mutants, deficient in cellulose production, exhibited significantly reduced level of attachment to tomatoes. Trans complementation of the mutation restored adhesion to the wild-type level.

Use of organic acids to inactivate Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh apples and lettuce

This study was undertaken to investigate the antimicrobial effect of organic acids against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on whole red organic apples and lettuce. Several studies have been conducted to evaluate organic acids as sanitizers. However, no studies have compared antimicrobial effects of various organic acids on organic fresh produce, including evaluation of color changes of produce. Apples and lettuce were inoculated with a cocktail of 3 strains each of 3 foodborne pathogens provided above and treated with 1% and 2% organic acids (propionic, acetic, lactic, malic, and citric acid) for 0, 0.5, 1, 5, and 10 min. With increasing treatment time and acid concentration, organic acid treatments showed significant reduction compared to the control treatment (distilled water), and differences in antimicrobial effects between organic acids were observed. After 10 min of treatment with 1% and 2% organic acids in apples, propionic (0.92 to 2.75 log reduction), acetic (0.52 to 2.78 log reduction), lactic (1.69 to >3.42 log reduction), malic (1.48 to >3.42 log reduction), and citric acid (1.52 to >3.42 log reduction) exhibited significant (P < 0.05) antibacterial effects against 3 foodborne pathogens compared to the control treatment. In lettuce, propionic (0.93 to 1.52 log reduction), acetic (1.13 to 1.74 log reduction), lactic (1.87 to 2.54 log reduction), malic (2.32 to 2.98 log reduction), and citric acid (1.85 to 2.86 log reduction) showed significant (P < 0.05) effects compared to the control treatment. Changes in sample color subjected to organic acids treatment were not significant during storage.

PRACTICAL APPLICATION

It is suggested that organic acids have a potential as sanitizers for organic fresh produce. These data may help the organic produce industry provide safe fresh produce for consumers.

Natural-light labeling of tomatoes does not facilitate growth or penetration of Salmonella into the fruit

The survival-growth capacity of Salmonella populations on tomato epidermis labeled by a natural-light labeling system was investigated after persistent fears of such marks serving as possible entryways for the pathogenic organisms, alone and in the presence of Pectobacterium carotovorum subsp. carotovorum, a soft-rot organism. Different treatments involving natural-light labeling, fruit waxing, and a five-strain cocktail of Salmonella were applied to mature green tomato surfaces in different sequences prior to storage at 4, 12, or 25°C. Fruit was sampled every 3 days, and Salmonella was enumerated from all treatments and unlabeled fruit, which served as controls. There were no significant differences between treatments or between treatments and controls throughout. The results indicate that the cuticle and epidermal interruptions caused by natural-light labeling do not facilitate the penetration and colonization of the tomato pericarp. In a separate set of experiments, the capacity of Salmonella to penetrate tomato in the presence of a potential synergism with P. carotovorum subsp. carotovorum was investigated. The addition of P. carotovorum at higher, lower, or equal population densities to Salmonella did not significantly alter the behavior of Salmonella on tomatoes stored at 25°C, regardless of natural-light labeling. The inability of P. carotovorum and Salmonella to colonize natural-light-etched surfaces of tomato fruit indicates that the use of this technology does not adversely compromise the surface of tomatoes.

Colonization of tomato plants by Salmonella enterica is cultivar dependent, and type 1 trichomes are preferred colonization sites

Nontyphoid salmonellosis caused by Salmonella enterica is the most common bacterial food-borne illness in humans, and fresh produce, including tomatoes, is a common vehicle. Accumulating data indicate that human enteric pathogenic bacteria, including S. enterica, interact actively with plants. Tomato plants were inoculated with S. enterica to evaluate plausible contamination routes and to determine if the tomato cultivar affects S. enterica colonization. S. enterica population levels on tomato leaves were cultivar dependent. S. enterica levels on Solanum pimpinellifolium (West Virginia 700 [WVa700]) were lower than on S. lycopersicum cultivars. S. enterica preferentially colonized type 1 trichomes and rarely interacted with stomata, unlike what has been reported for cut lettuce leaves. Early S. enterica leaf colonization led to contamination of all fruit, with levels as high as 10(5) CFU per fruit. Reduced bacterial speck lesion formation correlated with reduced S. enterica populations in the phyllosphere. Tomato pedicels and calyxes also harbored large S. enterica populations following inoculation via contaminated water postharvest. WVa700 green fruit harbored significantly smaller S. enterica populations than did red fruit or S. lycopersicum fruit. We found that plants irrigated with contaminated water had larger S. enterica populations than plants grown from seeds planted in infested soil. However, both routes of contamination resulted in detectable S. enterica populations in the phyllosphere. Phyllosphere S. enterica populations pose a risk of fruit contamination and subsequent human disease. Restricting S. enterica phyllosphere populations may result in reduced fruit contamination. We have identified WVa700 as a tomato cultivar that can restrict S. enterica survival in the phyllosphere.

Fresh fruit and vegetables as vehicles for the transmission of human pathogens

Much research into food-borne human pathogens has focused on transmission from foods of animal origin. However, recent investigations have identified fruits and vegetables are the source of many disease outbreaks. Now believed to be a much larger contributor to produce-associated outbreaks than previously reported, norovirus outbreaks are commonly caused by contamination of foods from hands of infected workers. Although infections with Shiga toxin-producing E. coli O157 have been linked to beef more often than to any other food product, severe outbreaks have been traced to consumption of contaminated radish sprouts and pre-packaged spinach. Similarly, while infections with Salmonella have mainly been linked to consumption of foods of animal origin, many outbreaks have been traced to contaminated fresh produce. E. coli O157 binds to lettuce leaves by alternative mechanisms involving the filamentous type III secretions system, flagella and the pilus curli. Association of Salmonella with fresh produce appears to be serovar-specific involving flagella, curli, cellulose, and O antigen capsule. A better understanding of plant, microbiological, environmental, processing and food handling factors that facilitate contamination will allow development of evidence-based policies, procedures and technologies aimed at reducing the risk of contamination of fresh produce.

Survival of Salmonella and Staphylococcus aureus in Mexican Red Salsa in a Food Service Setting

Mexican red salsa is one of the most common side dishes in Mexican cuisine. According to data on foodborne illnesses collected by the Centers for Disease Control and Prevention, salsa was associated with 70 foodborne illness outbreaks between 1990 and 2006. Salsa ingredients such as tomatoes, cilantro, and onions often have been implicated in foodborne illness outbreaks. Mexican-style restaurants commonly prepare a large batch of red salsa, store it at refrigeration temperatures, and then serve it at room temperature. Salmonella is one of the top etiologies in foodborne illness outbreaks associated with salsa, and our preliminary studies revealed the consistent presence of Staphylococcus aureus in restaurant salsa. In the present study, we evaluated the survival of Salmonella Enteritidis and S. aureus inoculated into restaurant-made salsa samples stored at ambient (20°C) and refrigeration (4°C) temperatures. These test temperature conditions represent best-case and worst-case scenarios in restaurant operations. Salmonella survived in all samples stored at room temperature, but S. aureus populations significantly decreased after 24 h of storage at room temperature. No enterotoxin was detected in samples inoculated with S. aureus at 6.0 log CFU/g. Both microorganisms survived longer in refrigerated samples than in samples stored at room temperature. Overall, both Salmonella and S. aureus survived a sufficient length of time in salsa to pose a food safety risk.

Tomato handling practices in restaurants

In recent years, multiple outbreaks of Salmonella infection have been associated with fresh tomatoes. Investigations have indicated that tomato contamination likely occurred early in the farm-to-consumer chain, although tomato consumption occurred mostly in restaurants. Researchers have hypothesized that tomato handling practices in restaurants may contribute to these outbreaks. However, few empirical data exist on how restaurant workers handle tomatoes. This study was conducted to examine tomato handling practices in restaurants. Members of the Environmental Health Specialists Network (EHS-Net) observed tomato handling practices in 449 restaurants. The data indicated that handling tomatoes appropriately posed a challenge to many restaurants. Produce-only cutting boards were not used on 49% of tomato cutting observations, and gloves were not worn in 36% of tomato cutting observations. Although tomatoes were washed under running water as recommended in most (82%) of the washing observations, tomatoes were soaked in standing water, a practice not recommended by the U.S. Food and Drug Administration (FDA), in 18% of observations, and the temperature differential between the wash water and tomatoes did not meet FDA guidelines in 21% of observations. About half of all batches of cut tomatoes in holding areas were above 41 degrees F (5 degrees C), the temperature recommended by the FDA. The maximum holding time for most (73%) of the cut tomatoes held above 41 degrees F exceeded the FDA recommended holding time of 4 h for unrefrigerated tomatoes (i.e., tomatoes held above 41 degrees F). The information provided by this study can be used to inform efforts to develop interventions and thus prevent tomato-associated illness outbreaks.

Risk factors for typhoid in Darjeeling, West Bengal, India: evidence for practical action

OBJECTIVE

To identify risk factors for typhoid and propose prevention measures.

METHODS

Case-control study; we compared hospital-based typhoid cases defined as fever>38 degrees C for >or=3 days with four-fold rise in 'O' antibodies on paired sera (Widal) with community, age and neighbourhood matched controls. We obtained information on drinking water, fruits, vegetables, milk products and sanitation; and calculated matched odds ratios (MOR) and attributable fractions in the population (AFP) for the risk factors or failure to use prevention measures.

RESULTS

The 123 typhoid cases (median age: 25 years, 47% female) and 123 controls did not differ with respect to baseline characteristics. Cases were less likely to store drinking water in narrow-mouthed containers (MOR: 0.4, 95% CI: 0.2-0.7, AFP 29%), tip containers to draw water (MOR: 0.4, 95% CI: 0.2-0.7, AFP 33%) and have home latrines (MOR: 0.5, 95% CI: 0.3-0.8, AFP 23%). Cases were more likely to consume butter (OR: 2.3, 95% CI: 1.3-4.1, AFP 28%), yoghurt (OR: 2.3, 95% CI: 1.4-3.7, AFP 34%) and raw fruits and vegetables, including onions (MOR: 2.1, 95% CI: 1.2-3.9, AFP 34%), cabbages (OR: 2.8, 95% CI: 1.7-4.8, AFP 44%) and unwashed guavas (OR: 1.9, 95% CI: 1.2-3, AFP 25%).

CONCLUSION

Typhoid was associated with unsafe water and sanitation practices as well as with consumption of milk products, fruits and vegetables. We propose to chlorinate drinking water at the point of use, wash/cook raw fruits and vegetables and ensure safer preparation/storage of local milk products.

Proteolytic activity of molds and their metabiotic association with Salmonella in a model system

The aim of this study was to investigate the proteolytic ability of some strains of aspergilli, fusaria, and penicillia and the metabiotic effect of Fusarium oxysporum and Penicillium expansum on Salmonella. The proteolytic activity of the target molds was determined on tomato juice agar and tomato juice, whereas the metabiotic effect of F. oxysporum and P. expansum on Salmonella was assessed in a model system consisting of tryptone soy broth with different amounts of tomato juice added. Fusaria, some aspergilli, and one strain of penicillium showed a proteolytic activity on tomato juice agar. In addition, Salmonella survival was enhanced in tryptone soy broth plus 20 or 50% tomato juice in the model system previously inoculated with F. oxysporum.

Differential attachment to and subsequent contamination of agricultural crops by Salmonella enterica

U.S. salmonellosis outbreaks have occurred following consumption of tomato and cantaloupe but not lettuce. We report differential contamination among agricultural seedlings by Salmonella enterica via soil. Members of the family Brassicaceae had a higher incidence of outbreak than carrot, lettuce, and tomato. Once they were contaminated, phyllosphere populations were similar, except for tomato. Contamination differences exist among tomato cultivars.

Survival and growth of acid-adapted and unadapted Salmonella in and on raw tomatoes as affected by variety, stage of ripeness, and storage temperature

Consumption of raw round and Roma tomatoes has been associated with outbreaks of salmonellosis. A study was done to determine whether survival and growth of Salmonella in and on tomatoes is affected by variety of tomato, stage of ripeness, and storage temperature. The influence of acid adaptation of cells and site of inoculation on survival and growth was studied. Salmonella grew in stem scar and pulp tissues of round, Roma, and grape tomatoes stored at 12 and 21 degrees C but not in those tomatoes stored at 4 degrees C. Survival and growth was largely unaffected by variety and stage of ripeness at the time of inoculation. The pathogen did not grow on the skin of grape tomatoes stored at 4, 12, and 21 degrees C. Survival and growth of Salmonella inoculated into stem scar and pulp tissues of round and Roma tomatoes were unaffected by exposure of cells to an acidic (pH 4.75) environment before inoculation. Results emphasize the importance of preventing contamination of tomatoes with Salmonella at all stages of ripeness, regardless of variety or previous exposure of cells to an acidic environment.

Metabiotic effects of Fusarium spp. on Escherichia coli O157:H7 and Listeria monocytogenes on raw portioned tomatoes

The metabiotic effects of Fusarium proliferatum, F. avenaceum, and F. oxysporum on Escherichia coli O157:H7 and Listeria monocytogenes in fresh tomatoes were investigated. Tomatoes were preinoculated with the molds and incubated at 15 degrees C for 7 days; then they were inoculated separately with the pathogens, packaged in air and modified atmosphere (5% O2, 30% CO2, and 65% N2), and stored at 4, 8, and 12 degrees C for 9 days. The cell loads of pathogens and lactic acid bacteria and the pH were evaluated periodically. The data were modeled through some different mathematical models to assess the shoulder length, i.e., the time before the beginning of the exponential death phase, the 1-log reduction time (s), and the pathogen death time (deltastand). The preinoculation of tomatoes with the molds enhanced the survival of E. coli O157:H7 by prolonging shoulder length and 8 parameters; this effect, however, was not observed for L. monocytogenes. pH values did not undergo significant changes within the storage time, and the lactic acid bacteria increased from 5 to 7 log CFU/g, without significant differences among the storage temperatures or the packaging atmospheres. The results of this research showed that the use of fresh tomatoes colonized by fusaria (even if the contamination is not visible) could increase significantly the risk of outbreaks due to some pathogens that could be on the surface of fruits and vegetables as a result of cross-contamination at home or incorrect postharvest operations.

Animal and environmental impact on the presence and distribution of Salmonella and Escherichia coli in hydroponic tomato greenhouses

From 2003 to 2004, we studied the impact of environmental influences on the microbiological quality of a hydroponic tomato farm. The presence of Salmonella was investigated on 906 samples of tomatoes and 714 environmental samples. The farm comprised 14 greenhouses and a technologically advanced packinghouse, and operated under a sanitary agricultural practices plan. The objective of the present study was to determine the operating sources of contamination. During the course of the study, two independent natural events affected the farm. In 2003, water runoff entered some of the greenhouses. A year later, wild animals (opossums, mice, and sparrows) gained entry into several of the greenhouses. Salmonella and Escherichia coli were found in samples of tomatoes, water puddles, soil, shoes, and the feces of local wild and farm animals. Salmonella Montevideo, Salmonella Newport, and strains of the F serogroup were isolated from tomatoes. Almost all of the Salmonella Newport strains were isolated from samples collected during or immediately after the flood. Analysis by pulsed-field gel electrophoresis revealed that some Salmonella Montevideo isolates from tomatoes, opossums, and mice displayed identical XbaI or AvrII patterns, suggesting that these wild animals represented one source of contamination. F serogroup strains were found mostly on samples of goat feces and personnel shoes when standard working practices were in place. Shoes were found to be an important vehicle for dissemination of Salmonella into the greenhouses. The level of protection provided by hydroponic greenhouses does not exclude the eventuality that enteric pathogenic bacteria can gain access through various avenues.

Efficacy of neutral electrolyzed water (NEW) for reducing microbial contamination on minimally-processed vegetables

Consumption of minimally-processed, or fresh-cut, fruit and vegetables has rapidly increased in recent years, but there have also been several reported outbreaks associated with the consumption of these products. Sodium hypochlorite is currently the most widespread disinfectant used by fresh-cut industries. Neutral electrolyzed water (NEW) is a novel disinfection system that could represent an alternative to sodium hypochlorite. The aim of the study was to determine whether NEW could replace sodium hypochlorite in the fresh-cut produce industry. The effects of NEW, applied in different concentrations, at different treatment temperatures and for different times, in the reduction of the foodborne pathogens Salmonella, Listeria monocytogenes and Escherichia coli O157:H7 and against the spoilage bacterium Erwinia carotovora were tested in lettuce. Lettuce was artificially inoculated by dipping it in a suspension of the studied pathogens at 10(8), 10(7) or 10(5) cfu ml(-1), depending on the assay. The NEW treatment was always compared with washing with deionized water and with a standard hypochlorite treatment. The effect of inoculum size was also studied. Finally, the effect of NEW on the indigenous microbiota of different packaged fresh-cut products was also determined. The bactericidal activity of diluted NEW (containing approximately 50 ppm of free chlorine, pH 8.60) against E. coli O157:H7, Salmonella, L. innocua and E. carotovora on lettuce was similar to that of chlorinated water (120 ppm of free chlorine) with reductions of 1-2 log units. There were generally no significant differences when treating lettuce with NEW for 1 and 3 min. Neither inoculation dose (10(7) or 10(5) cfu ml(-1)) influenced the bacterial reduction achieved. Treating fresh-cut lettuce, carrot, endive, corn salad and 'Four seasons' salad with NEW 1:5 (containing about 50 ppm of free chlorine) was equally effective as applying chlorinated water at 120 ppm. Microbial reduction depended on the vegetable tested: NEW and sodium hypochlorite treatments were more effective on carrot and endive than on iceberg lettuce, 'Four seasons' salad and corn salad. The reductions of indigenous microbiota were smaller than those obtained with the artificially inoculated bacteria tested (0.5-1.2 log reduction). NEW seems to be a promising disinfection method as it would allow to reduce the amount of free chlorine used for the disinfection of fresh-cut produce by the food industry, as the same microbial reduction as sodium hypochlorite is obtained. This would constitute a safer, 'in situ', and easier to handle way of ensuring food safety.

Role of soil, crop debris, and a plant pathogen in Salmonella enterica contamination of tomato plants

BACKGROUND

In the U.S., tomatoes have become the most implicated vehicle for produce-associated Salmonellosis with 12 outbreaks since 1998. Although unconfirmed, trace backs suggest pre-harvest contamination with Salmonella enterica. Routes of tomato crop contamination by S. enterica in the absence of direct artificial inoculation have not been investigated.

METHODOLOGY/PRINCIPAL FINDINGS

This work examined the role of contaminated soil, the potential for crop debris to act as inoculum from one crop to the next, and any interaction between the seedbourne plant pathogen Xanthomonas campestris pv. vesicatoria and S. enterica on tomato plants. Our results show S. enterica can survive for up to six weeks in fallow soil with the ability to contaminate tomato plants. We found S. enterica can contaminate a subsequent crop via crop debris; however a fallow period between crop incorporation and subsequent seeding can affect contamination patterns. Throughout these studies, populations of S. enterica declined over time and there was no bacterial growth in either the phyllosphere or rhizoplane. The presence of X. campestris pv. vesicatoria on co-colonized tomato plants had no effect on the incidence of S. enterica tomato phyllosphere contamination. However, growth of S. enterica in the tomato phyllosphere occurred on co-colonized plants in the absence of plant disease.

CONCLUSIONS/SIGNIFICANCE

S. enterica contaminated soil can lead to contamination of the tomato phyllosphere. A six week lag period between soil contamination and tomato seeding did not deter subsequent crop contamination. In the absence of plant disease, presence of the bacterial plant pathogen, X. campestris pv. vesicatoria was beneficial to S. enterica allowing multiplication of the human pathogen population. Any event leading to soil contamination with S. enterica could pose a public health risk with subsequent tomato production, especially in areas prone to bacterial spot disease.

Recurrent multistate outbreak of Salmonella Newport associated with tomatoes from contaminated fields, 2005

Salmonella Newport causes more than an estimated 100,000 infections annually in the United States. In 2002, tomatoes grown and packed on the eastern shore of Virginia contaminated with a pan-susceptible S. Newport strain caused illness in 510 patients in 26 states. In July-November 2005, the same strain caused illness in at least 72 patients in 16 states. We conducted a case-control study during the 2005 outbreak, enrolling 29 cases and 140 matched neighbourhood controls. Infection was associated with eating tomatoes (matched odds ratio 9.7, 95% confidence interval 3.3-34.9). Tomatoes were traced back to the eastern shore of Virginia, where the outbreak strain was isolated from pond water used to irrigate tomato fields. Two multistate outbreaks caused by one rare strain, and identification of that strain in irrigation ponds 2 years apart, suggest persistent contamination of tomato fields. Further efforts are needed to prevent produce contamination on farms and throughout the food supply chain.

Microbiological profile of greenhouses in a farm producing hydroponic tomatoes

Produce, including tomatoes, has been implicated in several outbreaks of foodborne illness. A number of the sources of contamination for produce grown in open fields are known. However, as an alternative agricultural system, hydroponic greenhouses are reasonably expected to reduce some of these sources. The objective of the present study was to determine the microbiological profile of tomatoes grown in greenhouses at a Mexican hydroponic farm with a high technological level and sanitary agricultural practices (SAPs) in place. Tomatoes and other materials associated with the farm were analyzed for the presence of Salmonella enterica and populations of Escherichia coli, coliforms, and Enterobacteriaceae. Tomatoes showed median levels of 0.8 log CFU per tomato for Enterobacteriaceae, < 0.5 log CFU per tomato for coliforms, and 0.5 most probable number per tomato for E. coli. Despite the physical barriers that the facilities provide and the implemented SAPs, we found that 2.8% of tomatoes were contaminated with Salmonella and 0.7% with E. coli. Other Salmonella-positive materials were puddles, soil, cleaning cloths, and sponges. Samples from the nursery and greenhouses were positive for E. coli, whereas Salmonella was found only in the latter. Although hydroponic greenhouses provide physical barriers against some sources of enteric bacterial contamination, these results show that sporadic evidence of fecal contamination and the presence of Salmonella can occur at the studied greenhouse farm.

Outbreak of Salmonella Braenderup infections associated with Roma tomatoes, northeastern United States, 2004: a useful method for subtyping exposures in field investigations

Salmonella Braenderup is an uncommon serotype in the United States. In July 2004, a multistate outbreak of Salmonella Braenderup diarrhoeal infections occurred, with 125 clinical isolates identified. To investigate, we conducted a case-control study, enrolling 32 cases and 63 matched controls. Cheese, lettuce and tomato eaten at restaurants all appeared to be associated with illness. To further define specific exposures, we conducted a second study and asked managers of restaurants patronized by patients and controls about cheese, lettuce and tomato varieties used in dishes their patrons reported consuming. This information was obtained for 27 cases and 29 controls. Roma tomatoes were the only exposure significantly associated with illness (odds ratio 4.3, 95% confidence interval 1.2-15.9). Roma tomatoes from two restaurants were traced back to a single tomato packing house. The methods used in this field investigation to define specific exposures may be useful for other foodborne outbreaks.

Efficacy of chlorinated and ozonated water in reducing Salmonella typhimurium attached to tomato surfaces

The purpose of this study was to compare chlorinated and ozonated water in reducing Salmonella typhimurium inoculated onto fresh ripe tomatoes. Surface-inoculated tomatoes were immersed/sprayed with chlorinated (200 mg l(-1)) and ozonated water (1 and 2 mg l(-1)) under 2 and 100 nefelometric turbidity units (NTU). Contact times were 120 and 30 s for immersing and spraying applications, respectively. Immersing in chlorinated water and low turbidity resulted in the most effective application with 3.61 log(10) bacterial reduction, while 1 and 2 mg l(-1) of ozone reduced 2.32 and 2.53 log(10), respectively. High turbidity and chlorine reduced the bacterial counts by 3.39 log(10), while 1 and 2 mg l(-1) of ozonated water and low turbidity reduced the bacteria by 1.48 and 1.92 log(10), respectively. Spraying chlorinated water reduced bacteria by 3 log(10), and ozonated water at 1 and 2 mg l(-1) reduced counts by 1.84 and 2.40 log(10), respectively. No statistical differences were found between chlorine and ozone (2 mg l(-1)) during spraying applications (p < 0.05). The use of ozonated water both in immersing and spraying applications is suggested when water turbidity remains low.

1% calcium chloride treatment in combination with gamma irradiation improves microbial and physicochemical properties of diced tomatoes

The purpose of this study was to determine the effect of a combination of a 1% calcium chloride dip with low dose irradiation on microbial populations, and biochemical and physical properties, of fresh diced tomatoes during a two-week storage period. Vine tomatoes at the light-red stage (trial 1) and Celebrity tomatoes at the table ripe stage (trial 2) were diced, dipped in 1% CaCl(2), and irradiated at 1 kGy from a Co(60) source. Tomatoes were also contaminated with cocktail of nalidixic-acid resistant Salmonella strains (S. Poona, S. Hartford, S. Gaminara, S. Michigan, and S. Montevideo) and subjected to gamma irradiation. Calcium treatment alone stimulated ethylene production in the diced tomatoes, whereas irradiation treatment alone suppressed ethylene production. The combination of calcium and irradiation treatments resulted in no change in ethylene production compared to the nontreated control, but respiration rate was suppressed by both irradiation and calcium treatment. The calcium dip was found to limit irradiation-induced loss of firmness. Irradiation, by itself and in combination with calcium treatment, resulted in a >3 log CFU/g decrease in total aerobic counts and psychrotrophs. Additionally, irradiation at 1.5 kGy eliminated > 3 log CFU/g of Salmonella organisms from tomatoes contaminated with Salmonella. Counts continued to decrease to an undetectable level over the 11 day storage period. The results indicate that the combination of calcium treatment and irradiation can reduce the risk of disease due to pathogenic organisms such as Salmonella and can eliminate the problem of softening induced by irradiation.

Using aqueous chlorine dioxide to prevent contamination of tomatoes with Salmonella enterica and erwinia carotovora during fruit washing

Chlorine dioxide (ClO2) is an antimicrobial agent recognized for its disinfectant properties. In this study, the sanitizing effects of ClO2 solutions against Salmonella enterica and Erwinia carotovora in water, on tomato surfaces, and between loads of tomatoes were evaluated. In water, ClO2 at 5, 10, and 20 ppm caused a > or = 5-log reduction of S. enterica within 6, 4, and 2 s, respectively. Higher lethality was observed with E. carotovora; a 5-log reduction was achieved after only 2 s with 10 ppm ClO2. On fruit surfaces, however, the sanitizing effects were compromised. A full minute of contact with ClO2 at 20 and 10 ppm was required to achieve a 5-log reduction in S. enterica and E. carotovora counts, respectively, on freshly spot-inoculated tomatoes. On inoculated fruit surfaces, populations decreased > 3 log CFU/cm2 during desiccation at 24 +/- 1 degrees C for 24 h. Populations of air-dried Salmonella and Erwinia were not significantly reduced (P > 0.05) by ClO2 at < or = 20 ppm after 1 min. Either wet or dry inoculum of these two pathogens could contaminate immersion water, which in turn can cross-contaminate a subsequent load of clean fruit and water. ClO2 at 5 ppm used for immersion effectively prevented cross-contamination. Pathogen contamination during fruit handling is best prevented with an effective disinfectant. Once a load of fruit is contaminated with pathogens, even a proven disinfectant such as ClO2 cannot completely eliminate such contaminants, particularly when they are in a dehydrated state on fruit.

Irradiation D Values ofSalmonellaspp. in Diced Tomatoes Dipped in 1% Calcium Chloride

Outbreaks of salmonellosis have been associated with eating raw domestic tomatoes. In this study, we examined the efficiency of combined irradiation and a 1% calcium chloride dip to reduce the population of Salmonella enterica strains on diced tomatoes. Tomatoes were contaminated with nalidixic acid-resistant strains of S. Hartford, S. Montevideo, or a mixture of 5 strains (S. Hartford, S. Montevideo, S. Poona, S. Michigan, S. Gaminara). We irradiated tomatoes at various doses up to 0.9 kGy from an electron beam source to conduct a D-value study (decimal reduction time required to eliminate 90% of the organism). Surviving Salmonella populations were detected by standard and recovery plating methods. D-value results ranged from 0.26 to 0.39 kGy, indicating that a 5 log(10) CFU/g reduction in Salmonella spp. in diced tomatoes would require a dose of 1.3-1.95 kGy.

The key role of pulsed-field gel electrophoresis in investigation of a large multiserotype and multistate food-borne outbreak of Salmonella infections centered in Pennsylvania

Five different serotypes of Salmonella enterica were implicated in a large outbreak linked to fresh Roma tomatoes served at gas station deli counters in Pennsylvania and nearby states during July 2004: S. enterica serotypes Javiana, Anatum, Thompson, Typhimurium, and Muenchen. One of these serotypes, Anatum, was isolated from both tomatoes and patients. Pulsed-field gel electrophoresis (PFGE) played a key role in identifying the outbreak-associated isolates and distinguishing them from unrelated sporadic isolates. It also demonstrated that the genetic fingerprints of serotype Anatum isolates derived from patients were indistinguishable from those derived from tomatoes. Rapid communication of PFGE fingerprints with other public health laboratories through the Centers for Disease Control and Prevention's PulseNet USA national molecular surveillance network for bacterial food-borne pathogens facilitated the tracking of this outbreak in other states. The work described in this report emphasizes the laboratory's role in core public health functions and services, thereby providing a highly visible example of public health in action.

Survival of Enterobacter sakazakii on fresh produce as affected by temperature, and effectiveness of sanitizers for its elimination

A study was done to determine the survival characteristics of Enterobacter sakazakii on the surface of apples, cantaloupes, strawberries, lettuce, and tomatoes stored at 4, 12, and 25 degrees C for 8-28 days. Populations significantly decreased (p<or=0.05) on all test produce at all storage temperatures. The efficacy of chlorine, chlorine dioxide, and a peroxyacetic acid-based sanitizer (Tsunami 200) treatments (1 and 5 min) in killing the bacterium on apples, tomatoes, and lettuce was determined. Chlorine and chlorine dioxide, at >or=50 microg/ml, were equivalent in killing E. sakazakii on apples. Populations of E. sakazakii on apples treated with 10 microg/ml chlorine dioxide for 1 or 5 min were significantly reduced (p<or=0.05) by 3.38 and 3.77 log CFU/apple, respectively, compared to the number remaining on apples after washing with water. Treatment with Tsunami 200 at 40 microg/ml for 1 min caused reductions of >or=4.00 log CFU/apple. Reductions of >or=3.70 log CFU/tomato were achieved by treatment with 10 microg/ml chlorine or chlorine dioxide or 40 microg/ml Tsunami 200 for 5 min. Reductions in populations of E. sakazakii on lettuce treated with chlorine at 10, 50, and 100 microg/ml for 1 min ranged from 1.61 to 2.50 log CFU/sample (26+/-4 g), compared to populations remaining on lettuce washed with water. Chlorine was less effective in killing E. sakazakii on lettuce than on apples or tomatoes. Treatment of lettuce with Tsunami 200 (40 and 80 microg/ml) for 5 min caused a reduction of >or=5.31 log CFU/sample. Results provide insights to predicting survival characteristics of E. sakazakii on produce and the efficacy of sanitizers in killing the bacterium.

Improving the microbiological quality and safety of fresh-cut tomatoes by low-dose electron beam irradiation

The effect of electron beam irradiation on microbiological quality and safety of fresh-cut tomatoes was studied. Fresh tomatoes were obtained from a local supplier and then cut into cubes that were separated from the stem scars. Both cubes and stem scars were inoculated with a rifampin-resistant strain of either Salmonella Montevideo or Salmonella Agona, separated into treatment groups, and treated by electron beam irradiation at 0.0 (control), 0.7, or 0.95 kGy. The effect of electron beam irradiation on Salmonella, lactic acid bacteria, yeast, and mold counts and pH of tomato cubes and stem scars was determined over a 15-day storage period at 4 degrees C. Results indicated that although irradiation treatment significantly reduced most microbial populations on tomato samples, there were no differences in the reduction of microbial populations between treatments of 0.7 and 0.95 kGy. Irradiation at either dose resulted in a significant reduction in Salmonella when compared with the control (P < 0.05). Lactic acid bacteria, yeasts, and molds were more resistant to irradiation than were Salmonella. No differences were detected between the two Salmonella serotypes in response to irradiation treatment. These results indicate that irradiation at doses of at least 0.7 kGy can be used for pathogen reduction in fresh-cut tomatoes. If the use of doses greater than 1 kGy were approved, this technology might be very effective for use in fresh-cut tomatoes to eliminate significant populations of pathogens and to ensure the microbial quality of the product.

Standardization of Pulsed-Field Gel Electrophoresis Protocols for the Subtyping ofEscherichia coliO157:H7,Salmonella, andShigellafor PulseNet

Standardized rapid pulsed-field gel electrophoresis (PFGE) protocols for the subtyping of Escherichia coli O157:H7, Salmonella serotypes, and Shigella species are described. These protocols are used by laboratories in PulseNet, a network of state and local health departments, and other public health laboratories that perform real-time PFGE subtyping of these bacterial foodborne pathogens for surveillance and outbreak investigations. Development and standardization of these protocols consisted of a thorough optimization of reagents and reaction conditions to ensure that the protocols yielded consistent results and high-quality PFGE pattern data in all the PulseNet participating laboratories. These rapid PFGE protocols are based on the original 3-4-day standardized procedure developed at Centers for Disease Control and Prevention that was validated in 1996 and 1997 by eight independent laboratories. By using these rapid standardized PFGE protocols, PulseNet laboratories are able to subtype foodborne pathogens in approximately 24 h, allowing for the early detection of foodborne disease case clusters and often aiding in the identification of the source responsible for the infections.

Fitness of human enteric pathogens on plants and implications for food safety

The continuous rise in the number of outbreaks of foodborne illness linked to fresh fruit and vegetables challenges the notion that enteric pathogens are defined mostly by their ability to colonize the intestinal habitat. This review describes the epidemiology of produce-associated outbreaks of foodborne disease and presents recently acquired knowledge about the behavior of enteric pathogens on plants, with an emphasis on Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes. The growth and survival of enteric pathogens on plants are discussed in the light of knowledge and concepts in plant microbial ecology, including epiphytic fitness, the physicochemical nature of plant surfaces, biofilm formation, and microbe-microbe and plant-microbe interactions. Information regarding the various stresses that affect the survival of enteric pathogens and the molecular events that underlie their interactions in the plant environment provides a good foundation for assessing their role in the infectious dose of the pathogens when contaminated fresh produce is the vehicle of illness.

Survival and growth of Enterobacter sakazakii on fresh-cut fruits and vegetables and in unpasteurized juices as affected by storage temperature

Enterobacter sakazakii is an emerging foodborne pathogen that has caused illnesses and deaths in infants and elderly immunocompromised adults. Outbreaks of E. sakazakii infection have been associated with infant formulas, but the documented presence of this pathogen in a wide variety of ready-to-eat foods, including lettuce and other raw vegetables, makes it important to learn more about its behavior in these environments. We investigated the survival and growth characteristics of E. sakazakii on fresh-cut apple, cantaloupe, strawberry, watermelon, cabbage, carrot, cucumber, lettuce, and tomato and in juices prepared from these fruits and vegetables. Produce and juices were inoculated with E. sakazakii at 2 to 3 log CFU/g and 1 to 2 log CFU/ml, respectively, and stored at 4, 12, or 25 degrees C. Populations either did not change or gradually decreased in fresh-cut produce and juices stored at 4 degrees C but grew at 12 degrees C on fresh-cut apple, cantaloupe, watermelon, cucumber, and tomato and in all juices except apple, strawberry, cabbage, and tomato juices. All fresh-cut fruits and vegetables except strawberry supported growth of E. sakazakii at 25 degrees C. Growth occurred in all juices except apple, strawberry, and cabbage juices, followed by decreases in population to < 1 CFU/ml after 48 to 72 h, which coincided with decreases in pH and an increase in the population of lactic acid bacteria. Increases in total counts occurred in all juices except strawberry juice stored at 25 degrees C and apple and strawberry juices stored at 12 degrees C. Total counts increased in cantaloupe, carrot, cucumber, and lettuce juices stored at 4 degrees C. Populations of molds and yeasts increased in apple and tomato juices stored at 25 degrees C but decreased to <1 CFU/ml in cabbage, lettuce, and cucumber juices. Further characterization of the behavior of E. sakazakii on fresh produce and in unpasteurized juice as affected by commercial packaging and handling practices is needed.

An outbreak of hepatitis A associated with green onions

BACKGROUND

In November 2003, a large hepatitis A outbreak was identified among patrons of a single Pennsylvania restaurant. We investigated the cause of the outbreak and factors that contributed to its unprecedented size.

METHODS

Demographic and clinical outcome data were collected from patients with laboratory confirmation of hepatitis A, and restaurant workers were tested for hepatitis A. A case-control study was conducted among patrons who dined at the restaurant between October 3 and October 6, 2003. Sequence analysis was performed on a 315-nucleotide region of viral RNA extracted from serum specimens.

RESULTS

Of 601 patients identified, 3 died; at least 124 were hospitalized. Of 425 patients who recalled a single dining date at the restaurant, 356 (84 percent) had dined there between October 3 and October 6. Among 240 patients in the case-control study, 218 had eaten mild salsa (91 percent), as compared with 45 of 130 controls (35 percent) (odds ratio, 19.6; 95 percent confidence interval, 11.0 to 34.9) for whom data were available. A total of 98 percent of patients and 58 percent of controls reported having eaten a menu item containing green onions (odds ratio, 33.3; 95 percent confidence interval, 12.8 to 86.2). All restaurant workers were tested, but none were identified who could have been the source of the outbreak. Sequences of hepatitis A virus from all 170 patients who were tested were identical. Mild salsa, which contained green onions grown in Mexico, was prepared in large batches at the restaurant and provided to all patrons.

CONCLUSIONS

Green onions that were apparently contaminated before arrival at the restaurant caused this unusually large foodborne outbreak of hepatitis A. The inclusion of contaminated green onions in large batches that were served to all customers contributed to the size of the outbreak.

Irrigation water as source of foodborne pathogens on fruit and vegetables

Awareness is growing that fresh or minimally processed fruit and vegetables can be sources of disease-causing bacteria, viruses, protozoa, and helminths. Irrigation with poor-quality water is one way that fruit and vegetables can become contaminated with foodborne pathogens. Groundwater, surface water, and human wastewater are commonly used for irrigation. The risk of disease transmission from pathogenic microorganisms present in irrigation water is influenced by the level of contamination; the persistence of pathogens in water, in soil, and on crops; and the route of exposure. Groundwater is generally of good microbial quality, unless it is contaminated with surface runoff; human wastewater is usually of very poor microbial quality and requires extensive treatment before it can be used safely to irrigate crops; surface water is of variable microbial quality. Bacteria and protozoa tend to show the poorest survival outside a human host, whereas viruses and helminths can remain infective for months to years. Guidelines governing irrigation water quality and strategies to reduce the risk of disease transmission by foodborne pathogens in irrigation are discussed.

Occurrence of antibiotic-resistant enterobacteria in agricultural foodstuffs

Antibiotic-resistant bacteria or their corresponding resistance determinants are known to spread from animals to humans via the food chain. We screened 20 vegetable foods for antibiotic-resistant coliform bacteria and enterococci. Isolates were directly selected on antibiotic-containing selective agar (color detection). Thirteen "common vegetables" (tomato, mushrooms, salad) possessed 10(4)-10(7) cfu/g vegetable of coliform bacteria including only few antibiotic-resistant variants (0-10(5) cfu/g). All seven sprout samples showed a some orders of magnitude higher contamination with coliform bacteria (10(7)-10(9) cfu/g) including a remarkable amount of resistant isolates (up to 10(7) cfu/g). Multiple resistances (up to 9) in single isolates were more common in sprout isolates. Resistant bacteria did not originate from sprout seeds. The most common genera among 92 isolates were: 25 Enterobacter spp. (19 E. cloacae), 22 Citrobacter spp. (8 C. freundii), and 21 Klebsiella spp. (9 K. pneumoniae). Most common resistance phenotypes were: tetracycline (43%), streptomycin (37%), kanamycin (26%), chloramphenicol (29%), co-trimoxazol (9%), and gentamicin (4%). The four gentamicin-resistant isolates were investigated in molecular details. Only three (chloramphenicol) resistant, typical plant-associated enterococci were isolated from overnight enrichment cultures. In conclusion, a contribution of sprouts contaminated with multiresistant, Gram-negative enterobacteria to a common gene pool among human commensal and pathogenic bacteria cannot be excluded.

Survival of Salmonella Montevideo on tomato leaves and mature green tomatoes

Survival of Salmonella Montevideo on tomato leaf and mature green tomatoes was examined. When spiked on the surfaces of excised leaves from greenhouse tomato plants, Salmonella Montevideo survived poorly if the leaves were allowed to dry. After spot inoculation of 6.6 log CFU per leaflet and incubation for 48 h at 60% relative humidity, the leaflets were dried, and surviving Salmonella were reduced 2.8 to 3.7 log CFU per leaflet. However, when leaves spiked with Salmonella were supported on hydroponic nutrient medium and incubated at 100% relative humidity, there was no significant reduction of surviving bacteria for at least 6 days. Exposure of mature green tomatoes to ethylene (100 ppm at 100% relative humidity and 20 degrees C for 6 days) did not significantly affect the survival of Salmonella on their surfaces. The significance of these data to pre- and postharvest safety of tomato is discussed.

Inactivation of Salmonella during drying and storage of Roma tomatoes exposed to predrying treatments including peeling, blanching, and dipping in organic acid solutions

The objective of this study was to evaluate the influence of predrying treatments, i.e., peeling, blanching prior to inoculation, and dipping in organic acid solutions, on inactivation of Salmonella during drying (60 degrees C for 14 h) and aerobic storage (25 degrees C for 28 days) of inoculated (five-strain composite, 7.1 to 7.4 log CFU/g) Roma tomato halves. Four predrying treatments groups were established. One group received no treatment (C). In the other three groups, unpeeled-unblanched, unpeeled-blanched (steam blanched at 88 degrees C for 3 min), peeled-unblanched, and peeled-blanched tomato halves were immersed for 10 min in water (W), ascorbic acid solution (AA; 3.40%, pH 2.48), or citric acid solution (CA; 0.21%, pH 2.51). Appropriate dilutions of homogenized tomato samples were spread plated on tryptic soy agar with 0.1% pyruvate and XLT4 agar for bacterial enumeration during drying and storage. Ten minutes of immersion in W, AA, or CA reduced bacterial populations by 0.7 to 1.6 log CFU/g. After 14 h of dehydration, total log reductions in the populations of bacteria were 3.2 to 4.5 (C), 3.7 to 4.9 (W), > 5.6 to > 6.1 (AA), and 4.5 to 5.5 (CA) log CFU/g, depending on type of agar used and condition of tomato samples. During drying and storage, the order of pathogen inactivation for predrying dipping treatments was AA > CA > W > C, with AA and CA rendering bacterial populations below detectable levels ( < 1.3 log CFU/g) prior to storage and between 7 and 14 days of storage, respectively. The results also indicated that peeling and blanching of tomatoes prior to inoculation may not necessarily affect destruction of Salmonella during the drying process. Use of predrying acid dipping treatments of tomatoes, especially in AA, may improve destruction of Salmonella during the dehydration process.

Internalization of bacterial pathogens in tomatoes and their control by selected chemicals

The effect of different washing or sanitizing agents was compared for preventing or reducing surface and internal contamination of tomatoes by Salmonella Typhimurium and Escherichia coli O157:H7. The tomatoes were inoculated by dipping them in a bacterial suspension containing approximately 6.0 log CFU/ml of each pathogen and then rinsing them with tap water, hypochlorite solution (250 mg/liter), or lactic acid solution (2%, wt/vol). All treatments were applied by dipping or spraying, and solutions were applied at 5, 25, 35, and 55 degrees C. With the exception of the lactic acid dip at 5 degrees C, all treatments reduced both pathogens on the surfaces of the tomatoes by at least 2.9 cycles. No significantly different results were obtained (P > 0.05) with the dipping and spraying techniques. For internalized pathogens, the mean counts for tomatoes treated with water alone or with chlorine ranged from 0.8 to 2.1 log CFU/g. In contrast, after lactic acid spray treatment, all core samples of tomatoes tested negative for Salmonella Typhimurium and, except for one sample with a low but detectable count, all samples tested negative for E. coli O157:H7 with a plate count method. When the absence of pathogens was verified by an enrichment method, Salmonella was not recovered from any samples, whereas two of four samples tested positive for E. coli O157:H7 even though the counts were negative. Few cells of internalized pathogens were able to survive in the center of the tomato during storage at room temperature (25 to 28 degrees C). The average superficial pH of tomatoes treated with tap water, chlorine, or lactic acid was 4.9 to 5.2, 4.1 to 4.3, and 2.5, respectively (P < 0.05), whereas no differences were observed in the internal pH (3.6 to 3.7) of the tomatoes treated with different sanitizers. The general practice in the tomato industry is to wash the tomatoes in chlorinated water. However, chlorine is rapidly degraded by organic matter usually present in produce. Therefore, lactic acid sprays may be a more effective alternative for decontaminating tomato surfaces. The use of warm (55 degrees C) sprays could reduce pathogen internalization during washing.

Evaluation of inoculation method and inoculum drying time for their effects on survival and efficiency of recovery of Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes inoculated on the surface of tomatoes

A study was undertaken to evaluate methods for applying inoculum and to examine the effect of inoculum drying time on survival and recovery of foodborne pathogens inoculated onto the surface of raw, ripe tomatoes. Five-strain mixtures of Escherichia coli O157:H7, Salmonella, or Listeria monocytogenes were applied to tomatoes by dip, spot, or spray inoculation methods. Inocula were dried for 1 or 24 h at 22 degrees C before tomatoes were treated with water (control) or chlorine (200 micrograms/ml). Significantly (alpha = 0.05) larger populations (CFU per tomato) of E. coli O157:H7 and Salmonella were recovered from dipinoculated tomatoes than from spot- or spray-inoculated tomatoes. This difference was attributed to larger numbers of cells adhering to tomatoes subjected to dip inoculation. Populations of E. coli O157:H7 and Salmonella recovered from spot- and spray-inoculated tomatoes containing the same initial number of cells were not significantly different. Significantly different L. monocytogenes population sizes were recovered from inoculated tomatoes (dip > spot > spray). Populations of pathogens recovered from tomatoes were significantly larger when inocula were dried for 1 h compared with 24 h. Significant differences (water > chlorine) were observed in the sizes of populations for all pathogens recovered from tomatoes treated with chlorine, regardless of inoculation method or drying time. Results indicate that inoculation method, drying time, and treatment affect survival and/or recovery of foodborne pathogens inoculated onto the surface of tomatoes. We recommend that spot inoculation with a drying time of 24 h at 22 degrees C be used with standard methods to determine the efficacy of chlorine and other sanitizers for killing foodborne pathogens on tomatoes.

A Bayesian approach to quantify the contribution of animal-food sources to human salmonellosis

Based on the data from the integrated Danish Salmonella surveillance in 1999, we developed a mathematical model for quantifying the contribution of each of the major animal-food sources to human salmonellosis. The model was set up to calculate the number of domestic and sporadic cases caused by different Salmonella sero and phage types as a function of the prevalence of these Salmonella types in the animal-food sources and the amount of food source consumed. A multiparameter prior accounting for the presumed but unknown differences between serotypes and food sources with respect to causing human salmonellosis was also included. The joint posterior distribution was estimated by fitting the model to the reported number of domestic and sporadic cases per Salmonella type in a Bayesian framework using Markov Chain Monte Carlo simulation. The number of domestic and sporadic cases was obtained by subtracting the estimated number of travel- and outbreak-associated cases from the total number of reported cases, i.e., the observed data. The most important food sources were found to be table eggs and domestically produced pork comprising 47.1% (95% credibility interval, CI: 43.3-50.8%) and 9% (95% CI: 7.8-10.4%) of the cases, respectively. Taken together, imported foods were estimated to account for 11.8% (95% CI: 5.0-19.0%) of the cases. Other food sources considered had only a minor impact, whereas 25% of the cases could not be associated with any source. This approach of quantifying the contribution of the various sources to human salmonellosis has proved to be a valuable tool in risk management in Denmark and provides an example of how to integrate quantitative risk assessment and zoonotic disease surveillance.

Metabiosis of proteolytic moulds and Salmonella in raw, ripe tomatoes

AIMS

The aim of this study was to determine the survival and growth characteristics of Salmonella enterica in sound and chill-injured tomatoes as influenced by co-infection with proteolytic moulds.

METHODS AND RESULTS

Sound (not chill injured) raw, ripe tomatoes (Lycopersicon esculentum Mill. 'Roma') were inoculated with a five-serotype mixture of S. enterica and/or Alternata alternata (two strains), Cladosporium herbarum and C. cladosporioides. Simultaneous and delayed (3 days) inoculation of tomatoes with Salmonella and each mould was studied. Growth of moulds in sound tomatoes stored at 15 and 25 degrees C for up to 10 days was accompanied by increased pH of radial pericarp tissue (pulp), which enhanced the growth of Salmonella. Growth of moulds and Salmonella at 25 degrees C was enhanced in chill-injured tomatoes compared with sound tomatoes.

CONCLUSIONS

Growth of proteolytic moulds in tomatoes stored at conditions simulating those commonly used in commercial postharvest storage and handling promotes the growth of Salmonella that may be an incidental contaminant.

SIGNIFICANCE AND IMPACT OF THE STUDY

Discarding tomatoes that are infected by moulds is important in handling and minimal processing practices designed to minimize the risk of human salmonellosis.

Comparison of culture, polymerase chain reaction (PCR), TaqMan Salmonella, and Transia Card Salmonella assays for detection of Salmonella spp. in naturally-contaminated ground chicken, ground turkey, and ground beef

Four types of assays were evaluated for the detection of Salmonella spp. in retail ground chicken (86 packages), ground turkey (104 packages), and ground beef (54 packages). Two 25 g samples from each package were separately subjected to pre-enrichment in buffered peptone water for 20 h at 37 degrees C followed by enrichment in Rappaport Vassiliadis (RV) broth for 20 h at 42 degrees C. The RV enrichments were plated onto Rambach agar, Rainbow Agar Salmonella, and XLT4 agar, and were also tested by a PCR assay targeting the Salmonella invA gene, as well as by the TaqMan Salmonella PCR assay. Additionally, the RV enrichments were tested using the Transia Card Salmonella immunoassay. Results showed that 16.8, 24.0, 28.8, and 26.4% of turkey samples were positive for Salmonella spp. by culture, PCR, TaqMan PCR, and Transia Card Salmonella assays, respectively. Eighteen, 28.5, 35.5, and 34.9% of chicken samples were positive by culture, PCR, TaqMan PCR, and Transia Card Salmonella assays, respectively, and 6.5, 6.5, 6.5, and 18.5% of ground beef samples were positive by the four assays, respectively. Analysis of the data using the kappa statistic showed that there was substantial to excellent agreement between the PCR and TaqMan PCR assays and between the PCR and culture assays (kappa coefficients ranging from 0.67 to 0.87), while there was poor to fair agreement between the results of the Transia Card Salmonella assay and the other methods (kappa coefficients ranging from 0.28 to 0.32). Overall, results showed that the PCR-based assays were more sensitive than the culture method, and the culture and PCR-based assays were more specific than the immunoassay for detection of Salmonella in ground chicken, turkey, and beef due to the occurrence of false positive results using the immunoassay.

Effect of inoculum size, relative humidity, storage temperature, and ripening stage on the attachment of Salmonella Montevideo to tomatoes and tomatillos

The influence of inoculum populations and environmental factors on attachment of Salmonella Montevideo to the surface of tomatoes and tomatillos was evaluated. To study the effect of inoculum size, red, ripe tomatoes were spot-inoculated with bacterial suspensions (10(5) and 10(8) CFU/fruit) and stored at 22 degrees C under 100% relative humidity. The effects of temperature (12, 22, and 30 degrees C) and relative humidity (75, 85, and 97%) on attachment of the pathogen (10(7) CFU/fruit) to tomatoes (red and green) and ripe tomatillos were also evaluated. Inoculated fruits were stored for 90 min at all combinations of temperature and relative humidity, and after rinsing with water, the number of cells attached to the surface was determined. Salmonella Montevideo attached to the surface of tomatoes within 90 min. A direct correlation between the number of attached cells and the population in the inoculum was observed. The percentage of cells that attached immediately after inoculation was approximately 0.3% for the three test products. After storage for 90 min at various temperature and relative humidity conditions, the number of adhering cells ranged from 4.0 to 5.4 log CFU/fruit (1.2% of inoculum). Both the type of product and the temperature/relative humidity combination had a significant (P < 0.05) effect on attachment of Salmonella Montevideo to the surfaces of tomatoes and tomatillos. Scanning electron micrographs of the cuticles of inoculated washed tomatoes and tomatillos revealed typical skin cell patterns, and only a few randomly dispersed Salmonella Montevideo were observed. Deposition of Salmonella Montevideo on the surface of tomatoes and tomatillos could result in attachment and subsequent colonization under suitable conditions.

Potential of a plant-parasitic nematode to facilitate internal contamination of tomato plants by Salmonella

The objective of this study was to determine whether tomato plants infested with a plant-parasitic nematode, Meloidogne incognita, can internalize Salmonella. Tomato plants (Lycopersicon esculentum Mill. 'Rutgers') were grown in soil infested with M. incognita and/or inoculated with a six-serotype mixture of Salmonella enterica. M. incognita, upon wounding roots when parasitizing the tomato plant, does not result in the entry and survival of Salmonella. Analysis of roots, galls, stems, and leaves 2 and 4 weeks after inoculation of the soil failed to reveal the presence of Salmonella. Salmonella remained viable in soil for at least 4 weeks. The potential for the presence of Salmonella in the tissues of tomato fruits via root entrance facilitated by M. incognita appears to be remote.