Enterohemorrhagic Escherichia coli O157:H7 present in radish sprouts

Characterization of the binding of diarrheagenic strains of E. coli to plant surfaces and the role of curli in the interaction of the bacteria with alfalfa sprouts

Diarrheagenic Escherichia coli were able to bind to plant surfaces, including alfalfa sprouts and open seed coats, and tomato and Arabidopsis thaliana seedlings incubated in water. The characteristics of the binding differed with the bacterial strain examined. Laboratory K12 strains of E. coli failed to show significant binding to any of the plant surfaces examined, suggesting that some of the genes present and expressed in pathogenic strains and absent or unexpressed in K12 strains may be required for binding to plants. When a plasmid carrying the mlrA gene (a positive regulator of curli biosynthesis) or a plasmid carrying the operons that encode the synthesis of curli (csgA-G) was introduced into K12 strains, the bacteria acquired the ability to bind to sprouts. CsgA mutants of an avian pathogenic E. coli and an O157:H7 strain showed no reduction in their ability to bind to sprouts. Thus, the production of curli appears to be sufficient to allow K12 strains to bind, but curli are not necessary for the binding of pathogenic strains, suggesting that pathogenic strains may have more than one mechanism for binding to plant surfaces.

Temporal clusters of Salmonella serovars in humans in Alberta, 1990-2001

BACKGROUND

Temporal clusters of salmonellosis are believed to occur but have not been quantitatively explored, thus, our objectives were to describe trends, seasonal patterns, and clusters of salmonellosis in humans in Alberta by examining isolates reported through passive surveillance systems.

METHODS

Cases of salmonellosis reported through Notifiable Disease Records between January 1990 and December 2001 were obtained from Alberta Health and Wellness. Least squares regression was used to characterize the distribution (long-term trends and seasonal patterns) of isolates. A cluster detection test was used to determine if and when isolates of specific serovars aggregated in time, over and above the background distribution. Comparisons were made to temporal patterns in Alberta livestock and to known outbreaks in humans.

RESULTS

S. Typhimurium, Enteritidis, Hadar, Heidelberg, and Thompson were the five most common serovars of the 9,188 isolates reported. The annual number of isolates was relatively stable over time, with a distinct summertime seasonal pattern. Clusters were observed in 23 of 32 serovars examined. More clusters occurred in September and October than in other months, and in 1998 through 2001 than in the early to mid-1990s. Also, more clusters were of short duration than long.

INTERPRETATION

Short-duration clusters likely indicate a point source of infection, while long-duration clusters may indicate exposure to a persistent common source or the occurrence of secondary infections. A sharp increasing trend and a large cluster of S. Heidelberg may have public health implications. Surveillance activities focussed on similarities between common serovars, trends, and temporal clusters in humans and animals, and studies on factors associated with autumn clusters may be useful in preventing outbreaks in humans.

Efficacy of aerosolized peroxyacetic acid as a sanitizer of lettuce leaves

Aerosolized sanitizer was investigated as a potential alternative to aqueous and gaseous sanitizers for produce. Peroxyacetic acid was aerosolized (5.42 to 11.42 microm particle diameter) by a commercially available nebulizer into a model cabinet. Iceberg lettuce leaves were inoculated with three strains each of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Typhimurium and then treated with aerosolized peroxyacetic acid for 10, 30, or 60 min in a model aerosol cabinet at room temperature (22 +/- 2 degrees C). After treatment, surviving healthy and injured bacterial cells were enumerated on appropriate selective agars or using the overlay agar method. Inoculated iceberg lettuce leaves exposed to aerosolized peroxyacetic acid for 10 min exhibited a 0.8-log reduction in E. coli O157:H7, a 0.3-log reduction in Salmonella Typhimurium, and a 2.5-log reduction in L. monocytogenes when compared with the control. After 30 min of treatment, the three pathogens were reduced by 2.2, 3.3, and 2.7 log, and after 60 min, the reductions were 3.4, 4.5, and 3.8 log, respectively. Aerosolization may be a new and convenient method for sanitizing produce for storage or shipping.

Use of fluorescent microspheres as a tool to investigate bacterial interactions with growing plants

Foodborne pathogens may exist as endophytes of growing plants. The internalization of Escherichia coli O157:H7 or other foodborne pathogens in growing lettuce plants may be independent of microbial factors. Mature lettuce plants were surface irrigated with E. coli O157:H7 or with FluoSpheres (fluorescent microspheres) and harvested 1, 3, and 5 days post-exposure. FluoSpheres were utilized as a bacterial surrogate. Microscopic examination of root, stem, and leaf tissue sections revealed that FluoSpheres were internalized into growing plants. Laser scanning confocal microscopy revealed that FluoSpheres were present within the root tissue and leaf stem tissue. The presence of FluoSpheres in internal portions of stem and leaf tissue suggests transport of the spheres from the root upward into the edible tissue. The level of uptake of FluoSpheres and E. coli O157:H7 was quantified using filtration. Numbers of FluoSpheres and E. coli O157:H7 cells in plant tissue were similar. The entry of E. coli O157:H7 into lettuce plants may be a passive event because the concentration of FluoSpheres was similar to that of the pathogen.

Interactions of Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes plants cultivated in a gnotobiotic system

The growth and persistence of Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes on a diverse range of plant types over extended cultivation periods was studied. When introduced on the seed of carrot, cress, lettuce, radish, spinach and tomato all the pathogens became rapidly established shortly after germination, attaining cell densities of the order of 5.5-6.5 log cfu/g. In general, Es. coli O157:H7 and L. monocytogenes became established and persisted at significantly higher levels on seedlings (9 days post-germination) than Salmonella. Es. coli O157:H7 became internalized in cress, lettuce, radish and spinach seedlings but was not recovered within the tissues of mature plants. Internalization of Salmonella was also observed in lettuce and radish but not cress or spinach seedlings. In contrast, L. monocytogenes did not internalize within seedlings but did persist on the surface of plants throughout the cultivation period. Co-inoculation of isolates recovered from the rhizosphere of plants did not significantly affect the numbers or persistence of human pathogens. The only exception was with Enterobacter cloacae, which reduced Es. coli O157:H7 Ph1 and L. monocytogenes levels by ca. 1 log cfu/g on lettuce. With the bioluminescent phenotype of Es. coli O157:H7 Ph1, it was demonstrated that the human pathogen became established on the roots of growing plants. Scanning electron micrographs of root seedlings suggested that Es. coli O157:H7 Ph1 preferentially colonized the root junctions of seedlings. It is proposed that such colonization sites enhanced the persistence of Es. coli O157:H7 on plants and facilitated internalization within developing seedlings. The results suggest that the risk associated with internalized human pathogens in salad vegetables at harvest is low. Nevertheless, the introduction of human pathogens at an early stage of plant development could enhance their persistence in the rhizosphere. The implications of the study with regards to on-farm food safety initiatives are discussed.

Factors affecting production of extracellular carbohydrate complexes by Escherichia coli O157:H7

Production of extracellular carbohydrate complexes (ECC) by foodborne pathogens on raw fruits and vegetables may result in protection against removal or inactivation by sanitizers. The influence of environmental conditions on cell growth, the total amount of ECC produced, and the amount of ECC produced on a per cell basis by Escherichia coli O157:H7 strains ATCC 43895 (wild type) and 43895-exopolysaccharides (EPS) (natural mutant, extensive EPS producer) was studied. To determine the effects of pH on the production of ECC on a per cell basis, E. coli O157:H7 was grown aerobically at 12 and 22 degrees C on tryptic soy agar (TSA) acidified to pH 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, and 4.0. Lettuce, alfalfa sprout, cantaloupe, tomato, and apple juice agars (pH 4.46-6.50) were also evaluated for their support of the ECC production. Conditions generally favorable for growth of E. coli O157:H7 were a rich nutrient medium (TSA) vs. heated lettuce juice agar (HLJA) or minimal salts medium (MSM), 22 degrees C compared to 12 degrees C, and an aerobic atmosphere compared to modified atmosphere (1% O(2), 10% CO(2), and 89% N(2)). Conditions favorable for production of ECC on a per cell basis were HLJA, 12 degrees C, and an aerobic atmosphere. This suggests that modified atmosphere packaging of lettuce may not only decrease the growth of E. coli O157:H7 but also its propensity to form biofilm. There was a negative relationship between cell growth and production of ECC on a per cell basis, and environmental conditions that affected the total amount of ECC produced based on initial population reflected a combination of environmental conditions influencing both cell growth and ECC production on a per cell basis. A relative growth index factor (RGIF) was calculated to better understand ECC production as affected by various environmental conditions simultaneously. The production of ECC on a per cell basis by strain 43895-EPS showed a negative linear relationship with pH of TSA at both 12 and 22 degrees C. This strain generally produced a greater amount of ECC on fresh juice agar than on TSA at the same pH, but production of ECC on alfalfa sprout juice agar (FJA, pH 6.45) at 22 degrees C was significantly less than on TSA (pH 6.50). This indicates that nutrient limitation is not based only on nutrient availability. There may be other factors that repress the production of ECC on FJA, and the effects of those factors may be temperature dependent. Further studies will be required to better understand the relationship between nutrient availability and other factors on the production of ECC by E. coli O157:H7 on raw produce.

Fresh produce: a growing cause of outbreaks of foodborne illness in the United States, 1973 through 1997

Fresh produce is an important part of a healthy diet. During the last three decades, the number of outbreaks caused by foodborne pathogens associated with fresh produce consumption reported to the Centers for Disease Control and Prevention has increased. To identify trends, we analyzed data for 1973 through 1997 from the Foodborne Outbreak Surveillance System. We defined a produce-associated outbreak as the occurrence of two or more cases of the same illness in which epidemiologic investigation implicated the same uncooked fruit, vegetable, salad, or juice. A total of 190 produce-associated outbreaks were reported, associated with 16,058 illnesses, 598 hospitalizations, and eight deaths. Produce-associated outbreaks accounted for an increasing proportion of all reported foodborne outbreaks with a known food item, rising from 0.7% in the 1970s to 6% in the 1990s. Among produce-associated outbreaks, the food items most frequently implicated included salad, lettuce, juice, melon, sprouts, and berries. Among 103 (54%) produce-associated outbreaks with a known pathogen, 62 (60%) were caused by bacterial pathogens, of which 30 (48%) were caused by Salmonella. During the study period, Cyclospora and Escherichia coli O157:H7 were newly recognized as causes of foodborne illness. Foodborne outbreaks associated with fresh produce in the United States have increased in absolute numbers and as a proportion of all reported foodborne outbreaks. Fruit and vegetables are major components of a healthy diet, but eating fresh uncooked produce is not risk free. Further efforts are needed to better understand the complex interactions between microbes and produce and the mechanisms by which contamination occurs from farm to table.

Autoinducer-2-like activity associated with foods and its interaction with food additives

The autoinducer-2 (AI-2) molecule produced by bacteria as part of quorum sensing is considered to be a universal inducer signal in bacteria because it reportedly influences gene expression in a variety of both gram-negative and gram-positive bacteria. The objective of this study was to determine whether selected fresh produce and processed foods have AI-2-like activity and whether specific food additives can act as AI-2 mimics and result in AI-2-like activity. The luminescence-based response of the reporter strain Vibrio harveyi BB170 was used as the basis for determining AI-2 activity in the selected foods and food ingredients. Maximum AI-2 activity was seen on the frozen fish sample (203-fold, compared with the negative control) followed by tomato, cantaloupe, carrots, tofu, and milk samples. Interestingly, some samples were capable of inhibiting AI-2 activity. Turkey patties showed the highest inhibition (99.8% compared with the positive control) followed by chicken breast (97.5%), homemade cheeses (93.7%), beef steak (90.6%), and beef patties (84.4%). AI-2 activity was almost totally inhibited by sodium propionate, whereas sodium benzoate caused 93.3% inhibition, compared with 75% inhibition by sodium acetate. Sodium nitrate did not have any appreciable effect, even at 200 ppm. Understanding the relationships that exist between AI-2 activity on foods and the ecology of pathogens and food spoilage bacteria on foods could yield clues about factors controlling food spoilage and pathogen virulence.

Isolation, identification, and selection of lactic acid bacteria from alfalfa sprouts for competitive inhibition of foodborne pathogens

Several studies have investigated the control of pathogens on alfalfa sprouts, and some treatments have been shown to be effective in reducing pathogen populations. However, control methods investigated thus far only provide pathogen control at a given point in the sprouting process and can affect germination. Competitive inhibition of pathogens with lactic acid bacteria might provide pathogen control throughout the sprouting process and up to consumption. The purpose of this study was to isolate and identify lactic acid bacteria from alfalfa sprouts to inhibit the growth of foodborne pathogens. Fifty-eight lactic acid bacteria isolates were obtained from alfalfa seeds and sprouts. These isolates were evaluated for inhibitory action against Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes by agar spot tests. All pathogens were inhibited by 32 (55%) of the isolates, S. enterica by 56 (97%), E. coli O157:H7 by 49 (84%), and L. monocytogenes by 41 (71%). The isolates were identified by the Analytical Profile Index evaluation of carbohydrate utilization. Isolates obtained from a sample of alfalfa seeds and identified as Lactococcus lactis subsp. lactis showed zones of inhibition of 4.0 mm or greater for all pathogens. One of these isolates, Lactococcus lactis subsp. lactis (L7), and an isolate previously obtained, Pediococcus acidilactici (D3), were evaluated for competitive inhibition of S. enterica, E. coli O157:H7, and L. monocytogenes in deMan Rogosa Sharpe agar and broth. Pathogen populations were significantly reduced by day 5. The selected isolates will be further evaluated in future studies for inhibitory action toward S. enterica, E. coli O157:H7, and L. monocytogenes during sprouting.

Development of method to quantify extracellular carbohydrate complexes produced by Escherichia coli O157:H7

AIMS

The aim of this study was to optimize conditions to separate extracellular carbohydrate complexes (ECC) produced by Escherichia coli O157:H7 and to standardize the amount of ECC produced on a per cell basis.

METHODS AND RESULTS

ECC fraction I was removed from E. coli O157:H7 cells produced on tryptic soya agar and lettuce juice agar by centrifugation. To remove ECC fraction II, cells were heated at 100 degrees C for 10 min, then centrifuged. The sum of ECC fractions I and II was considered as the total ECC produced by E. coli O157:H7. A correlation between cell mass and turbidity (O.D. 750 nm) of cell suspensions was determined. Cell mass has a linear relationship (R2 = 0.93) with turbidity of cell suspensions from which ECC is removed. The amount of ECC produced on a per cell basis was calculated by dividing total amount of ECC (microgram ml-1) produced by the turbidity (O.D. 750 nm) of heated cell suspension after removal ECC fractions I and II.

CONCLUSIONS

A method for separating ECC from cells of E. coli O157:H7 has been developed and conditions have been optimized. A standard method to estimate the amount of ECC produced on a per cell basis was also developed.

SIGNIFICANCE AND IMPACT OF THE STUDY

Using these procedures to prepare extract of ECC from E. coli O157:H7 and to standardize values, production of ECC on a per cell basis can be estimated and a comparison of the amount of ECC produced by the pathogen grown under different environmental conditions can be accurately measured.

Occurrence of inhibitory compounds in spent growth media that interfere with acid-tolerance mechanisms of enteric pathogens

Understanding the acid-tolerance ability of enteric human pathogens is critical in determining microbial food safety and the associated risk. We have discovered naturally occurring compounds in the spent growth media, which inhibit the acid-tolerance ability of several enteric human pathogens when challenged at pH 3.0 for 2 h at 37 degrees C. The compounds were detected in the spent growth media obtained from Escherichia coli O157:H7, Salmonella spp., and Vibrio cholera, but not from Shigella flexneri. The compounds were effective in reducing pathogen survival by 5-logs during acid-challenge assay. The low molecular weight (<300 Da) and heat resistant nature (121 degrees C, 15 min, at 15 psi) of the compounds make them excellent candidates to explore their suitability as food additives that would increase microbial food safety.

Internalization of bioluminescent Escherichia coli and Salmonella Montevideo in growing bean sprouts

AIMS

Investigate the interaction of bioluminescent Escherichia coli and Salmonella Montevideo with germinating mung bean sprouts.

METHODS AND RESULTS

E. coli or Salm. Montevideo introduced on mung beans became established both internally and externally on sprouts after the initial 24 h germinating period. In both cases the inoculated bacterium formed the predominant microflora on the sprouted beans throughout. From the bioluminescent profile of inoculated sprouting beans, bacterial growth was found to be in close proximity to the roots but not on the hypocotyls. Clumps (biofilms) of cells with low viability were observed within the grooves between epidermal cells on hypocotyls. Treatment with 20,000 ppm sodium hypochlorite removed the majority of bacteria from the surface of hypocotyls although nonviable single cells were occasionally observed. However, viable bacteria were recovered from the apoplastic fluid, and extracts of surface-sterilized sprouts indicating that the internal bacterial populations had been protected. This was confirmed using in situ beta-glucuronidase staining of surface-sterilized sprouts where cleaved enzyme substrate (by the action of internalized E. coli) was visualized within the plant vascular system.

CONCLUSIONS

E. coli or Salmonella present on seeds become internalized within the subsequent sprouts and cannot be removed by postharvest biocidal washing.

SIGNIFICANCE AND IMPACT OF THE STUDY

Mung bean production should be carefully controlled to prevent contamination occurring in order to minimize the health risk associated with raw bean sprouts.

Efficacy of chlorine and calcinated calcium treatment of alfalfa seeds and sprouts to eliminate Salmonella

The efficacy of a 20,000 ppm calcium hypochlorite treatment of alfalfa seeds artificially contaminated with Salmonella was studied. Salmonella populations reached >7.0 log on sprouts grown from seeds artificially contaminated with Salmonella and then treated with 20,000 ppm Ca(OCl)(2). The efficacy of spray application of chlorine (100 ppm) to eliminate Salmonella during germination and growth of alfalfa was assessed. Alfalfa seed artificially contaminated with Salmonella was treated at germination, on day 2 or day 4, or for the duration of the growth period. Spray application of 100 ppm chlorine at germination, day 2, or day 4 of growth was minimally effective resulting in approximately a 0.5-log decrease in population of Salmonella. Treatment on each of the 4 days of growth reduced populations of Salmonella by only 1.5 log. Combined treatment of seeds with 20,000 ppm Ca(OCl)(2) and followed by 100 ppm chlorine or calcinated calcium during germination and sprout growth did not eliminate Salmonella.

Evaluation of increased incubation temperature and cefixime-tellurite treatment for the isolation of Escherichia coli O157:H7 from minced beef

To improve enrichment and isolation of Escherichia coli O157:H7, this study evaluated increased incubation temperature and cefixime-tellurite (CT) on five strains of each of the following bacteria, E. coli, Hafnia alvei, Enterobacter spp., Citrobacter freundii and E. coli O157:H7, and two strains of E. coli O157:nH7. These were grown in pure culture in LST broth with varying cefixime-tellurite concentrations. A range of incubation temperatures from 37 to 46 degrees C was investigated for the inhibition of cohabitant microorganisms. Minced beef, spiked with E. coli O157:H7 and cohabitant microorganisms was investigated. Increased incubation temperature (42 degrees C) and treatment with half of the prescribed amount of cefixime-tellurite by BAM for SMAC agar in enrichment step were the most effective in selectively growing E. coli O157:H7. The results show that E. coli O157:H7 is more resistant to these two conditions than the other cohabitant bacteria.

Interaction of Escherichia coli with growing salad spinach plants

In this study, the interaction of a bioluminescence-labeled Escherichia coli strain with growing spinach plants was assessed. Through bioluminescence profiles, the direct visualization of E. coli growing around the roots of developing seedlings was accomplished. Subsequent in situ glucuronidase (GUS) staining of seedlings confirmed that E. coli had become internalized within root tissue and, to a limited extent, within hypocotyls. When inoculated seeds were sown in soil microcosms and cultivated for 42 days, E. coli was recovered from the external surfaces of spinach roots and leaves as well as from surface-sterilized roots. When 20-day-old spinach seedlings (from uninoculated seeds) were transferred to soil inoculated with E. coli, the bacterium became established on the plant surface, but internalization into the inner root tissue was restricted. However, for seedlings transferred to a hydroponic system containing 10(2) or 10(3) CFU of E. coli per ml of the circulating nutrient solution, the bacterium was recovered from surface-sterilized roots, indicating that it had been internalized. Differences between E. coli interactions in the soil and those in the hydroponic system may be attributed to greater accessibility of the roots in the latter model. Alternatively, the presence of a competitive microflora in soil may have restricted root colonization by E. coli. The implications of this study's findings with regard to the microbiological safety of minimally processed vegetables are discussed.

Antimicrobial performance of alkaline ionic fluid (GC-100X) and its ability to remove Escherichia coli O157:H7 from the surface of tomatoes

An efficacy test of GC-100X, a noncorrosive alkaline ionic fluid (pH 12) composed of free radicals and supplemented with xylitol, was carried out against six major foodborne pathogens-Staphylococcus aureus FRI 913, Salmonella enterica serovar Enteritidis ATCC 13076, S. enterica serovar Typhimurium DT104 Korean isolate, Vibrio parahaemolyticus ATCC 17803, Escherichia coli O157:H7 ATCC 43894, and Pseudomonas aeruginosa KCTC 1637-at three different temperatures (4, 25, and 36 degrees C) with or without organic load (2% yeast extract). Results revealed a more than 4-log10 (CFU/ml) reduction (1.0 x 10(4) CFU/ml reduction) against all pathogens reacted at 37 degrees C for 3 h in the absence of organic material. GC-100X solution diluted with an equal volume of distilled or standard hard water (300 ppm CaCO3) showed effective bactericidal activity, particularly against gram-negative bacteria. Washing efficacy of GC-100X solution was compared against E. coli O157:H7 on cherry tomato surfaces with those of a commercially used detergent and chlorine water (100 ppm). Viable cell counts of E. coli O157:H7 that had penetrated to the cores of tomatoes after sanitizing treatment revealed that GC-100X stock and its 5% diluted solutions had similar washing effects to 100-ppm chlorine water and were more effective than the other kitchen detergent. These results indicate that GC-100X has good bactericidal and sanitizing activities and is useful as a new sanitizer for food safety and kitchen hygiene.

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.

Colonization of Arabidopsis thaliana with Salmonella enterica and enterohemorrhagic Escherichia coli O157:H7 and competition by Enterobacter asburiae

Enteric pathogens, such as Salmonella enterica and Escherichia coli O157:H7, have been shown to contaminate fresh produce. Under appropriate conditions, these bacteria will grow on and invade the plant tissue. We have developed Arabidopsis thaliana (thale cress) as a model system with the intention of studying plant responses to human pathogens. Under sterile conditions and at 100% humidity, S. enterica serovar Newport and E. coli O157:H7 grew to 10(9) CFU g(-1) on A. thaliana roots and to 2 x 10(7) CFU g(-1) on shoots. Furthermore, root inoculation led to contamination of the entire plant, indicating that the pathogens are capable of moving on or within the plant in the absence of competition. Inoculation with green fluorescent protein-labeled S. enterica and E. coli O157:H7 showed invasion of the roots at lateral root junctions. Movement was eliminated and invasion decreased when nonmotile mutants of S. enterica were used. Survival of S. enterica serovar Newport and E. coli O157:H7 on soil-grown plants declined as the plants matured, but both pathogens were detectable for at least 21 days. Survival of the pathogen was reduced in unautoclaved soil and amended soil, suggesting competition from indigenous epiphytes from the soil. Enterobacter asburiae was isolated from soil-grown A. thaliana and shown to be effective at suppressing epiphytic growth of both pathogens under gnotobiotic conditions. Seed and chaff harvested from contaminated plants were occasionally contaminated. The rate of recovery of S. enterica and E. coli O157:H7 from seed varied from undetectable to 19% of the seed pools tested, depending on the method of inoculation. Seed contamination by these pathogens was undetectable in the presence of the competitor, Enterobacter asburiae. Sampling of 74 pools of chaff indicated a strong correlation between contamination of the chaff and seed (P = 0.025). This suggested that contamination of the seed occurred directly from contaminated chaff or by invasion of the flower or silique. However, contaminated seeds were not sanitized by extensive washing and chlorine treatment, indicating that some of the bacteria reside in a protected niche on the seed surface or under the seed coat.

Simultaneous detection of Escherichia coli O157:H7, Listeria monocytogenes and Salmonella strains by real-time PCR

A protocol enabling simultaneous detection of Escherichia coli O157:H7, Listeria monocytogenes and Salmonella strains was devised and evaluated using artificially contaminated fresh produce. Association of Official Analytical Chemists (AOAC)-approved polymerase chain reaction (PCR) detection methods for three human pathogens were modified to enable simultaneous and real-time detection with high throughput capability. The method includes a melting-curve analysis of PCR products, which serves as confirmatory test. The modified protocol successfully detected all three pathogens when fresh produce was washed with artificially contaminated water containing E. coli O157:H7 and S. typhimurium down to the predicted level of 1 to 10 cells/ml and L. monocytogenes at 1000 cells/ml. The ability to monitor several pathogens simultaneously will save time and increase our ability to assure food safety.

Alfalfa seed decontamination in a Salmonella outbreak

Based on in vitro data, the U.S. Food and Drug Administration recommends chemical disinfection of raw sprout seeds to reduce enteric pathogens contaminating the seed coats. However, little is known about the effectiveness of decontamination at preventing human disease. In 1999, an outbreak of Salmonella enterica serotype Mbandaka occurred in Oregon, Washington, Idaho, and California. Based on epidemiologic and pulsed-field gel electrophoresis evidence from 87 confirmed cases, the outbreak was linked to contaminated alfalfa seeds grown in California's Imperial Valley. Trace-back and trace-forward investigations identified a single lot of seeds used by five sprout growers during the outbreak period. Cases of salmonellosis were linked with two sprout growers who had not employed chemical disinfection; no cases were linked to three sprout growers who used disinfection. This natural experiment provides empiric evidence that chemical disinfection can reduce the human risk for disease posed by contaminated seed sprouts.

Evidence of association of salmonellae with tomato plants grown hydroponically in inoculated nutrient solution

The possibility of uptake of salmonellae by roots of hydroponically grown tomato plants was investigated. Within 1 day of exposure of plant roots to Hoagland nutrient solution containing 4.46 to 4.65 log(10) CFU of salmonellae/ml, the sizes of the pathogen populations were 3.01 CFU/g of hypocotyls and cotyledons and 3.40 log(10) CFU/g of stems for plants with intact root systems (control) and 2.55 log(10) CFU/g of hypocotyls and cotyledons for plants from which portions of the roots had been removed. A population of > or =3.38 log(10) CFU/g of hypocotyls-cotyledons, stems, and leaves of plants grown for 9 days was detected regardless of the root condition. Additional studies need to be done to unequivocally demonstrate that salmonellae can exist as endophytes in tomato plants grown under conditions that simulate commonly used agronomic practices.

Differences in growth of Salmonella enterica and Escherichia coli O157:H7 on alfalfa sprouts

Sprout producers have recently been faced with several Salmonella enterica and Escherichia coli O157:H7 outbreaks. Many of the outbreaks have been traced to sprout seeds contaminated with low levels of human pathogens. Alfalfa seeds were inoculated with S. enterica and E. coli O157:H7 strains isolated from alfalfa seeds or other environmental sources and sprouted to examine growth of these human pathogens in association with sprouting seeds. S. enterica strains grew an average of 3.7 log(10) on sprouting seeds over 2 days, while E. coli O157:H7 strains grew significantly less, an average of 2.3 log(10). The initial S. enterica or E. coli O157:H7 inoculum dose and seed-sprouting temperature significantly affected the levels of both S. enterica and E. coli O157:H7 on the sprouts and in the irrigation water, while the frequency of irrigation water replacement affected only the levels of E. coli O157:H7. Colonization of sprouting alfalfa seeds by S. enterica serovar Newport and E. coli O157:H7 strains transformed with a plasmid encoding the green fluorescent protein was examined with fluorescence microscopy. Salmonella serovar Newport colonized both seed coats and sprout roots as aggregates, while E. coli O157:H7 colonized only sprout roots.

Microbial risk assessment of source-separated urine used in agriculture

A screening-level quantitative microbial risk assessment (QMRA) was undertaken for a urine separating sewerage system. Exposures evaluated included the handling of stored and unstored urine as well as consumption of crops fertilised with urine. Faecal cross-contamination was the source of risk and Campylobacter jejuni, Cryptosporidium parvum and rotavirus were the organisms chosen to represent different groups of enteric pathogens. Accidental ingestion of unstored urine implied a high risk (Pinf = 0.56) for infection from rotavirus whereas the risks for infection from bacteria and protozoa were approximately 1:10 000. After six months storage at 20 degrees C the risk for viral infections by accidental ingestion of 1 ml of urine was < 10(-3), the suggested acceptable risk benchmark. Ingestion of crops contaminated with urine resulted in risks of < 10(-5) after a 3-week (<10(-7) after 4 weeks) withholding period between fertilising and harvesting.

Reduction of the native microflora on alfalfa sprouts during propagation by addition of antimicrobial compounds to the irrigation water

Alfalfa and other types of sprouts are known to harbor large populations of native microorganisms. As some of these microbes may be causes of reduced shelf life of the product (plant pathogens and other spoilage organisms) and sprouts may, on occasion, harbor bacteria pathogenic towards humans, the addition of antimicrobial compounds to the irrigation water may be warranted. In this study, we tested the efficacy of several antimicrobial compounds for reducing the native microbial populations on alfalfa sprouts during propagation. These compounds included H2O2, peroxyacetic acid+hydrogen peroxide (Tsunami 100), acidified NaClO2, NaClO2 (Aquatize), EDTA, Na3PO4 and NaOCl. When added to the irrigation water at vanous concentrations, none of the antimicrobial compounds reduced the levels of any class of native microflora by more than 1 log10 without evidence of phytotoxicity.

Evaluation of techniques for enrichment and isolation of Escherichia coli O157:H7 from artificially contaminated sprouts

Because sprouted seed products are kept wet during and after production, have high levels of nutrients, and a neutral pH, they are subject to the outgrowth of pathogens such as Escherichia coli O157:H7. For these same reasons, these products also contain high levels of heterotrophic organisms and in particular coliform bacteria. Recent outbreaks have focused attention on the need to improve methodology for isolating this pathogen from sprouts. When 40 E. coli O157:H7 strains were grown in pure culture in enterohemorrhagic E. coli enrichment broth (EEB) as prescribed in the U.S. FDA-Bacteriological Analytical Manual (FDA-BAM) and in EEB modified by varying the cefixime concentration, outgrowth for all strains in EEB was inhibited at 0.05 mg/l but for only 2 of 40 strains when the cefixime level was adjusted to 0.0125 mg/l. These two enrichment formulae were compared to modified E. coli broth (mEC), modified Tryptic Soy Broth with 20 mg/l novobiocin (mTSB + N), modified Buffered Peptone Water (mBPW), and mBPW with added 10 mg/l acriflavin, 10 mg/l cefsulodin, and 8 mg/l vancomycin (mBPW + ACV) for isolation of E. coli O157:H7 from sprouts. These comparisons were performed using low-level (0.12 to 0.42 cfu/g) artificially contaminated alfalfa and mixed salad sprouts. After enrichment, two isolation methods were compared for recovery; direct plating to Tellurite-Cefixime Sorbitol MacConkey agar (TCSMAC) and immunomagnetic separation (IMS) (Dynabeads anti-E. coli O157, Dynal, Oslo, Norway) followed by plating to TCSMAC. In addition, an immunoprecipitin detection kit, VIP (BioControl, Bellevue, WA), was evaluated for detection after enrichment. We found that five of the six enrichments were equivalent for detection or recovery while one enrichment (mTSB + N without agitation) was less productive. Incubation for 24 h was more effective in recovering E. coli O157:H7 from sprouts than 6 h for all enrichment broths. Plating after IMS was more productive than direct plating at these low levels of contamination, yielding recovery in 70 of 90 trials compared to 37 of 90 trials without IMS for six enrichments. The sensitivity of VIP for detection of E. coli O157:H7 varied depending on the enrichment broth. Because of the rapid rate of growth of E. coli O157:H7 in mBPW, the high productivity of mBPW + ACV after 24-h enrichment and its compatibility with both IMS and detection with immunoprecipitin tests, mBPW + ACV at 42 degrees C with agitation was found to be the most promising enrichment protocol for testing sprouts.

Use of green fluorescent protein expressing Salmonella Stanley to investigate survival, spatial location, and control on alfalfa sprouts

Laser scanning confocal microscopy (LSCM) was used to observe the interaction of Salmonella Stanley with alfalfa sprouts. The green fluorescent protein (gfp) gene was integrated into the chromosome of Salmonella Stanley for constitutive expression, thereby eliminating problems of plasmid stability and loss of signal. Alfalfa seeds were inoculated by immersion in a suspension of Salmonella Stanley (ca. 10(7) CFU/ml) for 5 min at 22 degrees C. Epifluorescence microscopy demonstrated the presence of target bacteria on the surface of sprouts. LSCM demonstrated bacteria present at a depth of 12 microm within intact sprout tissue. An initial population of ca. 10(4) CFU/g seed increased to 7.0 log CFU/g during a 24-h germination period and then decreased to 4.9 log CFU/g during a 144-h sprouting period. Populations of Salmonella Stanley on alfalfa seeds decreased from 5.2 to 4.1 log CFU/g and from 5.2 to 2.8 log CFU/g for seeds stored 60 days at 5 and 22 degrees C, respectively. The efficacy of 100, 200, 500, or 2,000 ppm chlorine in killing Salmonella Stanley associated with sprouts was determined. Treatment of sprouts in 2,000 ppm chlorine for 2 or 5 min caused a significant reduction in populations of Salmonella Stanley. Influence of storage on Salmonella Stanley populations was investigated by storing sprouts 4 days at 4 degrees C. The initial population (7.76 log CFU/g) of Salmonella Stanley on mature sprouts decreased (7.67 log CFU/g) only slightly. Cross-contamination during harvest was investigated by harvesting contaminated sprouts, then directly harvesting noncontaminated sprouts. This process resulted in the transfer of ca. 10(5) CFU/g Salmonella Stanley to the noncontaminated sprouts.

Reduction of Escherichia coli O157:H7 and Salmonella typhimurium in artificially contaminated alfalfa seeds and mung beans by fumigation with ammonia

Sprouts eaten raw are increasingly perceived as hazardous foods because they have been vehicles in outbreaks of foodborne disease, often involving Escherichia coli O157:H7 and Salmonella Typhimurium. Although the source of these pathogens has not been established, it is known that the seeds usually are already contaminated at the time sprouting begins. Earlier studies had shown that ammonia was lethal to these same pathogens in manure, so it seemed reasonable to determine whether ammonia was effective against them when associated with seeds to be used for sprouting. Experimentally contaminated (10(8) to 10(9) CFU/g) and dried seeds, intended for sprouting, were sealed in glass jars in which 180 or 300 mg of ammonia/liter of air space was generated by action of ammonium sulfate and sodium hydroxide. Samples were taken after intervals up to 22 h at 20 degrees C. Destruction of approximately 2 to 3 logs was observed with both bacteria associated with alfalfa seeds, versus 5 to 6 logs with mung beans. Greater kills are apparently associated with lower initial bacterial loads. Germination of these seeds was unaffected by the treatment. It appears that this simple treatment could contribute significantly to the safety of sprout production from alfalfa seeds and mung beans.

Multistate outbreak of Salmonella serovar Muenchen infections associated with alfalfa sprouts grown from seeds pretreated with calcium hypochlorite

During September 1999, a multistate outbreak of Salmonella serovar Muenchen infection associated with eating raw alfalfa sprouts was identified in Wisconsin. Despite use of a calcium hypochlorite sanitizing procedure to pretreat seeds before sprouting, at least 157 outbreak-related illnesses were identified in seven states having sprouters who received alfalfa seed from a specific lot. The continued occurrence of sprout-related outbreaks despite presprouting disinfection supports the concern that no available treatment will eliminate pathogens from seeds before sprouting and reinforces the need for additional safeguards to protect the public. A lack of consumer knowledge regarding exposure to sprouts documented in this investigation suggests that more-targeted outreach to high-risk individuals may be needed to reduce their risk.

Survival of Escherichia coli O157:H7 on strawberry fruit and reduction of the pathogen population by chemical agents

Survival of Escherichia coli O157:H7 was studied on strawberry, a fruit that is not usually washed during production, harvest, or postharvest handling. Two strains of the bacteria were tested separately on the fruit surface or injected into the fruit. Both strains of E. coli O157:H7 survived externally and internally at 23 degrees C for 24 h and at 10, 5, and -20 degrees C for 3 days. The largest reduction in bacterial population occurred at -20 degrees C and on the fruit surface during refrigeration. In all experiments, the bacteria inside the fruit either survived as well as or better than bacteria on the surface, and ATCC 43895 frequently exhibited greater survival than did ATCC 35150. Two strains of E. coli also survived at 23 degrees C on the surface and particularly inside strawberry fruit. Chemical agents in aqueous solution comprising NaOCl (100 and 200 ppm), Tween 80 (100 and 200 ppm), acetic acid (2 and 5%), Na3PO4 (2 and 5%), and H2O2 (1 and 3%) were studied for their effects on reduction of surface-inoculated (10(8) CFU/ml) E. coli O157:H7 populations on strawberry fruit. Dipping the inoculated fruit in water alone reduced the pathogen population about 0.8 log unit. None of the compounds with the exception of H2O2 exhibited more than a 2-log CFU/g reduction of the bacteria on the fruit surface. Three percent H202, the most effective chemical treatment, reduced the bacterial population on strawberries by about 2.2 log CFU/g.

Evaluation of volatile chemical treatments for lethality to Salmonella on alfalfa seeds and sprouts

A study was done to evaluate natural volatile compounds for their ability to kill Salmonella on alfalfa seeds and sprouts. Acetic acid, allyl isothiocyanate (AIT), trans-anethole, carvacrol, cinnamic aldehyde, eugenol, linalool, methyl jasmonate, and thymol were examined for inhibitory and lethal activity against Salmonella by exposing inoculated alfalfa seeds to compounds (1,000 mg/liter of air) for 1, 3, and 7 h at 60 degrees C. Only acetic acid, cinnamic aldehyde, and thymol caused significant reductions in Salmonella populations (>3 log10 CFU/g) compared with the control (1.9 log10 CFU/g) after treatment for 7 h. Treatment of seeds at 50 degrees C for 12 h with acetic acid (100 and 300 mg/liter of air) and thymol or cinnamic aldehyde (600 mg/liter of air) significantly reduced Salmonella populations on seeds (>1.7 log10 CFU/g) without affecting germination percentage. Treatment of seeds at 50 degrees C with AIT (100 and 300 mg/liter of air) and cinnamic aldehyde or thymol (200 mg/liter of air) did not significantly reduce populations compared with the control. Seed germination percentage was largely unaffected by treatment with gaseous acetic acid, AIT, cinnamic aldehyde, or thymol for up to 12 h at 50 degrees C. The number of Salmonella on seeds treated at 70 degrees C for 80 min with acetic acid (100 and 300 mg/liter of air), AIT (100 mg/liter of air), and cinnamic aldehyde and thymol (600 mg/liter of air) at water activity (a(w)) 0.66 was not significantly different than the number inactivated on seeds at a(w) 0.49. Acetic acid at 200 and 500 mg/liter of air reduced an initial population of 7.50 log10 CFU/g of alfalfa sprouts by 2.33 and 5.72 log10 CFU/g, respectively, within 4 days at 10 degrees C. whereas AIT at 200 and 500 mg/liter of air reduced populations to undetectable levels; however, both treatments caused deterioration in sensory quality. Treatment of sprouts with 1 or 2 mg of AIT per liter of air adversely affected sensory quality but did not reduce Salmonella populations after 11 days of exposure at 10 degrees C.

Expression of red-shifted green fluorescent protein by Escherichia coli O157:H7 as a marker for the detection of cells on fresh produce

Escherichia coli O157:H7 was transformed with a plasmid vector red-shifted green fluorescence protein (pEGFP) to express red-shifted green fluorescence protein (EGFP) from Aequorea victoria. The EGFP expression among total cells and nonviable cells was determined at the cellular level by microscopic observation of immunostained and membrane-impermeable, dye-stained cultures, respectively. E. coli O157:H7 retained pEGFP during frozen storage at -80 degrees C. The percentage of EGFP expression was improved by repeated subculturing, reaching 83.4 +/- 0.1%, although the fluorescence intensity varied among cells. A low percentage of EGFP-expressing cells was nonviable. The percentage of EGFP decreased when the culture plate was kept at 4 degrees C, suggesting that some cells lost pEGFP during refrigeration. The storage of the culture suspension in sterile deionized water at 4 degrees C for 24 h reduced the percentage of EGFP expression, indicating that some EGFP was denatured. The application of EGFP as a marker for E. coli O157:H7 on green leaf lettuce, cauliflower, and tomato was evaluated using confocal scanning laser microscopy. EGFP-transformed cells were readily visible under confocal scanning laser microscopy on all produce types. The numbers of E. coli O157:H7 cells detected with EGFP were equivalent to those detected with immunostaining for green leaf lettuce and cauliflower but less for tomato. E. coli O157:H7 attached preferentially to damaged tissues of green leaf lettuce and tomato over intact tissue surfaces and to flowerets of cauliflower than to stem surfaces. EGFP can serve as a marker to characterize E. coli O157:H7 attachment on green leaf lettuce and cauliflower but may not be suitable on tomato.

A multistate outbreak of Escherichia coli O157:H7 infections linked to alfalfa sprouts grown from contaminated seeds

A multistate outbreak of Escherichia coli O157:H7 infections occurred in the United States in June and July 1997. Two concurrent outbreaks were investigated through independent case-control studies in Michigan and Virginia and by subtyping isolates with pulsed-field gel electrophoresis (PFGE). Isolates from 85 persons were indistinguishable by PFGE. Alfalfa sprouts were the only exposure associated with E. coli O157:H7 infection in both Michigan and Virginia. Seeds used for sprouting were traced back to one common lot harvested in Idaho. New subtyping tools such as PFGE used in this investigation are essential to link isolated infections to a single outbreak.

The effect of different grain diets on fecal shedding of Escherichia coli O157:H7 by steers

Three groups of six yearling steers (three rumen fistulated plus three nonfistulated) fed one of three different grain diets (85% cracked corn, 15% whole cottonseed and 70% barley, or 85% barley) were inoculated with 10(10) CFU of Escherichia coli O157:H7 strain 3081, and the presence of the inoculated strain was followed in the rumen fluid and feces for a 10-week period. E. coli O157:H7 was rapidly eliminated from the rumen of the animals on all three diets but persisted in the feces of some animals up to 67 days after inoculation, suggesting that the bovine hindgut is the site of E. coli O157:H7 persistence. A significant difference existed in the levels of E. coli O157:H7 shed by the animals among diets on days 5, 7, 49, and 63 after inoculation (P < 0.05). No significant difference was found between the levels shed among diets on days 9 through 42 and on day 67 (P > 0.05). The number of animals that were culture positive for E. coli O157:H7 strain 3081 during the 10-week period was significantly higher for the barley fed group (72 of 114 samplings) as opposed to the corn fed group (44 of 114 samplings) (P < 0.005) and the cottonseed and barley fed group (57 of 114 samplings) (P < 0.05). The fecal pH of the animals fed the corn diet was significantly lower (P < 0.05) than the fecal pH of the animals fed the cottonseed and barley and barley diets, likely resulting in a less suitable environment for E. coli O157:H7 in the hindgut of the corn fed animals. E. coli O157:H7 strain 3081 was present in 3 of 30 (corn, 1 of 10; cottonseed, 1 of 10; barley, 1 of 10) animal drinking water samples, 3 of 30 (corn, 1 of 10; cottonseed, 0 of 10; barley, 2 of 10) water trough biofilm swabs, 5 of 30 (corn, 0 of 10; cottonseed, 2 of 10; barley, 3 of 10) feed samples, and 30 of 30 manure samples taken from the pens during the entire experimental period. Mouth swabs of the steers were also culture positive for E. coli O157:H7 strain 3081 in 30 of 180 samples (corn, 7 of 60; cottonseed, 4 of 60; barley, 19 of 60) taken during the 10-week period. Minimizing environmental dissemination of E. coli O157:H7 in conjunction with diet modification may reduce numbers of E. coli O157:H7-positive cattle.

Efficacy of novel organic acid and hypochlorite treatments for eliminating Escherichia coli O157:H7 from alfalfa seeds prior to sprouting

This study investigated novel two-step organic acid/hypochlorite treatments as alternatives to 20000 ppm active chlorine (from calcium hypochlorite) for eliminating Escherichia coli O157:H7 from alfalfa seeds prior to sprouting. Commercially available alfalfa seeds were inoculated with a five-strain E. coli O157:H7 mixture and dried to attain ca. 10(6) CFU/g of seeds. Seeds then underwent one of several soak treatments including: (1) 5% (v/v) lactic acid for 10 min at 42 degrees C; (2) 5% acetic acid (v/v) for 10 min at 42 degrees C; (3) 2.5% lactic acid for 10 min at 42 degrees C followed by 2000 ppm active chlorine (from calcium hypochlorite) for 15 min at 25 degrees C; (4) 5% lactic acid for 10 min at 42 degrees C followed by 2000 ppm active chlorine for 15 min at 25 degrees C; or (5) 20000 ppm active chlorine for 15 min at 25 degrees C. Each treatment reduced numbers of inoculum cells by about 6.0 log10 CFU/g as determined by plating on Sorbitol MacConkey agar (SMac). Plating on non-selective brain heart infusion agar (BHI) showed that treatments 1-4 reduced counts by 2.3-4.1 log10 CFU/g, thus indicating a large proportion of injured cells. Successive lactic acid and hypochlorite treatments (3 and 4) were more lethal than either organic acid alone (1 and 2). No surviving cells were detected on SMac or BHI following treatment with 20000 ppm active chlorine (treatment 5). Regardless of the previous treatment, E. coli O157:H7 counts increased to 10(7)-10(8) CFU/g during sprouting. Germination of seeds was not adversely affected by any of the treatments (germination > 90%). Results of this study show that: (a) non-lethal cell injury must be considered when evaluating intervention treatments against E. coli O157:H7 on alfalfa seeds; (b) reductions of 2-4 log10 CFU/g can be attained without using 20000 ppm active chlorine; (c) successive lactic acid and hypochlorite treatments have greater lethality than organic acid treatments alone; and (d) none of the treatments tested can prevent regrowth of surviving E. coli O157:H7 during sprouting.

Naturally occurring biofilms on alfalfa and other types of sprouts

Scanning electron microscopy was used to examine the cotyledons, hypocotyls, and roots of alfalfa, broccoli, clover, and sunflower sprouts purchased from retail outlets as well as alfalfa sprouts grown in the laboratory using a tray system equipped with automatic irrigation. Biofilms were observed on all plant parts of the four types of commercially grown sprouts. Biofilms were also commonly observed on alfalfa sprouts grown in the laboratory by 2 days of growth. Rod-shaped bacteria of various sizes were predominant on all sprouts examined both as free-living cells and as components of biofilms. Occasionally, cocci-shaped bacteria as well as yeast cells were also present in biofilms. The microbes contained in the biofilms appeared to be attached to each other and to the plant surface by a matrix, most likely composed of bacterial exopolysaccharides. Biofilms were most abundant and of the largest dimensions on cotyledons, sometimes covering close to the entire cotyledon surface (approximately 2 mm in length). Naturally occurring biofilms on sprouts may afford protected colonization sites for human pathogens such as Salmonella and Escherichia coli O157:H7, making their eradication with antimicrobial compounds difficult.

Volatile compounds from Escherichia coli O157:H7 and their absorption by strawberry fruit

Volatile compounds emitted by cultures of two strains of the pathogenic bacterium Escherichia coliO157:H7 and a nonpathogenic strain of E. coli were trapped on Super-Q porous polymer and identified by GC-MS. The predominant compound produced by all three strains was indole with lesser amounts of other components including methyl ketones, 2-heptanone, 2-nonanone, 2-undecanone, and 2-tridecanone. The vapor-phase profiles of these strains were similar for most chemicals identified but differed with regard to ketones. Strawberry fruit was shown to be a suitable host for E. coli O157:H7 with the population of the bacterium either increasing or remaining stable after 3 days depending on inoculation level. Headspace analysis of the volatile compounds from inoculated fruit yielded no detectable quantity of indole. Strawberry fruit readily absorbed indole and other volatile compounds produced by the bacteria and in some cases metabolized the compounds to new volatile products. Thus, headspace "marker" compounds indicating possible bacterial contamination of fruit were largely removed from the vapor phase by the strawberries.

Microbiological safety evaluations and recommendations on sprouted seeds. National Advisory Committee on Microbiological Criteria for Foods

In 1997, the National Advisory Committee on Microbiological Criteria for Foods (NACMCF/the Committee) was asked to review the current literature on sprout-associated outbreaks: identify the organisms and production practices of greatest public health concern: prioritize research needs: and provide recommendations on intervention and prevention strategies. In response to this charge, the Fresh Produce Work Group (FPWG) documented the relevant epidemiology and microbial ecology of sprout-associated outbreaks and reviewed current industry practices and initiatives related to the growing of seed and the production of sprouts. Sprouts have been identified as a special problem because of the potential for pathogen growth during the sprouting process. If pathogens are present on or in the seed, sprouting conditions may favor their proliferation. There is no inherent step in the production of raw sprouts to reduce or eliminate pathogens. Contaminated seed is the likely source for most reported sprout-associated outbreaks. Research has been initiated on methods to reduce or eliminate pathogenic bacteria on seeds and sprouts and some treatments show promise. However, to date, no single treatment has been shown to completely eliminate pathogens under experimental conditions used. Finally, the Committee found that, at the time of the charge, there was a lack of fundamental food safety knowledge along the continuum from seed production through sprout consumption. More recently, many have become aware of the potential for this food to be a vehicle for foodborne illness and the need for appropriate controls: however, such awareness is not universal. Although seed appears to be the most likely source of contamination in sprout associated outbreaks, practices and conditions at the sprouting facility may also impact on the safety of the finished product. In recent sprout-associated outbreak investigations, facilities associated with outbreaks did not consistently apply seed disinfection treatments prior to sprouting. Conversely, facilities that used seed from the same lot as an implicated facility, but had not been associated with reported illnesses, appear to have been consistently using seed disinfection treatments, such as 20,000 ppm calcium hypochlorite, to disinfect seed prior to sprouting. The Committee has developed a number of specific recommendations, including: 1. The knowledge of all interested parties pertaining to the microbiological safety of sprouted seeds must be enhanced; government and industry should develop education programs for seed and sprout producers on basic principles for microbiological food safety, good agricultural practices, good manufacturing practices, and hazard analysis and critical control point (HACCP) systems. 2. Good agricultural practices should be systematically implemented to reduce the potential for microbial contamination of seeds for sprout production. 3. Seed cleaning, storage, and handling practices that minimize the potential for microbial contamination should be developed and implemented. 4. Seeds should be treated with one or more treatments that have been shown to reduce pathogenic bacteria that may be present. Intervention strategies that deliver less than a given reduction (at this time, 5-log) in levels of Salmonella spp. and enterohemorrhagic Escherichia coli O157 should be coupled with a microbiological testing program. 5. Establish good manufacturing practices and food safety systems, including regulatory oversight, microbial testing, adoption of HACCP, and improved traceback, that systematically look for means to prevent seeds from serving as the vehicle for foodborne disease, and 6. Conduct research related to the microbiological safety of sprouted seeds, particularly in the areas of pathogen reduction or elimination, sources of contamination and its prevention, and preventing or retarding pathogen growth during sprouting.

Infections associated with eating seed sprouts: an international concern

Recent outbreaks of Salmonella and Escherichia coli O157:H7 infections associated with raw seed sprouts have occurred in several countries. Subjective evaluations indicate that pathogens can exceed 107 per gram of sprouts produced from inoculated seeds during sprout production without adversely affecting appearance. Treating seeds and sprouts with chlorinated water or other disinfectants fails to eliminate the pathogens. A comprehensive approach based on good manufacturing practices and principles of hazard analysis and critical control points can reduce the risk of sprout-associated disease. Until effective measures to prevent sprout-associated illness are identified, persons who wish to reduce their risk of foodborne illness from raw sprouts are advised not to eat them; in particular, persons at high risk for severe complications of infections with Salmonella or E. coli O157:H7, such as the elderly, children, and those with compromised immune systems, should not eat raw sprouts.