Survival and growth of Escherichia coli O157:H7 inoculated onto cut lettuce before or after heating in chlorinated water, followed by storage at 5 or 15 degrees C

Growth models for Salmonella, E. coli O157:H7 and L. monocytogenes give different predictions for pathogen growth in cut leafy greens transportation, but are consistent in identifying higher risk conditions

Leafy greens are frequently implicated in foodborne disease outbreaks and cut-leafy greens are a food that requires time and temperature control for safety. Predictive microbiology uses mathematical models to predict the growth of bacteria based on environmental conditions. The objective of our study was to compare published square root growth models for Salmonella (n = 6), pathogenic E. coli (n = 6) and Listeria monocytogenes (n = 4) using real world transport temperature data. Data from trucks transporting fresh-cut leafy greens during cross-country shipments were used as temperature inputs to the models. Bacterial growth was computed using the temperatures from each probe in every truck over the duration of transit, which resulted in 12-18 growth predictions per truck for each model. Each model generally gave significantly different predictions than other models for the same organism. The exception was for the two Salmonella models predicting the least growth and the two Salmonella models predicting the most growth which gave predictions that were not significantly different. Although different models tended to give different predictions, their ability to rank risk by truck was generally consistent across models. While absolute risk might be dependent upon choice of model, relative risk is independent of model choice.

Nanoemulsified Carvacrol as a Novel Washing Treatment Reduces Escherichia coli O157:H7 on Spinach and Lettuce

ABSTRACT

Fresh produce continues to be the main source of foodborne illness outbreaks in the United States, implicating bacterial pathogens such as Escherichia coli O157:H7 (EHEC). The efficacy of nanoemulsified carvacrol (NCR) as a washing treatment in reducing EHEC on fresh produce was investigated. Fresh baby spinach, romaine lettuce, and iceberg lettuce leaves (2.5-cm-diameter cores) were spot inoculated with a five-strain cocktail of nalidixic acid-resistant EHEC at ∼6 log CFU/cm2. After air drying for 1 h, 20 pieces of each inoculated produce leaf were immersed in water-based treatment solutions (200 mL per group), including water alone, 25 or 50 ppm of free chlorine, and 0.25 or 0.75% NCR for 2 min. Inoculated produce leaves without any treatment served as baseline. Produce leaves were stored at 10°C, and surviving EHEC populations were enumerated on days 0, 2, 7, and 14. The viability of EHEC following NCR treatments on the fresh produce was visualized under a fluorescence microscope. NCR treatment at 0.75% immediately reduced EHEC populations on iceberg lettuce by 1.3 log CFU/cm2 as compared with the produce treated with water alone (P < 0.05). Antimicrobial activity of NCR against EHEC was comparable to chlorine treatments on day 0 for all produce (P > 0.05). After 14 days of storage at 10°C, populations of EHEC on 0.75% NCR-treated romaine lettuce were reduced by 2.3 log CFU/cm2 compared with the recovery from 50 ppm of chlorine-treated samples (P < 0.05). Microscopic images revealed that EHEC cells were observed to be clustered on the baseline samples, indicating the development of cell aggregation, compared with the scattered cells seen on NCR-treated leaf surfaces. Treatments with NCR did not significantly affect the color of the fresh produce leaves during 14 days of storage at 10°C. Results of this study support the potential use of NCR as a water-soluble natural antimicrobial wash treatment for controlling EHEC on fresh produce.

HIGHLIGHTS

Nature versus Nurture: Assessing the Impact of Strain Diversity and Pregrowth Conditions on Salmonella enterica, Escherichia coli, and Listeria Species Growth and Survival on Selected Produce Items

Inoculation studies are important when assessing microbial survival and growth in food products. These studies typically involve the pregrowth of multiple strains of a target pathogen under a single condition; this emphasizes strain diversity. To gain a better understanding of the impacts of strain diversity ("nature") and pregrowth conditions ("nurture") on subsequent bacterial growth in foods, we assessed the growth and survival of Salmonella enterica (n = 5), Escherichia coli (n = 6), and Listeria (n = 5) inoculated onto tomatoes, precut lettuce, and cantaloupe rind, respectively. Pregrowth conditions included (i) 37°C to stationary phase (baseline), (ii) low pH, (iii) high salt, (iv) reduced water activity, (v) log phase, (vi) minimal medium, and (vii) 21°C. Inoculated tomatoes were incubated at 21°C; lettuce and cantaloupe were incubated at 7°C. Bacterial counts were assessed over three phases, including initial reduction (phase 1), change in bacterial numbers over the first 24 h of incubation (phase 2), and change over the 7-day incubation (phase 3). E. coli showed overall decline in counts (<1 log) over the 7-day period, except for a <1-log increase after pregrowth in high salt and to mid-log phase. In contrast, S. enterica and Listeria showed regrowth after an initial reduction. Pregrowth conditions had a substantial and significant effect on all three phases of S. enterica and E. coli population dynamics on inoculated produce, whereas strain did not show a significant effect. For Listeria, both pregrowth conditions and strain affected changes in phase 2 but not phases 1 and 3.IMPORTANCE Our findings suggest that inclusion of multiple pregrowth conditions in inoculation studies can best capture the range of growth and survival patterns expected for Salmonella enterica and Escherichia coli present on produce. This is particularly important for fresh and fresh-cut produce, where stress conditions encountered by pathogens prior to contamination can vary widely, making selection of a typical pregrowth condition virtually impossible. Pathogen growth and survival data generated using multiple pregrowth conditions will allow for more robust microbial risk assessments that account more accurately for uncertainty.

Leaf Surface Topography Contributes to the Ability of Escherichia coli on Leafy Greens to Resist Removal by Washing, Escape Disinfection With Chlorine, and Disperse Through Splash

The attachment of foodborne pathogens to leaf surfaces is a complex process that involves multiple physical, chemical, and biological factors. Here, we report the results from a study designed to specifically determine the contribution of spinach leaf surface topography as it relates to leaf axis (abaxial and adaxial) and leaf age (15, 45, and 75 days old) to the ability of Escherichia coli to resist removal by surface wash, to avoid inactivation by chlorine, and to disperse through splash impact. We used fresh spinach leaves, as well as so-called "replicasts" of spinach leaf surfaces in the elastomer polydimethylsiloxane to show that leaf vein density correlated positively with the failure to recover E. coli from surfaces, not only using a simple water wash and rinse, but also a more stringent wash protocol involving a detergent. Such failure was more pronounced when E. coli was surface-incubated at 24°C compared to 4°C, and in the presence, rather than absence, of nutrients. Leaf venation also contributed to the ability of E. coli to survive a 50 ppm available chlorine wash and to laterally disperse by splash impact. Our findings suggest that the topographical properties of the leafy green surface, which vary by leaf age and axis, may need to be taken into consideration when developing prevention or intervention strategies to enhance the microbial safety of leafy greens.

Evaluation by Flow Cytometry of Escherichia coli Viability in Lettuce after Disinfection

Foodborne outbreaks due to the consumption of ready-to-eat vegetables have increased worldwide, with Escherichia coli (E. coli) being one of the main sources responsible. Viable but nonculturable bacteria (VBNC) retain virulence even after some disinfection procedures and constitute a huge problem to public health due to their non-detectability through conventional microbiological techniques. Flow cytometry (FCM) is a promising tool in food microbiology as it enables the distinction of the different physiological states of bacteria after disinfection procedures within a short time. In this study, samples of lettuce inoculated with E. coli were subject to disinfection with sodium hypochlorite at free chlorine concentrations of 5, 10, 25, 50, and 100 mg·L⁻¹ or with 35% peracetic acid at concentrations of 5, 10, 25, and 50 mg·L⁻¹. The efficiency of these disinfectants on the viability of E. coli in lettuce was evaluated by flow cytometry with LIVE/DEAD stains. Results from this study suggest that FCM can effectively monitor cell viability. However, peracetic acid is more effective than sodium hypochlorite as, at half the concentration, it is enough to kill 100% of bacteria and always induces a lower percentage of VBNC. Finally, we can conclude that the recommended levels of chemical disinfectants for fresh fruit and vegetables are adequate when applied in lettuce. More importantly, it is possible to ensure that all cells of E. coli are dead and that there are no VBNC cells even with lower concentrations of those chemicals. These results can serve as guidance for lettuce disinfection, improving quality and the safety of consumption.

Microbes associated with fresh produce: Sources, types and methods to reduce spoilage and contamination

Global food security remains one of the most important challenges that needs to be addressed to ensure the increasing demand for food of the fast growing human population is satisfied. Fruits and vegetables comprise an essential component of a healthy balanced diet as they are the major source of both macro- and micronutrients. They are particularly important for communities in developing countries whose nutrition often relies solely on a plant-based diet. Recent advances in agriculture and food processing technologies have facilitated production of fresh, nutritious and safe food for consumers. However, despite the development of sophisticated chemical and physical methods of food and equipment disinfection, fresh-cut produce and fruit juice industry still faces significant economic losses due to microbial spoilage. Furthermore, fresh produce remains an important source of pathogens that have been causing outbreaks of human illness worldwide. This chapter characterizes common spoilage and human pathogenic microorganisms associated with fresh-cut produce and fruit juice products, and discusses the methods and technology that have been developed and utilized over the years to combat them. Substantial attention is given to highlight advantages and disadvantages of using these methods to reduce microbial spoilage and their efficacy to eliminate human pathogenic microbes associated with consumption of fresh-cut produce and fruit juice products.

Decontamination and survival of Enterobacteriaceae on shredded iceberg lettuce during storage

Enterobacteriaceae family can contaminate fresh produce at any stage of production either at pre-harvest or post-harvest stages. The objectives of the current study were to i) identify Enterobacteriaceae species on iceberg lettuce, ii) compare the decontamination efficiency of water, sodium hypochlorite (free chlorine 200 ppm), peroxyacetic acid (PA 80 ppm; Kenocid 2100®) or their combinations and ionizing radiation against Enterobacteriaceae on shredded iceberg lettuce and iii) determine the survival of Enterobacteriaceae post-treatment storage of shredded iceberg lettuce at 4, 10 and 25 °C, for up to 7 days. Klebsiella pneumonia spp. pneumonia, Enterobacter cloacae, Klebsiella oxytoca, Pantoea spp., Leclercia adecarboxylata and Kluyvera ascorbate were identified on iceberg lettuce. No significant difference (P≥ 0.05) among Enterobacteriaceae survival after washing with water or sanitizing with sodium hypochlorite or Kenocid 2100® (reduction ≤ 0.6 log CFU/g) were found. Combined sanitizer treatments were more effective against Enterobacteriaceae than single washing/sanitizing treatments. Sanitization of iceberg lettuce with combined washing/sanitizing treatments reduced Enterobacteriaceae by 0.85-2.24 CFU/g. Post-treatment growth of Enterobacteriaceae during storage on samples sanitized with sodium hypochlorite and Kenocid 2100® was more than on samples washed with water. The D10-value of Enterobacteriaceae on shredded iceberg lettuce was 0.21 KGy. The reduction of Enterobacteriaceae populations on iceberg after gamma radiation (0.6 KGy) was 3 log CFU/g, however, Enterobacteriaceae counts increased post-irradiation storage by 4-5 log CFU/g. Therefore, washing shredded iceberg lettuce with combined sanitizing treatment (sodium hypochlorite/sodium hypochlorite, sodium hypochlorite/Kenocid 2100®, or Kenocid 2100®/Kenocid 2100®) for total time of 6 min or exposing it to gamma irradiation (0.6 KGy) can decrease the risk of Enterobacteriaceae (reduction ≥ 2 log). Post-washing storage of sliced iceberg lettuce (4, 10, 25 °C) could increase the risk of Enterobacteriaceae as their counts increased during storage even at low temperatures.

Growth Kinetics of Listeria monocytogenes in Cut Produce

Cut produce continues to constitute a significant portion of the fresh fruit and vegetables sold directly to consumers. As such, the safety of these items during storage, handling, and display remains a concern. Cut tomatoes, cut leafy greens, and cut melons, which have been studied in relation to their ability to support pathogen growth, have been specifically identified as needing temperature control for safety. Data are needed on the growth behavior of foodborne pathogens in other types of cut produce items that are commonly offered for retail purchase and are potentially held without temperature control. This study assessed the survival and growth of Listeria monocytogenes in cut produce items that are commonly offered for retail purchase, specifically broccoli, green and red bell peppers, yellow onions, canned green and black olives, fresh green olives, cantaloupe flesh and rind, avocado pulp, cucumbers, and button mushrooms. The survival of L. monocytogenes strains representing serotypes 1/2a, 1/2b, and 4b was determined on the cut produce items for each strain individually at 5, 10, and 25°C for up to 720 h. The modified Baranyi model was used to determine the growth kinetics (the maximum growth rates and maximum population increases) in the L. monocytogenes populations. The products that supported the most rapid growth of L. monocytogenes, considering the fastest growth and resulting population levels, were cantaloupe flesh and avocado pulp. When stored at 25°C, the maximum growth rates for these products were 0.093 to 0.138 log CFU/g/h and 0.130 to 0.193 log CFU/g/h, respectively, depending on the strain. Green olives and broccoli did not support growth at any temperature. These results can be used to inform discussions surrounding whether specific time and temperature storage conditions should be recommended for additional cut produce items.

Effect of relevant environmental stresses on survival of enterohemorrhagic Escherichia coli in dry-fermented sausage

Dry-fermented sausages (DFSs) have been linked to several serious foodborne outbreaks of enterohemorrhagic Escherichia coli (EHEC). The ability of pathogens to utilize adaptive responses to different stressful conditions intended to control their growth in foods, food preparation and production processes may enhance their survival. In certain cases, induced tolerance to one type of stress may lead to enhanced resistance to the applied stress as well as to other stresses. We exposed two EHEC strains, MF3582 of serotype O157:H- and MF5554 of serogroup O145, to different stresses commonly encountered during a production process. The two EHEC strains, previously shown to have different abilities to survive DFS production process conditions, were subjected to low temperatures (4°C and 12°C), 5% NaCl or 1% lactic acid for 6days prior to being added to sausage batters. Survival of EHEC was recorded in salami of two recipes, fermented at two temperatures (20°C and 30°C). The results showed that recipe type had the largest impact on EHEC reductions where Moderate recipe (MR) salami batters containing increased levels of NaCl, glucose and NaNO2 provided enhanced EHEC reductions in salami (2.6 log10) compared to Standard recipe (SR) salami (1.7 log10). Effects of pre-exposure stresses were dependent both on strain and recipe. While acid adaptation of MF5554 provided enhanced log10 reductions from 2.0 to 3.0 in MR sausages, adaptation to a combination of acid and salt stress showed the opposite effect in SR sausages with reductions of only 1.1 log10 as compared to the average of 1.8 log10 for the other SR sausages. Otherwise, the salt and acid adaptation single stresses had relatively small effects on EHEC survival through the DFS production process and subsequent storage and freeze/thaw treatments. Growing cells and cells frozen in batter survived poorly in MR sausages with an average reduction of 3.4 and 3.2 log10, respectively. The reductions of EHEC after storage of DFS increased with higher temperature and storage time. Up to 3.7 log10 additional reduction was obtained when MF3582 was stored for 2months at 20°C. In conclusion, adaptation of EHEC to acid, salt and low temperatures prior to being introduced in a DFS production process has limited, but strain dependent effects on EHEC reductions. Producers should avoid conditions leading to acid and salt adapted cells that can contaminate the sausage batter. Recipe parameters had the largest impact on EHEC reductions while storage at 20°C is effective for enhanced reductions in finished products.

Tracing temperature patterns of cut leafy greens during service in North Carolina school food service

Contaminated fresh produce has been increasingly identified as a cause of foodborne illnesses. Because of concerns about pathogen growth on these food items at retail, the 2009 U.S. Food and Drug Administration Food Code established that cut leafy greens (lettuce, spinach, spring mix, cabbage, arugula, and kale) must have time and temperature controls for safety and hence should be kept at refrigerated temperatures (5°C or lower). The purpose of this study was to determine the temperature profiles of cut leafy greens in single-serving clamshell containers provided as part of the North Carolina School Lunch Program and to compare the two policies that North Carolina has in place to control the temperature of these products (the 3-day rule and time in lieu of temperature). Temperatures were recorded with data loggers in 24 schools during a 3-day period. In all cases, substantial temperature variability was found for these products, including temperatures above 5°C for at least 1 h on each of the 3 days. In some cases, temperatures reached above 5°C for more than 3 h throughout the serving time. The results demonstrate the importance of developing a protocol for continuous temperature monitoring of leafy greens served in school lunch programs.

Modeling growth of Escherichia coli O157:H7 in fresh-cut lettuce treated with neutral electrolyzed water and under modified atmosphere packaging

The purpose of this study was to evaluate and model the growth of Escherichia coli O157:H7 in fresh-cut lettuce submitted to a neutral electrolyzed water (NEW) treatment, packaged in passive modified atmosphere and subsequently stored at different temperatures (4, 8, 13, 16°C) for a maximum of 27 days. Results indicated that E. coli O157:H7 was able to grow at 8, 13, and 16°C, and declined at 4°C. However at 8°C, the lag time lasted 19 days, above the typical shelf-life time for this type of products. A secondary model predicting growth rate as a function of temperature was developed based on a square-root function. A comparison with literature data indicated that the growth predicted by the model for E. coli O157:H7 was again lower than those observed with other disinfection treatments or packaging conditions (chlorinated water, untreated product, NEW, etc.). The specific models here developed might be applied to predict growth in products treated with NEW and to improve existing quantitative risk assessments.

Efficacy of commercial produce sanitizers against nontoxigenic Escherichia coli O157:H7 during processing of iceberg lettuce in a pilot-scale leafy green processing line

Chemical sanitizers are routinely used during commercial flume washing of fresh-cut leafy greens to minimize cross-contamination from the water. This study assessed the efficacy of five commercial sanitizer treatments against Escherichia coli O157:H7 on iceberg lettuce, in wash water, and on equipment during simulated commercial production in a pilot-scale processing line. Iceberg lettuce (5.4 kg) was inoculated to contain 10(6) CFU/g of a four-strain cocktail of nontoxigenic, green fluorescent protein-labeled, ampicillin-resistant E. coli O157:H7 and processed after 1 h of draining at ~22 °C. Lettuce was shredded using a commercial slicer, step-conveyed to a flume tank, washed for 90 s using six different treatments (water alone, 50 ppm of peroxyacetic acid, 50 ppm of mixed peracid, or 50 ppm of available chlorine either alone or acidified to pH 6.5 with citric acid [CA] or T-128), and then dried using a shaker table and centrifugal dryer. Various product (25-g) and water (50-ml) samples collected during processing along with equipment surface samples (100 cm(2)) from the flume tank, shaker table, and centrifugal dryer were homogenized in neutralizing buffer and plated on tryptic soy agar. During and after iceberg lettuce processing, none of the sanitizers were significantly more effective (P ≤ 0.05) than water alone at reducing E. coli O157:H7 populations on lettuce, with reductions ranging from 0.75 to 1.4 log CFU/g. Regardless of the sanitizer treatment used, the centrifugal dryer surfaces yielded E. coli O157:H7 populations of 3.49 to 4.98 log CFU/100 cm(2). Chlorine, chlorine plus CA, and chlorine plus T-128 were generally more effective (P ≤ 0.05) than the other treatments, with reductions of 3.79, 5.47, and 5.37 log CFU/ml after 90 s of processing, respectively. This indicates that chlorine-based sanitizers will likely prevent wash water containing low organic loads from becoming a vehicle for cross-contamination.

Modelling growth of Escherichia coli O157:H7 in fresh-cut lettuce submitted to commercial process conditions: chlorine washing and modified atmosphere packaging

Fresh-cut iceberg lettuce inoculated with Escherichia coli O157:H7 was submitted to chlorine washing (150 mg/mL) and modified atmosphere packaging on laboratory scale. Populations of E. coli O157:H7 were assessed in fresh-cut lettuce stored at 4, 8, 13 and 16 °C using 6-8 replicates in each analysis point in order to capture experimental variability. The pathogen was able to grow at temperatures ≥8 °C, although at low temperatures, growth data presented a high variability between replicates. Indeed, at 8 °C after 15 days, some replicates did not show growth while other replicates did present an increase. A growth primary model was fitted to the raw growth data to estimate lag time and maximum growth rate. The prediction and confidence bands for the fitted growth models were estimated based on Monte-Carlo method. The estimated maximum growth rates (log cfu/day) corresponded to 0.14 (95% CI: 0.06-0.31), 0.55 (95% CI: 0.17-1.20) and 1.43 (95% CI: 0.82-2.15) for 8, 13 and 16 °C, respectively. A square-root secondary model was satisfactorily derived from the estimated growth rates (R(2) > 0.80; Bf = 0.97; Af = 1.46). Predictive models and data obtained in this study are intended to improve quantitative risk assessment studies for E. coli O157:H7 in leafy green products.

Dynamic effects of free chlorine concentration, organic load, and exposure time on the inactivation of Salmonella, Escherichia coli O157:H7, and non-O157 Shiga toxin-producing E. coli

This study evaluated the dynamic effects of free-chlorine (FC) concentration, contact time, and organic load on the inactivation of Salmonella, Escherichia coli O157:H7, and non-O157 Shiga toxin-producing E. coli (STEC) in suspension. Bacterial cells from four strains each of Salmonella, E. coli O157:H7, and non-O157 STEC were inoculated separately or as a multistrain cocktail into solutions with varying FC concentrations. Lettuce or tomato extract was used to simulate the organic matter present during commercial fresh and fresh-cut produce wash operations. After exposure to FC for various lengths of time, the bacterial survival and water-quality changes were determined. In the absence of organic matter in a wash solution, pathogen inactivation is primarily a function of initial FC concentration (P < 0.0001), exposure time (P < 0.0001), and pathogen strains (P < 0.0001). In general, an over 4.5-log CFU/ml pathogen reduction was found after exposure to >0.5 mg/liter FC for over 30 s, or to >1.0 mg/liter FC for over 5 s. When the combination of FC concentration and contact time were less than or equal to the above conditions, survival of pathogens was strain dependant and ranked as: Salmonella > E. coli O157:H7 > non-O157 STEC. When organic matter was present in the wash solution, pathogen inactivation efficacy was specifically dependent on the residual FC concentration, which directly relates to both the initial FC concentration and the organic load. Prevention of pathogen survival in chlorinated produce wash solutions can be achieved by maintaining sufficient FC concentration and reducing the accumulation of organic matter.

Fate of Escherichia coli O157:H7 in field-inoculated lettuce

Impact of drip and overhead sprinkler irrigation on the persistence of attenuated Escherichia coli O157:H7 in the lettuce phyllosphere was investigated using a split-plot design in four field trials conducted in the Salinas Valley, California, between summer 2007 and fall 2009. Rifampicin-resistant attenuated E. coli O157:H7 ATCC 700728 (BLS1) was inoculated onto the soil beds after seeding with a backpack sprayer or onto 2- or 4-week-old lettuce plant foliage with a spray bottle at a level of 7 log CFU ml⁻¹. When E. coli O157:H7 was inoculated onto 2-week-old plants, the organism was recovered by enrichment in 1 of 120 or 0 of 240 plants at 21 or 28 days post-inoculation, respectively. For the four trials where inoculum was applied to 4-week-old plants, the population size of E. coli O157:H7 declined rapidly and by day 7, counts were near or below the limit of detection (10 cells per plant) for 82% or more of the samples. However, in 3 out 4 field trials E. coli O157:H7 was still detected in lettuce plants by enrichment 4-weeks post-inoculation. Neither drip nor overhead sprinkler irrigation consistently influenced the survival of E. coli O157:H7 on lettuce.

Efficacy of washing with hydrogen peroxide followed by aerosolized antimicrobials as a novel sanitizing process to inactivate Escherichia coli O157:H7 on baby spinach

Aerosolization was investigated as a potential way to apply allyl isothiocyanate (AIT), hydrogen peroxide (H(2)O(2)), acetic acid (AA) and lactic acid (LA) on fresh baby spinach to control Escherichia coli O157:H7 during refrigeration storage. In this study, baby spinach leaves were dip-inoculated with E. coli O157:H7 to a level of 6 log CFU/g and stored at 4°C for 24 h before treatment. Antimicrobials were atomized into fog-like micro-particles by an ultrasonic nebulizer and routed into a jar and a scale-up model system where samples were treated. Samples were stored at 4°C for up to 10 days before the survival of the cells was determined. A 2-min treatment with 5% AIT resulted in a >5-log reduction of E. coli O157:H7 on spinach after 2 days refrigeration regardless if the samples were pre-washed or not; however, this treatment impaired the sensory quality of leaves. Addition of LA to AIT improved the antimicrobial efficacy of AIT. In the jar system, washing with 3% H(2)O(2) followed by a 2-min treatment of 2.5% LA+1% AIT or 2.5% LA+2% AIT reduced E. coli O157:H7 population by 4.7 and >5 log CFU/g, respectively, after 10 days refrigeration. In the scale-up system, up to 4-log reduction of bacterial population was achieved for the same treatments without causing noticeable adverse effect on the appearance of leaves. Thus, this study demonstrates the potential of aerosolized AIT+LA as a new post-washing intervention strategy to control E. coli O157:H7 on baby spinach during refrigeration storage.

Development of a dynamic growth-death model for Escherichia coli O157:H7 in minimally processed leafy green vegetables

Escherichia coli O157:H7, an occasional contaminant of fresh produce, can present a serious health risk in minimally processed leafy green vegetables. A good predictive model is needed for Quantitative Risk Assessment (QRA) purposes, which adequately describes the growth or die-off of this pathogen under variable temperature conditions experienced during processing, storage and shipping. Literature data on behaviour of this pathogen on fresh-cut lettuce and spinach was taken from published graphs by digitization, published tables or from personal communications. A three-phase growth function was fitted to the data from 13 studies, and a square root model for growth rate (μ) as a function of temperature was derived: μ=(0.023*(Temperature-1.20))(2). Variability in the published data was incorporated into the growth model by the use of weighted regression and the 95% prediction limits. A log-linear die-off function was fitted to the data from 13 studies, and the resulting rate constants were fitted to a shifted lognormal distribution (Mean: 0.013; Standard Deviation, 0.010; Shift, 0.001). The combined growth-death model successfully predicted pathogen behaviour under both isothermal and non-isothermal conditions when compared to new published data. By incorporating variability, the resulting model is an improvement over existing ones, and is suitable for QRA applications.

Efficacy of a novel sanitizer composed of lactic acid and peroxyacetic acid against single strains of nonpathogenic Escherichia coli K-12, Listeria innocua, and Lactobacillus plantarum in aqueous solution and on surfaces of romaine lettuce and spinach

A novel sanitizer composed of lactic acid and peroxyacetic acid (LA-PAA) was developed as an alternative to chlorinated water (CW) for fresh produce processing. Single strains of Lactobacillus plantarum, nonpathogenic Escherichia coli K-12, and Listeria innocua were used to demonstrate the microbial efficacy of LA-PAA. LA-PAA achieved a >7.8-log reduction of L. innocua and L. plantarum suspended in water at 4°C for 20 s, and LA, PAA, and CW achieved reductions of 0.4, 4.8, and 2.7 log, respectively. LA-PAA, when compared with LA, PAA, and CW, enhanced the reduction of L. innocua attached to romaine leaves by >2.2 log, and improved the removal of E. coli attached to spinach leaves by >2.4 log. The exponential improvement in the microbial efficacy of LA-PAA showed synergism between LA and PAA. LA-PAA microbial efficacy was inversely proportional to pH value and directly correlated with residence time and concentration. Despite an improvement in microbial reduction through the addition of surfactant to LA-PAA, the usage of surfactant in washing fresh produce was impeded by excessive foaming during actual processing. Effects of organic matter on the performance of LA-PAA were minimal. External sensory evaluations showed that LA-PAA had no negative effects on the quality of lettuce and tender leaves. Temperature-abuse studies demonstrated that LA-PAA reduced decay by ∼50% when compared with CW. Overall, these results support the premise that LA-PAA has significant potential to be an alternative to CW for fresh produce processing.

Quantitative assessment of the microbial risk of leafy greens from farm to consumption: preliminary framework, data, and risk estimates

This project was undertaken to relate what is known about the behavior of Escherichia coli O157:H7 under laboratory conditions and integrate this information to what is known regarding the 2006 E. coli O157:H7 spinach outbreak in the context of a quantitative microbial risk assessment. The risk model explicitly assumes that all contamination arises from exposure in the field. Extracted data, models, and user inputs were entered into an Excel spreadsheet, and the modeling software @RISK was used to perform Monte Carlo simulations. The model predicts that cut leafy greens that are temperature abused will support the growth of E. coli O157:H7, and populations of the organism may increase by as much a 1 log CFU/day under optimal temperature conditions. When the risk model used a starting level of -1 log CFU/g, with 0.1% of incoming servings contaminated, the predicted numbers of cells per serving were within the range of best available estimates of pathogen levels during the outbreak. The model predicts that levels in the field of -1 log CFU/g and 0.1% prevalence could have resulted in an outbreak approximately the size of the 2006 E. coli O157:H7 outbreak. This quantitative microbial risk assessment model represents a preliminary framework that identifies available data and provides initial risk estimates for pathogenic E. coli in leafy greens. Data gaps include retail storage times, correlations between storage time and temperature, determining the importance of E. coli O157:H7 in leafy greens lag time models, and validation of the importance of cross-contamination during the washing process.

Determination of free chlorine concentrations needed to prevent Escherichia coli O157:H7 cross-contamination during fresh-cut produce wash

This study was conducted to investigate the effect of free chlorine concentrations in wash water on Escherichia coli O157:H7 reduction, survival, and transference during washing of fresh-cut lettuce. The effectiveness of rewashing for inactivation of E. coli O157:H7 on newly cross-contaminated produce previously washed with solutions containing an insufficient amount of chlorine also was assessed. Results indicate that solutions containing a minimum of 0.5 mg/liter free chlorine were effective for inactivating E. coli O157:H7 in suspension to below the detection level. However, the presence of 1 mg/liter free chlorine in the wash solution before washing was insufficient to prevent E. coli O157:H7 survival and transfer during washing because the introduction of cut lettuce to the wash system quickly depleted the free chlorine. Although no E. coli O157:H7 was detected in the wash solution containing 5 mg/liter free chlorine before washing a mix of inoculated and uninoculated lettuce, low numbers of E. coli O157:H7 cells were detected on uninoculated lettuce in four of the seven experimental trials. When the prewash free chlorine concentration was increased to 10 mg/liter or greater, no E. coli O157:H7 transfer was detected. Furthermore, although rewashing newly cross-contaminated lettuce in 50 mg/liter free chlorine for 30 s significantly reduced (P = 0.002) the E. coli O157:H7 populations, it failed to eliminate E. coli O157:H7 on lettuce. This finding suggests that rewashing is not an effective way to correct for process failure, and maintaining a sufficient free chlorine concentration in the wash solution is critical for preventing pathogen cross-contamination.

Effect of storage temperature and duration on the behavior of Escherichia coli O157:H7 on packaged fresh-cut salad containing romaine and iceberg lettuce

This study investigated the impact of storage temperature and duration on the fate of Escherichia coli O157:H7 on commercially packaged lettuce salads, and on product quality. Fresh-cut Romaine and Iceberg lettuce salads of different commercial brands were obtained from both retail and wholesale stores. The packages were cut open at one end, the lettuce salad inoculated with E. coli O157:H7 via a fine mist spray, and resealed with or without an initial N(2) flush to match the original package atmospheric levels. The products were stored at 5 and 12 °C until their labeled "Best If Used By" dates, and the microbial counts and product quality were monitored periodically. The results indicate that storage at 5 °C allowed E. coli O157:H7 to survive, but limited its growth, whereas storage at 12 °C facilitated the proliferation of E. coli O157:H7. There was more than 2.0 log CFU/g increase in E. coli O157:H7 populations on lettuce when held at 12 °C for 3 d, followed by additional growth during the remainder of the storage period. Although there was eventually a significant decline in visual quality of lettuce held at 12 °C, the quality of this lettuce was still fully acceptable when E. coli O157:H7 growth reached a statistically significant level. Therefore, maintaining fresh-cut products at 5 °C or below is critical for reducing the food safety risks as E. coli O157:H7 grows at a rapid, temperature-dependent rate prior to significant quality deterioration.

PRACTICAL APPLICATION

Specific information regarding the effect of temperature on pathogen growth on leafy greens is needed to develop science-based food safety guidelines and practices by the regulatory agencies and produce industry. Temperature control is commonly thought to promote quality of leafy greens, not safety, based at least partially on a theory that product quality deterioration precedes pathogen growth at elevated temperatures. This prevalent attitude results in temperature abuse incidents being frequently overlooked in the supply chain. This study demonstrates that human pathogens, such as E. coli O157:H7, can grow significantly on commercially packaged lettuce salads while the product's visual quality is fully acceptable. Packaged fresh-cut salads are marketed as "ready-to-eat" while lacking an effective pathogen kill step during their preparation. Thus, maintaining storage temperature at 5 °C or below is critical to prevent pathogen proliferation and mitigate food safety risks should pathogen contamination inadvertently occur during crop growth or postharvest fresh-cut processing.

Whole-leaf wash improves chlorine efficacy for microbial reduction and prevents pathogen cross-contamination during fresh-cut lettuce processing

Currently, most fresh-cut processing facilities in the United States use chlorinated water or other sanitizer solutions for microbial reduction after lettuce is cut. Freshly cut lettuce releases significant amounts of organic matter that negatively impacts the effectiveness of chlorine or other sanitizers for microbial reduction. The objective of this study is to evaluate whether a sanitizer wash before cutting improves microbial reduction efficacy compared to a traditional postcutting sanitizer wash. Romaine lettuce leaves were quantitatively inoculated with E. coli O157:H7 strains and washed in chlorinated water before or after cutting, and E. coli O157:H7 cells that survived the washing process were enumerated to determine the effectiveness of microbial reduction for the 2 cutting and washing sequences. Whole-leaf washing in chlorinated water improved pathogen reduction by approximately 1 log unit over traditional cut-leaf sanitization. Similar improvement in the reduction of background microflora was also observed. Inoculated "Lollo Rossa" red lettuce leaves were mixed with noninoculated Green-Leaf lettuce leaves to evaluate pathogen cross-contamination during processing. High level (96.7% subsamples, average MPN 0.6 log CFU/g) of cross-contamination of noninoculated green leaves by inoculated red leaves was observed when mixed lettuce leaves were cut prior to washing in chlorinated water. In contrast, cross-contamination of noninoculated green leaves was significantly reduced (3.3% of subsamples, average MPN Collapse

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MESH Headings

• Bacterial Adhesion
• Chlorine/pharmacology
• Colony Count, Microbial
• Consumer Product Safety
• Escherichia coli O157/drug effects
• Escherichia coli O157/growth & development
• Food Contamination/prevention & control
• Food Handling/methods
• Food Microbiology
• Lactuca/microbiology
• Microbial Sensitivity Tests
• Treatment Outcome
• Water

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Affiliation(s)

Xiangwu Nou U.S. Dept. of Agriculture, Agricultural Research Service, Beltsville Agricultural Research Center, Environmental Microbiology and Food Safety Laboratory, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
Yaguang Luo

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Growth of Listeria spp. in shredded cabbage is enhanced by a mild heat treatment

Mild thermal processing can enhance the shelf life of cut fruits and vegetables by delaying the onset of spoilage and preserving the organoleptic properties of shredded cabbage. However, food safety issues related to this process have not been fully investigated. Therefore, the survival and growth of Listeria spp. on cabbage treated in this manner was examined. Experimentally, 24 strains of Listeria spp. (including L. monocytogenes) were inoculated onto cut and intact cabbage tissues and stored at 5 degrees C. All strains on intact tissues exhibited a moderate decline in numbers (up to 1.0 log CFU/cm(2)) over the 28-day storage period. Conversely, cut tissue supported growth of most strains during the first 7 to 14 days of incubation with maximum increases of 1.2 log CFU/cm(2). Subsequently, the survival or growth on heat-treated (50 degrees C for 3 min) and untreated shredded cabbage of four L. monocytogenes and four nonpathogenic Listeria spp. strains were compared during storage for 21 days at 5 degrees C. Growth on untreated shred for all strains was similar to the results observed on cut tissue with a maximum increase of approximately 1.0 log CFU/g. However, in the heat-treated cabbage shred all strains displayed a rapid increase in growth (up to 2.5 log CFU/g) during the first 7 days of incubation, which may be indicative of the destruction of an endogenous growth-inhibiting compound within the cabbage. In conclusion, this study shows that mild thermal treatments of cut cabbage may promote pathogen growth if other inimical barriers are not implemented downstream of the thermal treatment.

Behavior of Escherichia coli O157:H7 on damaged leaves of spinach, lettuce, cilantro, and parsley stored at abusive temperatures

Recent foodborne illness outbreaks associated with the consumption of leafy green produce indicates a need for additional information on the behavior of pathogenic bacteria on these products. Previous research indicates that pathogen growth and survival is enhanced by leaf damage. The objective of this study was to compare the behavior of Escherichia coli O157:H7 on damaged leaves of baby Romaine lettuce, spinach, cilantro, and parsley stored at three abusive temperatures (8, 12, and 15 degrees C). The damaged portions of leaves were inoculated with approximately 10(5) CFU E. coli O157:H7 per leaf. The pathogen grew on damaged spinach leaves held for 3 days at 8 and 12 degrees C (P < 0.05), with the population increasing by 1.18 and 2.08 log CFU per leaf, respectively. E. coli O157:H7 did not grow on damaged Romaine leaves at 8 or 12 degrees C, but growth was observed after 8 h of storage at 15 degrees C, with an increase of less than 1.0 log. Growth of E. coli O157:H7 on Romaine lettuce held at 8 or 12 degrees C was enhanced when inocula were suspended in 0.05% ascorbic acid, indicating the possibility of inhibition by oxidation reactions associated with tissue damage. Damaged cilantro and Italian parsley leaves held at 8 degrees C for 4 days did not support the growth of E. coli O157:H7. Behavior of the pathogen in leaf extracts differed from behavior on the damaged tissue. This study provides evidence that the damaged portion of a leafy green is a distinct growth niche that elicits different microbial responses in the various types of leafy greens.

Quantitative microbial risk assessment for Escherichia coli O157:H7, salmonella, and Listeria monocytogenes in leafy green vegetables consumed at salad bars

Fresh vegetables are increasingly recognized as a source of foodborne outbreaks in many parts of the world. The purpose of this study was to conduct a quantitative microbial risk assessment for Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes infection from consumption of leafy green vegetables in salad from salad bars in The Netherlands. Pathogen growth was modeled in Aladin (Agro Logistics Analysis and Design Instrument) using time-temperature profiles in the chilled supply chain and one particular restaurant with a salad bar. A second-order Monte Carlo risk assessment model was constructed (using @Risk) to estimate the public health effects. The temperature in the studied cold chain was well controlled below 5 degrees C. Growth of E. coli O157:H7 and Salmonella was minimal (17 and 15%, respectively). Growth of L. monocytogenes was considerably greater (194%). Based on first-order Monte Carlo simulations, the average number of cases per year in The Netherlands associated the consumption leafy greens in salads from salad bars was 166, 187, and 0.3 for E. coli O157:H7, Salmonella, and L. monocytogenes, respectively. The ranges of the average number of annual cases as estimated by second-order Monte Carlo simulation (with prevalence and number of visitors as uncertain variables) were 42 to 551 for E. coli O157:H7, 81 to 281 for Salmonella, and 0.1 to 0.9 for L. monocytogenes. This study included an integration of modeling pathogen growth in the supply chain of fresh leafy vegetables destined for restaurant salad bars using software designed to model and design logistics and modeling the public health effects using probabilistic risk assessment software.

Parameters for determining inoculated pack/challenge study protocols

The National Advisory Committee on Microbiological Criteria for Foods developed guidelines for conducting challenge studies on pathogen inhibition and inactivation studies in a variety of foods. The document is intended for use by the food industry, including food processors, food service operators, and food retailers; federal, state, and local food safety regulators; public health officials; food testing laboratories; and process authorities. The document is focused on and limited to bacterial inactivation and growth inhibition and does not make specific recommendations with respect to public health. The Committee concluded that challenge studies should be designed considering the most current advances in methodologies, current thinking on pathogens of concern, and an understanding of the product preparation, variability, and storage conditions. Studies should be completed and evaluated under the guidance of an expert microbiologist in a qualified laboratory and should include appropriate statistical design and data analyses. This document provides guidelines for choice of microorganisms for studies, inoculum preparation, inoculum level, methods of inoculation, incubation temperatures and times, sampling considerations, and interpreting test results. Examples of appropriately designed growth inhibition and inactivation studies are provided.

Effects of plant maturity and growth media bacterial inoculum level on the surface contamination and internalization of Escherichia coli O157:H7 in growing spinach leaves

The incidence of foodborne outbreaks linked to fresh produce has increased in the United States. Particularly noteworthy was the 2006 Escherichia coli O157:H7 outbreak associated with prepackaged baby spinach. This study aimed to determine whether E. coli O157:H7 would be present in the aerial leaf tissue of a growing spinach plant when introduced at various plant maturities and different inoculum levels in a greenhouse setting. Spinach seeds of a commercial variety were sown in 8-in. (20.32-cm) pots. After seed germination, two levels (10(3) and 10(7) CFU/ml) of an E. coli O157:H7 green fluorescent protein-expressing strain were introduced into the plant growth media weekly for a total of five times. Inoculated spinach plants were examined weekly for the presence of E. coli O157:H7 on leaves and in surrounding growth media. Among 120 spinach plant samples examined for internal leaf contamination, only one yielded a positive result. Surface leaf contamination occurred occasionally and clustered between 3 and 5 weeks of age, but not among leaves younger than 3 weeks of age. On the other hand, when inoculated at the 10(7) CFU/ml level, the E. coli O157:H7 green fluorescent protein strain survived the entire cultivation period, although with gradually reduced levels. The experiments demonstrated that internalization of E. coli O157:H7 of growing spinach plant leaves under greenhouse conditions was a rare event, but surface contamination did occur, primarily when the plants reached 3 weeks of age. The study provided important data to further assess the association between spinach age and potential contamination of E. coli O157:H7.

Fate of Escherichia coli O157:H7 in the presence of indigenous microorganisms on commercially packaged baby spinach, as impacted by storage temperature and time

This study investigated the effect of storage temperature and time on the survival and growth of Escherichia coli O157:H7, the growth of indigenous microorganisms, and the changes in product quality of packaged baby spinach. Commercial packages of spinach within 2 days of processing were cut open at one end, sprayed with fine mists of E. coli O157:H7 inoculum, resealed, and then stored at 1, 5, 8, and 12 degrees C for 12 days until their labeled best-if-used-by dates. Microbial enumeration and product quality evaluation were conducted on day(s) 0, 3, 6, 9, and 12 postinoculation. Spinach held at 12 degrees C supported significant (P < 0.001) E. coli O157:H7 growth, with a 1.0-log CFU/g increase within 3 days postinoculation, which was followed by additional growth during continued storage. E. coli O157:H7 grew slowly when held at 8 degrees C, with a significant (P < 0.01) level of growth reached after 6 days of storage. However, on products held at 1 and 5 degrees C, E. coli O157:H7 populations declined significantly (P < 0.01 and P < 0.001, respectively) within 3 days of storage. Aerobic mesophilic bacteria, psychrotrophic bacteria, and yeast and mold populations increased significantly at all storage temperatures, with more growth on products held at elevated temperatures. Product quality scores remained high within the first 6 days of storage, with a sharp decline noted on samples held at 12 degrees C on day 9. Results suggest that E. coli O157:H7 can grow significantly on commercially packaged spinach held at 8 degrees C or above before significant product quality deterioration occurs.

Inactivation of Escherichia coli O157:H7 on the intact and damaged portions of lettuce and spinach leaves by using allyl isothiocyanate, carvacrol, and cinnamaldehyde in vapor phase

Antimicrobials in the vapor phase might be more effective in inactivating Escherichia coli O157:H7 cells attached to leafy greens than aqueous antimicrobials. We determined the activity of allyl isothiocyanate (AIT), cinnamaldehyde, and carvacrol against E. coli O157:H7 on intact and damaged lettuce and spinach tissue. Samples were treated with various concentrations of antimicrobial in the vapor phase at 0, 4, and 10 degrees C in an enclosed container. On intact lettuce surface, the vapor of the lowest concentration of these antimicrobials inactivated >4 log of E. coli O157:H7 at 0 and 4 degrees C in 4 days and at 10 degrees C in 2 days. However, at the tissue damaged by cutting, the highest concentration reduced the population by 4 log at 0 degrees C and 2 to 4 log at 4 degrees C in 4 days. These concentrations also reduced the population of the pathogen by 1 to 3 log at 10 degrees C in 2 days. The pathogen population on spinach surface was reduced by 1 log less than on lettuce surface. However, reduction of the pathogen within spinach tissue was 2 and 3 log less than within lettuce tissue at 0 and 4 degrees C, respectively. Overall, greater inactivation occurred on lettuce than spinach leaves and on the leaf surfaces than at the damaged area. Using antimicrobials in the vapor phase may improve the safety of refrigerated leafy greens marketed in sealed packages.

Transcriptomic response of Escherichia coli O157:H7 to oxidative stress

Chlorinated water is commonly used in industrial operations to wash and sanitize fresh-cut, minimally processed produce. Here we compared 42 human outbreak strains that represented nine distinct Escherichia coli O157:H7 genetic lineages (or clades) for their relative resistance to chlorine treatment. A quantitative measurement of resistance was made by comparing the extension of the lag phase during growth of each strain under exposure to sublethal concentrations of sodium hypochlorite in Luria-Bertani or brain heart infusion broth. Strains in clade 8 showed significantly (P < 0.05) higher resistance to chlorine than strains from other clades of E. coli O157:H7. To further explore how E. coli O157:H7 responds to oxidative stress at transcriptional levels, we analyzed the global gene expression profiles of two strains, TW14359 (clade 8; associated with the 2006 spinach outbreak) and Sakai (clade 1; associated with the 1996 radish sprout outbreak), under sodium hypochlorite or hydrogen peroxide treatment. We found over 380 genes were differentially expressed (more than twofold; P < 0.05) after exposure to low levels of chlorine or hydrogen peroxide. Significantly upregulated genes included several regulatory genes responsive to oxidative stress, genes encoding putative oxidoreductases, and genes associated with cysteine biosynthesis, iron-sulfur cluster assembly, and antibiotic resistance. Identification of E. coli O157:H7 strains with enhanced resistance to chlorine decontamination and analysis of their transcriptomic response to oxidative stress may improve our basic understanding of the survival strategy of this human enteric pathogen on fresh produce during minimal processing.

Prevention of Escherichia coli cross-contamination by different commercial sanitizers during washing of fresh-cut lettuce

The efficacy of fresh-cut produce sanitizers has mainly been evaluated by measuring microbial reductions on produce. However, its suitability to ensure that pathogens are rapidly killed avoiding cross-contamination of subsequent product also needs to be considered. The efficacy of chlorine, Tsunami, Citrox and Purac on non pathogenic Escherichia coli reductions in processing water and on fresh-cut lettuce were studied. Selection of minimum effective doses was carried out in processing water, which contained a chemical oxygen demand (COD) within the range of 700-1000 mg/l and a total mesophilic load of about 7 log CFU/ml. The processing water was inoculated with two inoculum levels (3 and 5 log CFU/ml). It was observed that 40 mg/l of chlorine and 500 mg/l of Tsunami were effective in reducing the inoculum levels in the processing water to the detection limit (5 and 4 log units). However, Citrox and Purac were not effective in reducing E. coli population even at the highest manufacturer's recommended doses. Evaluation of cross-contamination in fresh-cut lettuce was carried out by measuring E. coli transfer from inoculated (~5 log CFU/g) to uninoculated lettuce after washing the contaminated product in the water containing different sanitizing agents. Chlorine and Tsunami were able to inactivate E. coli in wash water, avoiding cross-contamination between contaminated and non-contaminated product. However, Citrox and Purac at the recommended doses did not prevent transfer of E. coli cells between inoculated and uninoculated fresh-cut lettuce and therefore indicating cross-contamination. The results obtained show that chlorine and Tsunami are recommended as water disinfection agents preventing E. coli cross-contamination of produce during processing.

Impact of preinoculation culture conditions on the behavior of Escherichia coli O157:H7 inoculated onto romaine lettuce (Lactuca sativa) plants and cut leaf surfaces

Inoculum preparation methods can impact growth or survival of organisms inoculated into foods, thus complicating direct comparison of results among studies. The objective of this study was to evaluate preinoculation culture preparation for impact on Escherichia coli O157:H7 inoculated onto leaves of romaine lettuce plants and cut leaf surfaces. E. coli O157:H7 was grown quiescently or shaken at 15, 25, or 37 degrees C to different growth phases in tryptic soy or M9 minimal salts broth or agar. Cells were harvested, washed, and suspended in 0.1% peptone, Milli Q water, or well water and refrigerated for 0 or 18 h. Prepared inoculum was spotted onto cut romaine lettuce (10 microl; 3 x 10(4) CFU/10 g) or onto romaine lettuce plants (20 microl; 3 x 10(6) CFU per leaf). Cut lettuce was sealed in 100-cm2 bags (made from a commercial polymer film) and incubated at 5 or 20 degrees C. Lettuce plants were held at 23 degrees C for 24 h. For all tested conditions, levels of E. coli O157:H7 increased at 20 degrees C on cut lettuce and decreased on cut lettuce stored at 5 degrees C or on leaves of lettuce plants. At 20 degrees C, preinoculation culture conditions had little impact on growth of E. coli O157:H7 on cut lettuce. However, survival at 5 degrees C was significantly better (P < 0.05) for cultures grown at 15 or 37 degrees C in minimal medium and to late stationary phase. Impact of preinoculation handling on survival on lettuce plants was less clear due to relatively high standard deviations observed among samples.

Use of the systems approach to determine the fate of Escherichia coli O157:H7 on fresh lettuce and spinach

Lettuce and spinach inoculated with Escherichia coli O157:H7 were processed and handled in ways that might occur in commercial situations, including variations in holding times before and after product cooling, transportation conditions and temperatures, wash treatments, and product storage temperatures and times. Populations of background microflora and E. coli O157:H7 were enumerated after each step in the system. Data analysis was done to predict response variables with a combination of independent categorical variables. Field temperature, time before cooling, and wash treatment significantly affected E. coli O157:H7 populations on both products. The lowest populations of E. coli O157:H7 were encountered when precool time was minimal, lettuce was washed with chlorine, and storage temperature was 4 degrees C. For lettuce, field and transportation temperature were not important once the storage period started, whereas after 2 days E. coli O157:H7 populations on packaged baby spinach were not affected by field temperature. On chopped iceberg lettuce and whole leaf spinach that was packaged and stored at 4 degrees C, E. coli O157:H7 contamination could still be detected after typical handling practices, although populations decreased from initial levels in many cases by at least 1.5 log units. In abusive cases, where populations increased, the product quality quickly deteriorated. Although E. coli O157:H7 levels decreased on products handled and stored under recommended conditions, survivors persisted. This study highlights practices that may or may not affect the populations of E. coli O157:H7 on the final product.

Efficacy of sodium hypochlorite and peroxyacetic acid to reduce murine norovirus 1, B40-8, Listeria monocytogenes, and Escherichia coli O157:H7 on shredded iceberg lettuce and in residual wash water

The efficiency of sodium hypochlorite (NaOCl) and peroxyacetic acid (PAA) to reduce murine norovirus 1 (MNV-1), a surrogate for human norovirus, and Bacteroides fragilis HSP40-infecting phage B40-8 on shredded iceberg lettuce was investigated. The levels of removal of viruses MNV-1 and B40-8 were compared with the reductions observed for bacterial pathogens Listeria monocytogenes and Escherichia coli O157:H7. Two inoculation levels, one with a high organic load and the other containing a 10-fold lower number of pathogens and organic matter, showed that the effectiveness of NaOCl was greatly influenced by the presence of organic material, which was not observed for PAA. Moreover, the present study showed that 200 mg/liter NaOCl or 250 mg/liter PAA is needed to obtain an additional reduction of 1 log (compared with tap water) of MNV-1 on shredded iceberg lettuce, whereas only 250 mg/liter PAA achieved this for bacterial pathogens. None of the treatments resulted in a supplementary 1-log PFU/g reduction of B40-8 compared with tap water. B40-8 could therefore be useful as an indicator of decontamination processes of shredded iceberg lettuce based on NaOCl or PAA. Neither MNV-1, B40-8, nor bacterial pathogens could be detected in residual wash water after shredded iceberg lettuce was treated with NaOCl and PAA, whereas considerable numbers of all these microorganisms were found in residual wash water consisting solely of tap water. This study illustrates the usefulness of PAA and NaOCl in preventing cross-contamination during the washing process rather than in causing a reduction of the number of pathogens present on lettuce.

Persistence of Escherichia coli on injured iceberg lettuce in the field, overhead irrigated with contaminated water

Fresh produce is increasingly implicated in food-related illnesses. Escherichia coli can survive in soil and water and can be transferred onto plant surfaces through farm management practices such as irrigation. A trial was conducted to evaluate the impact of field conditions on E. coli persistence on iceberg lettuce irrigated with contaminated water, and the impact of plant injury on the persistence of E. coli. Lettuce heads were injured at 14, 7, 3, 2, 1, and 0 days before inoculation, with uninjured heads used as a control. All lettuce heads (including controls) were overhead irrigated with a mixture of nonpathogenic E. coli strains (10(7) CFU/ml). E. coli counts were measured on the day of inoculation and 5 days after, and E. coli was detected on all lettuce head samples. Injury immediately prior to inoculation and harvest significantly (P = 0.00067) increased persistence of E. coli on lettuce plants. Harsh environmental conditions (warm temperatures, limited rainfall) over 5 days resulted in a 2.2-log reduction in E. coli counts on uninjured lettuce plants, and lettuce plants injured more than 2 days prior to inoculation had similar results. Plants with more recent injuries (up to 2 days prior to inoculation) had significantly (P = 7.6 x 10(-6)) greater E. coli persistence. Therefore, growers should postpone contaminated water irrigation of lettuce crops with suspected injuries for a minimum of 2 days, or if unavoidable, use the highest microbiological quality of water available, to minimize food safety risks.