Environmental inactivation and irrigation-mediated regrowth of Escherichia coli O157:H7 on romaine lettuce when inoculated in a fecal slurry matrix

Pre-harvest biocontrol of Listeria and Escherichia coli O157 on lettuce and spinach by lactic acid bacteria

Recent outbreaks linked to contaminated leafy greens underline the need for identifying effective natural approaches to improve produce safety at pre-harvest level. Lactic acid bacteria (LAB) have been evaluated as biocontrol agents in food products. In this study, the efficacy of a cocktail of LAB including Lactococcus lactis, Lactiplantibacillus plantarum, Lactobacillus johnsonii, and Lactobacillus acidophilus as pre-harvest biocontrol agents against Listeria and Escherichia coli O157 on lettuce and spinach was investigated. Bacterial pathogens L. monocytogenes and E. coli O157:H7 and the non-pathogenic surrogates L. innocua and E. coli O157:H12 were used to spray-inoculate cultivars of lettuce and spinach grown in growth chamber and in field, respectively. Inoculated plants were spray-treated with water or a cocktail of LAB. On day 0, 3, and 5 post-inoculation, four samples from each group were collected and bacterial populations were determined by serial dilution and spiral plating on selective agars. LAB treatment exhibited an immediate antimicrobial efficacy against L. monocytogenes and E. coli O157:H7 on "Green Star" lettuce by ~2 and ~ 1 log reductions under growth chamber conditions, respectively (P < 0.05). The effect of LAB against E. coli O157:H7 on "New Red Fire" lettuce remained effective during the 5-day period in growth chamber (P < 0.05). Treatment of LAB delivered an effective bactericidal effect against E. coli O157:H12 immediately after application on the field-grown lettuce plants (P < 0.05). Approximately 1 log L. innocua reduction was observed on "Matador" and "Palco" spinach on day 5 (P < 0.05). Results of this study support that LAB could potentially be applied as biocontrol agents for controlling Listeria and E. coli O157 contamination on leafy greens at the pre-harvest level.

Bacterial Detection and Recovery From Poultry Litter

The level of pathogens in poultry litter used for raising broiler chickens is critical to the overall health of a broiler chicken flock and food safety. Therefore, it is imperative that methods used for determining bacterial concentration in litter are accurate and reproducible across studies. In this perspective, we discuss the shortcomings associated with current methods used for bacterial quantification and detection from litter and assess the efficacy of one method for pathogen and commensal (Campylobacter, Salmonella, Escherichia coli, and Enterococcus spp.) recovery. The limit of quantitation and detection for this method differed between pathogens, and the recovery rate (∼138–208%) was higher for Salmonella, E. coli, and Enterococcus compared to Campylobacter (24%). Our results suggest that pathogen recovery from litter is highly variable and pathogen concentrations need to be reported in dry weight before comparisons can be made between studies.

Persistence of Escherichia coli O157:H12 and Escherichia coli K12 as Non-pathogenic Surrogates for O157:H7 on Lettuce Cultivars Irrigated With Secondary-Treated Wastewater and Roof-Collected Rain Water in the Field

Treated wastewater (TW) and roof-collected rain water (RW) that meet the required microbial quality as per Food Safety Modernization Act (FSMA) regulation may serve as alternative irrigation water sources to decrease the pressure on the current water scarcity. Alternative water sources may have different water characteristics that influence the survival and transfer of microorganisms to the irrigated produce. Further, these water sources may contain pathogenic bacteria such as Shiga-toxigenic Escherichia coli. To evaluate the risk associated with TW and RW irrigation on the fresh produce safety, the effect of TW and RW irrigation on the transfer of two non-pathogenic E. coli strains as surrogates for E. coli O157:H7 to different lettuce cultivars grown in the field was investigated. Lettuce cultivars “Annapolis,” “Celinet,” and “Coastline” were grown in the field at the Fulton farm (Chambersburg, PA). Approximately 10 days before harvest, lettuce plants were spray-irrigated with groundwater (GW), TW, or RW containing 6 log CFU ml−1 of a mixture of nalidixic acid-resistant E. coli O157:H12 and chloramphenicol-resistant E. coli K12 in fecal slurry as non-pathogenic surrogates for E. coli O157:H7. On 0, 1, 3, 7, and 10 days post-irrigation, four replicate lettuce leaf samples (30 g per sample) from each group were collected and pummeled in 120 ml of buffered peptone water for 2 min, followed by spiral plating on MacConkey agars with antibiotics. Results showed that the recovery of E. coli O157:H12 was significantly greater than the populations of E. coli K12 recovered from the irrigated lettuce regardless of the water sources and the lettuce cultivars. The TW irrigation resulted in the lowest recovery of the E. coli surrogates on the lettuce compared to the populations of these bacteria recovered from the lettuce with RW and GW irrigation on day 0. The difference in leaf characteristics of lettuce cultivars significantly influenced the recovery of these surrogates on lettuce leaves. Populations of E. coli O157:H12 recovered from the RW-irrigated “Annapolis” lettuce were significantly lower than the recovery of this bacterium from the “Celinet” and “Coastline” lettuce (P &lt; 0.05). Overall, the recovery of specific E. coli surrogates from the RW and TW irrigated lettuce was comparable to the lettuce with the GW irrigation, where GW served as a baseline water source. E. coli O157:H12 could be a more suitable surrogate compared to E. coli K12 because it is an environmental watershed isolate. The findings of this study provide critical information in risk assessment evaluation of RW and TW irrigation on lettuce in Mid-Atlantic area.

Effect of Weather on the Die-Off of Escherichia coli and Attenuated Salmonella enterica Serovar Typhimurium on Preharvest Leafy Greens following Irrigation with Contaminated Water

The Food Safety Modernization Act (FSMA) includes a time-to-harvest interval following the application of noncompliant water to preharvest produce to allow for microbial die-off. However, additional scientific evidence is needed to support this rule. This study aimed to determine the impact of weather on the die-off rate of Escherichia coli and Salmonella on spinach and lettuce under field conditions. Standardized, replicated field trials were conducted in California, New York, and Spain over 2 years. Baby spinach and lettuce were grown and inoculated with an ∼104-CFU/ml cocktail of E. coli and attenuated Salmonella Leaf samples were collected at 7 time points (0 to 96 h) following inoculation; E. coli and Salmonella were enumerated. The associations of die-off with study design factors (location, produce type, and bacteria) and weather were assessed using log-linear and biphasic segmented log-linear regression. A segmented log-linear model best fit die-off on inoculated leaves in most cases, with a greater variation in the segment 1 die-off rate across trials (-0.46 [95% confidence interval {95% CI}, -0.52, -0.41] to -6.99 [95% CI, -7.38, -6.59] log10 die-off/day) than in the segment 2 die-off rate (0.28 [95% CI, -0.20, 0.77] to -1.00 [95% CI, -1.16, -0.85] log10 die-off/day). A lower relative humidity was associated with a faster segment 1 die-off and an earlier breakpoint (the time when segment 1 die-off rate switches to the segment 2 rate). Relative humidity was also found to be associated with whether die-off would comply with FSMA's specified die-off rate of -0.5 log10 die-off/day.IMPORTANCE The log-linear die-off rate proposed by FSMA is not always appropriate, as the die-off rates of foodborne bacterial pathogens and specified agricultural water quality indicator organisms appear to commonly follow a biphasic pattern with an initial rapid decline followed by a period of tailing. While we observed substantial variation in the net culturable population levels of Salmonella and E. coli at each time point, die-off rate and FSMA compliance (i.e., at least a 2 log10 die-off over 4 days) appear to be impacted by produce type, bacteria, and weather; die-off on lettuce tended to be faster than that on spinach, die-off of E. coli tended to be faster than that of attenuated Salmonella, and die-off tended to become faster as relative humidity decreased. Thus, the use of a single die-off rate for estimating time-to-harvest intervals across different weather conditions, produce types, and bacteria should be revised.

A Predictive Model for Survival of Escherichia coli O157:H7 and Generic E. coli in Soil Amended with Untreated Animal Manure

This study aimed at developing a predictive model that captures the influences of a variety of agricultural and environmental variables and is able to predict the concentrations of enteric bacteria in soil amended with untreated Biological Soil Amendments of Animal Origin (BSAAO) under dynamic conditions. We developed and validated a Random Forest model using data from a longitudinal field study conducted in mid-Atlantic United States investigating the survival of Escherichia coli O157:H7 and generic E. coli in soils amended with untreated dairy manure, horse manure, or poultry litter. Amendment type, days of rain since the previous sampling day, and soil moisture content were identified as the most influential agricultural and environmental variables impacting concentrations of viable E. coli O157:H7 and generic E. coli recovered from amended soils. Our model results also indicated that E. coli O157:H7 and generic E. coli declined at similar rates in amended soils under dynamic field conditions.The Random Forest model accurately predicted changes in viable E. coli concentrations over time under different agricultural and environmental conditions. Our model also accurately characterized the variability of E. coli concentration in amended soil over time by providing upper and lower prediction bound estimates. Cross-validation results indicated that our model can be potentially generalized to other geographic regions and incorporated into a risk assessment for evaluating the risks associated with application of untreated BSAAO. Our model can be validated for other regions and predictive performance also can be enhanced when data sets from additional geographic regions become available.

Shiga Toxin-Producing Escherichia coli (STEC) O157:H7 and Romaine Lettuce: Source Labeling, Prevention, and Business

From May to November most romaine lettuce shipments in the United States come from California's Central Coast region, whereas from December to April most come from the Yuma, Arizona, region. During 2017-2018, the 3 outbreaks of Shiga toxin-producing Escherichia coli (STEC) O157:H7 in US romaine lettuce all occurred at the tail end of a region's production season. During the fall 2018 outbreak, the US Food and Drug Administration (FDA) recommended that suppliers begin labeling romaine packaging so that consumers can identify the product's harvest region.We used publicly available data to show that labels may not avert many illnesses in future outbreaks but may reduce suppliers' financial losses and reduce food loss.Market data available during both 2018 outbreak investigations showed that there was no romaine production from one of the 2 regions when the first illness onset occurred. That is, at the beginning of an outbreak investigation, market data may allow the FDA to quickly rule out an entire production region as a source of contamination.

Experimental In-Field Transfer and Survival of Escherichia coli from Animal Feces to Romaine Lettuce in Salinas Valley, California

This randomized controlled trial characterized the transfer of E. coli from animal feces and/or furrow water onto adjacent heads of lettuce during foliar irrigation, and the subsequent survival of bacteria on the adaxial surface of lettuce leaves. Two experiments were conducted in Salinas Valley, California: (1) to quantify the transfer of indicator E. coli from chicken and rabbit fecal deposits placed in furrows to surrounding lettuce heads on raised beds, and (2) to quantify the survival of inoculated E. coli on Romaine lettuce over 10 days. E. coli was recovered from 97% (174/180) of lettuce heads to a maximal distance of 162.56 cm (5.33 ft) from feces. Distance from sprinklers to feces, cumulative foliar irrigation, and lettuce being located downwind of the fecal deposit were positively associated, while distance from fecal deposit to lettuce was negatively associated with E. coli transference. E. coli exhibited decimal reduction times of 2.2 and 2.5 days when applied on the adaxial surface of leaves within a chicken or rabbit fecal slurry, respectively. Foliar irrigation can transfer E. coli from feces located in a furrow onto adjacent heads of lettuce, likely due to the kinetic energy of irrigation droplets impacting the fecal surface and/or impacting furrow water contaminated with feces, with the magnitude of E. coli enumerated per head of lettuce influenced by the distance between lettuce and the fecal deposit, cumulative application of foliar irrigation, wind aspect of lettuce relative to feces, and time since final irrigation. Extending the time period between foliar irrigation and harvest, along with a 152.4 cm (5 ft) no-harvest buffer zone when animal fecal material is present, may substantially reduce the level of bacterial contamination on harvested lettuce.