Scientifically valid corrective actions for multiple harvest shade-house production systems

From 1996 to 2016, multiple outbreaks associated with consumption of shade-house or greenhouse grown fresh produce, resulting in over 1200 illnesses, 260 hospitalizations and at least six deaths, were reported. Outbreak environmental investigations boldly underscored the need for science-based practices to prevent or respond to a detected contamination on multiple-harvest crops produced in enclosed cropping systems (protected culture). A common response to pathogen detection may be to destroy the remaining crop as the practical economic loss containment decision; however, a better knowledge foundation for pathogen die-off expectations and development of systematic sampling regimes has broad industry support. At this time, there is very sparse science-based guidance for assessing the risk of contamination of fresh produce grown under protected culture. Closing this knowledge gap is critical to decision-making and application of validated corrective actions in the case of pathogen detection in product or environmental samples. Our specific goal is the validation of die-off expectations for bacterial pathogens and corrective action options for shade-house grown crops. We propose also to evaluate corrective actions necessary to minimize the risk of transference and persistence of bacterial pathogens in the shade-house. We anticipate a high degree of transferability to diverse protected crop culture systems.

From 1996 to 2016, seven events of foodborne illness associated with the consumption of raw cucumbers have been reported. Six of the outbreaks were attributed to Salmonella spp., and one to E. coli, resulting in over 1200 illnesses and 260 hospitalizations. Outbreaks in the five most recent incidents attributed to cucumbers were grown in protected culture (3 Salmonella and 1 E. coli O157:H7) and one Salmonella outbreak in an open farm environment. In the final update of the large multistate outbreak of Salmonella enterica sv. Poona associated with consumption of fresh cucumber, CDC reported that more than 907 people from 40 states were infected with the outbreak strains of Salmonella Poona. Of these, 204 ill people were hospitalized, and six deaths directly attributed to the infection. The implicated cucumbers were grown in protected culture, specifically shade-houses. The outcomes of the on-going outbreak environmental investigation of S. Poona on cucumber boldly underscored the need to develop a specific science-basis for the critical decisions necessary by any shade-house producer and affiliated shipper to prevent or respond and recover in the event of a detected contamination on a multiple harvest crop. Similar to open-field poled or staked production systems, most protected culture involves crops with multiple harvests over at least a few months period. Within these production units, each standing crop at harvest maturity, and each harvested lot in distribution, is intimately linked to the remaining crop. A common response to a positive postharvest pathogen detection may be to destroy the remaining shade- house production as the practical economic loss containment decision; however, a better knowledge foundation for die-off expectations and systematic sampling regime may shift the risk burden to packing operations. At this time, there is very sparse science-based guidance for systematically assessing the risk of contamination of fresh produce grown under protected culture. Closing this knowledge gap is critical to decision-making and application of validated corrective actions in the case of presumptive or confirmed pathogen detection in preharvest product or environmental samples. Our goal is the validation of die-off expectations for bacterial pathogens and corrective action options for shade-house crops. To accomplish this goal we propose to determine the comparative die-off kinetics of indicator E. coli, attenuated Salmonella, attenuated shiga-toxin E. coli, and indicator Listeria innocua contaminants in controlled research shade-houses. We propose also to evaluate the efficacy of various corrective actions to minimize the risk of transference and persistence of bacterial pathogens within and on the standing crop. Environmental monitoring guidance will result from studies that comparatively evaluate sampling protocols, sample processing protocols, and detection sensitivity for bacterial pathogens. Overall, we proposed to elucidate system-wide improvements in preventive controls, environmental monitoring programs, and recall-response management for shade-house grown fruit vegetables and the development of science-based responses to routine monitoring and early detection of contamination events. We anticipate a high degree of transferability to other protected crop culture systems and multiple harvest crops.

1. Determine the die-off kinetics of E. coli, EHEC*, Salmonella, and indicator Listeria*, in controlled research protected culture, on cucumber, roma-type tomato, and jalapeño peppers following simulated leaf and fruit contamination from irrigation and soil surface and other shadehouse environmental sources, including trellising and harvest activities. 

2. Comparatively evaluate methods, sampling protocols, sample processing protocols, and sensitivity of detection of bacterial pathogens in preharvest testing designed for shadehouse EMPs. 

3. Evaluate the efficacy of various corrective actions** to minimize the risk of transference and minimize persistence of bacterial pathogens within and on the standing crop. 

NOTE: Objectives 1 and 3 were modified in spring 2019 due to reductions in technical staff and farm management challenges with this project. * For Objective 1, enterohemorrhagic E. coli (EHEC) and indicator Listeria spp. were removed from the inoculation’s matrix. ** For Objective 3, all corrective action experiments focus on shadehouse crops and exclude field trials.

• This study provides science-based die-off kinetics data of E. coli and Salmonella under field and protected production (hoop houses) on cucumbers, tomatoes and jalapeno peppers. 

• Recovery of both E. coli and Salmonella from crops growing in the open-field environment showed a rapid decline within the first day after inoculation; however surviving bacteria may establish and persist at very low cell numbers that require concentration or enrichment for their detection (Figure 10). 

• Results from studies on the behavior of E. coli and Salmonella inoculated onto cucumber, tomato and jalapeno plants in hoop houses showed that given the conditions (temperature and high humidity in a closed system resulting in condensation) bacteria cannot only survive but may grow on the crops (Figure 11). 

• Transfer of E. coli and Salmonella from utensils (bins and clippers) and human hands (gloves) to the fruit was observed frequently on inoculation day; however, in experiments conducted within hoop houses the transference was observed until 8 days post inoculation. While the risk of contamination in commercial shadehouses will likely be reduced as compared to this study, these findings show the potential for pathogens to survive/transfer in protected environments. 

• Preharvest treatment can reduce significantly the number of viable bacteria, as observed using 1% Oxidate 5.0 with a 24-h contact time. 

• Salmonella contamination surviving to postharvest phases can be increased by waxing and MAP bags. Wash-line injuries (brushed cucumbers) were shown to have the potential to enhance Salmonella survival and growth on non-waxed and non-MAP bagged cucumbers. 

• If contamination occurs in protected environments, such as shadehouses, crosscontamination of produce is likely to continue to be a food safety concern.