Microbial food safety issues are rare events and tracking the source of disease outbreaks is extremely complex, making it difficult to predict or determine their cause . Thus, the best way to minimize these events is to perform risk assessment analyses . As discussed above, it has become evident that the plant is not a passive vehicle for microbial food hazards, hence providing opportunities to breed crops for enhanced food safety. The challenge remains to identify effective traits and genetic variability useful for breeding. It has long been possible to breed plant germplasm that is resistant to plant pathogens. For example, the Fusarium pathogen synthesizes toxic DON and/or fumonisins and reduces seed set and fill in wheat; Aspergillus flavus can cause ear rots of maize in environmental conditions suitable for fungal growth. In both cases, these fungi can reduce plant yield and germplasm resistant to these pathogens is available . However, in cases where the fitness of the plant is not as directly reduced by the presence of the pathogen, traits that could potentially increase food safety may be harder to find and may require indirect or more creative solutions. They also compete with priorities for crop production and quality in breeding programs. Edible plants carrying human pathogens generally do not show visual symptoms as they would when infected with plant pathogens, particularly when they occur at low levels. This creates a challenge in developing screening assays to identify phenotypes with useful variation to support breeding efforts. Unlike the challenges associated with microbial hazards,maceta 7 litros detection of elements such as nitrates or heavy metals is relatively easy with standard tissue analysis.
Allergens can often be detected by routine assays . However, for human pathogens, rapid and cost-effective assays still need to be developed for routine screening of breeding populations, although some efforts have been made in this direction 9 . These assays will allow large scale assessment of germplasm to find the best expression of useful traits and their introgression into cultivated varieties. Despite the challenges, variations in human pathogen colonization of lettuce, tomato, and spinach genotypes have already been determined. An additional hurdle comes from the fact that microbial colonization is a complex behavior influenced by the plant host– pathogen combination and crop management practice such as irrigation type and crop fertilization . Human pathogen–plant models should be developed for the purpose of breeding efforts to enhance food safety based on enteric pathogen strain– plant commodity variety pairs identified from prominent or recurring foodborne illness outbreaks. At the same time, plant genetic resources that may facilitate genome-wide association studies should not be excluded. Furthermore, the use of human pathogens in routine assays requires highly trained personnel and laboratory/greenhouse bio-safety conditions according to NIH guidelines, in addition to considerable costs associated with the handling of microbial hazards in contained facilities. These approaches will require collaborative efforts among food safety experts, plant–microbe interaction biologists, microbiologists, and crop breeders for successful advancements in the field. to enhance food safety based on enteric pathogen strain– plant commodity variety pairs identified from prominent or recurring foodborne illness outbreaks.
At the same time, plant genetic resources that may facilitate genome-wide association studies should not be excluded. Furthermore, the use of human pathogens in routine assays requires highly trained personnel and laboratory/greenhouse bio-safety conditions according to NIH guidelines, in addition to considerable costs associated with the handling of microbial hazards in contained facilities. These approaches will require collaborative efforts among food safety experts, plant–microbe interaction biologists, microbiologists, and crop breeders for successful advancements in the field. to enhance food safety based on enteric pathogen strain– plant commodity variety pairs identified from prominent or recurring foodborne illness outbreaks. At the same time, plant genetic resources that may facilitate genome-wide association studies should not be excluded. Furthermore, the use of human pathogens in routine assays requires highly trained personnel and laboratory/greenhouse bio-safety conditions according to NIH guidelines, in addition to considerable costs associated with the handling of microbial hazards in contained facilities. These approaches will require collaborative efforts among food safety experts, plant–microbe interaction biologists, microbiologists, and crop breeders for successful advancements in the field. The growth of the human population places an ever-increasing demand on freshwater resources and food supply. The nexus of water and food is now well recognized. One promising strategy to sustain food production in the face of competing water demands is to increase the reuse of treated human wastewater. Municipal wastewater reuse for food production has been successfully adopted in some regions of the world. For example, Israel uses ~84% treated wastewater in agriculture production . However, Southern California, a region that suffers from a similar degree of water shortage, currently uses less than ~3% of municipal wastewater in agriculture, while discharging ~1.5 million acre-feet effluent per year into the Pacific Ocean . Secondary municipal waste water effluent for ocean discharge is often sufficient to support both the nutrient and water needs for food production.
Water reuse in agriculture can bring municipal water reclamation effluent to nearby farms within the city limit,vertical grow rack thus promoting local agriculture and also reducing the rate of farmland loss to urban development. While the use of reclaimed water in agriculture offers a multitude of societal and agronomical benefits, broader adoption faces great challenges. One of the important challenges is ensuring the safety of food products in light of a plethora of human pathogens that may be present in recycled wastewater. Past studies have identified risks associated with irrigating food with recycled wastewater through the retention of the irrigation water on edible plant surfaces during overhead irrigation . With the emphasis on water conservation and reduction of evapotranspiration, subsurface drip irrigation is gaining popularity . Since there is lesser contact between water and the plant surface, the chance of surface contamination of pathogens is reduced. However, this new practice presents risk of uptake of microbial pathogens into plants. Such internalized pathogens are of greater concerns as washing, even with disinfectants, may not affect pathogens sheltered in the vasculature. Although pathogen transport through root uptake and subsequent internalization into the plant has been a growing research area, results vary due to differences in experimental design, systems tested, and pathogens and crops examined . Among the array of pathogens causing foodborne illness that may be carried by treated wastewater, viruses are of the greatest concern but least studied. According to the CDC, 60% of U.S. foodborne outbreaks associated with eating leafy greens were caused by noroviruses , while Salmonella and E. coli only accounted for 10% of the outbreaks . Estimates of global foodborne illness prevalence associated with NoV sur pass all other pathogens considered . Viruses are also of concern because they persist in secondary wastewater effluents in high concentrations . They do not settle well in sedimentation basins and are also more resistant to degradation than bacteria . Therefore, in the absence of solid scientific understanding of the risks involved, the public are likely less receptive to adopting treated wastewater for agricultural irrigation. NoV internalization in hydroponic systems has been quantified by DiCaprio et al. . Internalization in crops grown in soil is considered lesser but nevertheless occurs. However, the only risk assessment that considered the possibility of NoV internalization in plants assumed a simple ratio of viruses in the feed water over viruses in produce at harvest to account for internalization. The time dependence of viral loads in lettuce was not explored and such an approach did not permit insights into the key factors influencing viral uptake in plants. In this study, we introduce a viral transport model to predict the viral load in crisp head lettuce at harvest given the viral load in the feed water. It is parameterized for both hydroponic and soil systems. We demonstrate its utility by performing a quantitative microbial risk assessment . Strategies to reduce risk enabled by such a model are explored, and a sensitivity analysis highlights possible factors affecting risk.
Some parameters to complete the conceptual viral transport model were obtained from the literature. Others were estimated by fitting the model to published data from experiments using NoV seeded feed water to grow crisp head lettuces in a hydroponic system . The initial volume of 800 mL for the hydroponic growth medium was adopted based on these experiments.For the soil system, the volume of the growth medium equals the volume of water contained in the soil interstitial spaces in an envelope around the roots. This envelope is a region around the roots that the plant is assumed to interact with. Vg, s is given by Eq. 17, where θ is the volumetric water content obtained from Clapp and Hornberger . Estimates for Ve spanned a large range and a middle value of Ve = 80000 cm3 was adopted and assumed to be constant over the lettuce growth period. This assumed value was also verified to have minimal impact on the model outcome .The plant transpiration rate was adopted as the viral transport rate ) based on: 1) previous reports of passive bacterial trans port in plants , 2) the significantly smaller size of viruses compared tobacte ria, and 3) the lack of known specific interactions between human vi ruses and plant hosts . Accordingly, viral transport rate in hydroponically grown lettuce was deter mined from the previously reported transpiration model , in which the transpiration rate is proportional to the lettuce growth rate and is influenced by cultivar specific factors . These cultivar specific factors used in our model were predicted using the hydroponic crisp head lettuce growth experiment carried out by DiCaprio et al. described in Section 2.3 . Since the transpiration rate in soil grown lettuce is significantly higher than that in the hydroponic system, viral trans port rate in soil grown lettuce was obtained directly from the graphs published by Gallardo et al. using WebPlotDigitizer .In the absence of a published root growth model for lettuce in soil, a fixed root volume of 100 cm3 was used. In the viral transport model, viral transfer efficiency was used to account for the potential “barrier” between each compartment . The existence of such a “barrier” is evident from field experiments where some microbial pathogens were inter nalized in the root but not in the shoot of plants . In addition, viral transfer efficiencies also account for differing observations in pathogen internalization due to the type of pathogen or lettuce. For example, DiCaprio et al. reported the internalization of NoV into lettuce, while Urbanucci et al. did not detect any NoV in another type of lettuce grown in feed water seeded with viruses. The values of ηgr and ηrs were deter mined by fitting the model to experimental data reported by DiCaprio et al. and is detailed in Section 2.3. The values of ηgr and ηrs predicted for the hydroponic lettuce were assumed in the soil case. The viral removal in the growth medium includes both die-off and AD, while only natural die-off was considered in the lettuce root and shoot. AD kinetic constants as well as the growth medium viral decay constant in the hydroponic case were obtained by fitting the model to the data from DiCaprio et al. . Viral AD in soil has been investigated in both lab scale soil columns and field studies . In our model, viral AD constants in soil were obtained from the experiments of Schijven et al. , who investigated MS2 phage kinetics in sandy soil in field experiments. As the MS2 phage was transported with the water in soil, the AD rates changed with the distance from the source of vi ruses. To capture the range of AD rates, two scenarios of viral behavior in soils were investigated. Scenario 1 used the AD rates estimated at the site closest to the viral source , while scenario 2 used data from the farthest site . In contrast to lab scale soil column studies, field studies provided more realistic viral removal rates . Using surrogate MS2 phage for NoV provided conservative risk estimates since MS2 attached to a lesser extent than NoV in several soil types .